(PDF 6 MB)
Technical guide
Solar thermal systems
Technical guide
Solar thermal systems
Viessmann Werke
D-35107 Allendorf (Eder)
Tel. +49 6452 70-0
Fax +49 6452 70-2780
9449 829 GB 05/2009
Subject to technical modifications
Technical guide
Solar thermal systems
Table of contents
Table of contents
Application information
A.1 Potential benefits of utilising insolation thermally
A.1.1 Sun – source of radiation
A.1.2 Available radiation on earth
Fundamental parameters of collector systems
Collector efficiency
Idle temperature
Collector output
Collector yield
Solar coverage
Fundamental differences between operating modes
Pressurised system with antifreeze
Pressurised system with thermal frost protection
Drainback system
Table of contents
Construction and function
Area designations
Quality and certification
Selecting a suitable collector type
Aspects concerning collector fixing
Collectors as design features
Why store energy?
Principles of cylinder technology
Cylinder types
Cylinder heating
Heat exchangers/indirect coils
Primary circuit
Collector circuit
Heat transfer medium
Stagnation and safety equipment
System selection and sizing
Designing/engineering the collector array
Layout of single array systems
Layout of multi-array systems
Collector arrays with different orientation
Sizing a system for solar DHW heating
Sizing a system for solar central heating backup
Utilisation profiles in commercial applications
Swimming pool water heating
Cooling with solar backup
High temperature applications
136 C.3 Combinations with renewables
137 C.3.1 Solar thermal systems in combination with biomass boilers
138 C.3.2 Solar thermal systems in combination with heat pumps
140 C.4
System simulation with ESOP
144 D
Solar controllers
146 D.1
Solar controller functions
147 D.1.1 Standard functions
149 D.1.2 Auxiliary functions
154 D.2 Checking function and yield
155 D.2.1 Checking function
156 D.2.2 Checking the yield
160 E
System operation
Commissioning and maintenance
Pressure inside the solar thermal system
Preparing for commissioning
Commissioning steps
Maintenance of heat transfer medium containing glycol
172 E.2
Condensation in flat-plate collectors
176 Appendix
178 Viability considerations
182 Information regarding large system tenders
184 Information regarding the Energy Savings Order (EnEV) [Germany]
186 Keyword index
190 The Viessmann Group
192 The comprehensive product range from Viessmann
194 Production
This manual describes and explains essential principles for the engineering,
installation and operation of solar thermal systems. It is designed as a reference
guide, as a document for basic and advanced training, and it can also provide support
during consultations.
Application information
The selection and extent to which subjects
are discussed here are restricted to areas
that are relevant to planning/engineering –
information regarding their translation in
practical installations is included if this is
specifically relevant to the installation of
a solar thermal system. For example, the
"Pipework" section refers exclusively to
solar-specific subjects, such as longitudinal
expansion or protecting the thermal insulation
on the roof. Instructions regarding soldering
the solar circuit are therefore not included.
The illustrations in this manual serve to
increase the understanding of individual
components, the hydraulics and the control
of a solar thermal system, thereby making
the engineering decision in favour of a
specific system easier. For that reason, many
illustrations are shown as schematic drawings
and concentrate on what is essential.
For day to day information, this guide is
supplemented by the familiar Viessmann
product documentation. Datasheets with
process information, precise dimensions
or output details regarding specific
components are available for engineering
purposes. Similarly, there are also complete
system schemes with all the necessary
fittings. Some engineering steps in this text
refer to the availability of electronic aids
that are available on the internet at
The global energy situation is characterised
by finite natural gas and mineral oil reserves,
simultaneously increasing consumption and
drastically rising prices. Furthermore, ever
increasing CO2 emissions are heating up the
atmosphere, leading to dangerous climate
change. This forces us to handle energy
responsibly. We need greater efficiency
and an increase in the use of renewables.
The heating sector is the largest consumer
of energy. It can therefore make a major
contribution towards essential energy
savings and CO2 reduction through the use of
innovative and efficient heating technology.
The comprehensive product range from
Viessmann includes system solutions for every
type of energy that keep the use of resources
for reliable and convenient heat provision to a
minimum and protect the environment through
a reduction of CO2 emissions. Whether with
a condensing boiler for oil or gas, with a pellet
boiler or heat pump – the ideal supplement to
every heat source is a solar thermal system for
DHW heating and central heating backup.
A solar thermal system can provide approx.
60% of the energy required each year for
heating domestic hot water. Solar thermal
systems that can also provide central heating
backup, reduce energy costs further still.
Such systems can save up to 35% of the
annual costs for DHW and central heating.
The integration of solar thermal systems
requires precisely matched individual
components to achieve optimum heat yield
and to keep costs under control. This must be
supported by the right system engineering.
Viessmann began the development and
manufacture of powerful systems for the
utilisation of solar energy more than 30 years
ago, and can therefore call on extensive
experience. We would like you to benefit
from this experience through this compact
technical guide.
In selecting our subjects, we have given
priority to the reliability of engineering and
installation of solar thermal systems. After all,
correct engineering and implementation are
fundamental prerequisites, not only for the
trouble-free and efficient operation of a solar
thermal system, but also for the safety of the
building and its occupants.
I am convinced that this technical guide
will provide everyone who wants to take
advantage of the excellent opportunities
offered by the business of the future – solar
thermal systems – with the help they need. I
wish every success to those who use it.
Dr. Martin Viessmann
A Principles
Utilising the enormous potential offered by insolation requires excellent
components and proven systems.
The energy of the sun can be utilised actively
or passively. With passive utilisation of solar
energy, radiation is utilised directly (e.g.
windows, conservatory), in other words
without technical aids.
Various technologies are available for active
utilisation of solar energy. Apart from
generating heat (solar heating), the sun can
also be used to generate power (photovoltaic).
This manual deals exclusively with the solar
thermal topic.
One principle of the thermal utilisation of solar
energy is the level of insolation that is available on
Earth. This depends on the season, location and
the utilisation area.
The collector (Latin: collegere = to collect)
is the essential component required for the
utilisation of insolation. In this section it is shown
with its essential parameters. By connecting
various additional components, a solar thermal
system can be created that can be operated in
different modes.
Potential benefits of utilising insolation thermally
A.1.1 Sun – source of radiation
A.1.2 Available radiation on Earth
Fundamental parameters of collector systems
Collector efficiency
Idle temperature
Collector output
Collector yield
Solar coverage
Fundamental differences between operating modes
A.3.1 Pressurised system with antifreeze
A.3.2 Pressurised system with thermal frost protection
A.3.3 Drainback system
A.1 Potential benefits of utilising insolation thermally
Potential benefits of utilising insolation thermally
In the long term, the sun is the most reliable source of energy
available to mankind.
The opportunities for using this source of
energy for the everyday generation of heating
are, from a technical viewpoint, largely well
developed. However, the true potential for
the actual utilisation of solar energy is by no
means exhausted.
This chapter describes the composition of
useable insolation. It also explains what is
special about "solar fuel" and how the free
radiation energy can be utilised effectively. In
an initial overview, the most common solar
thermal systems are described and compared.
This information forms the basis for the
factually and technically correct handling of
solar thermal energy utilisation.
Fig. A.1.1–1
Spectral distribution of solar and infrared radiation
Irradiance [W/(m²·μm)]
1 000 million
100 million
Insolation of
the photosphere
10 million
1 million
A.1.1 Sun – source of radiation
Every source of radiation emits radiation in
different wavelengths. The length of waves
depends on temperature, radiation intensity
and increases with rising temperatures. Up to
a temperature of 400 °C, a body radiates in
the long wave, still invisible IR range; above
that temperature, radiation becomes visible.
Red glowing metals at 850 °C already radiate
visible light. Halogen lamps from approx.
1 700 °C emit almost white light and a small
proportion of invisible shortwave ultraviolet
radiation. The overall spectrum of the different
wavelengths of a source of radiation is
referred to as spectral distribution.
100 000
10 000
1 000
insolation at
1 700 °C
850 °C
300 °C
100 °C
30 °C
Insolation in
Earth's orbit
Wavelength (μm)
With increasing temperatures, the
strength of radiation and the proportion
of shortwave radiation increases.
Radiation level of the sun
Its high temperature makes the sun
a very strong source of radiation. The
visible range of insolation makes up only a
small part of the total radiation spectrum.
However, it represents the highest level of
radiation intensity.
Deep in the sun's interior, nuclear fusion
processes take place that fuse hydrogen
atoms into helium atoms. The resulting mass
defect (the mass of the helium nucleus
is smaller than the sum of the individual
components) in excess of four million tonnes
per second releases energy that heats the
interior of the sun to a temperature of approx.
15 million degrees Celsius.
At the sun's surface (photosphere) the
temperature is still just about 5 500 °C. From
the surface, energy in the form of radiation
is dissipated. The strength of this radiation
corresponds to an output of 63 MW/m2.
During a single day, 1 512 000 kWh energy
per square metre is radiated, which equates
to an energy content of approx. 151 200 litres
of fuel oil.
A.1 Potential benefits of utilising insolation thermally
Relation sun to Earth
1.4 million km
The sun is almost 5 000 million years old
and will last approx. as long again. It has
a diameter of 1.4 million kilometres; the
Earth's diameter is just 13 000 km. The vast
distance between Earth and sun (approx.
150 million km) reduces the enormous level
of the sun's radiation to a magnitude that
enables life on our planet.
63 000 kW
per m²
1 367 kW
per m²
This distance reduces the average radiation
power to the outer edge of the Earth's
atmosphere to an irradiance of 1 367 W/m2.
This is a fixed value that is referred to as
solar constant – is defined by the World
Meteorological Organization (WMO), a
body of the United Nations (UN). The actual
irradiance fluctuates by ± 3.5 percent. The
Earth's elliptical orbit around the sun means
that the distance between the Earth and the
sun is not constant – it is between
147 million and 152 million km. Solar activity,
too, fluctuates.
150 million km
September and March it is the southern
hemisphere. As a result days are of different
lengths in summer and winter.
The length of day is also subject to latitude,
i.e. the further north you go the longer (in
summer) or shorter (in winter) days will
become. For example, in Stockholm the
daylight hours of 21 June last 18 hours and
38 minutes, in Madrid they only last 15 hours
and 4 minutes. In the winter months the
opposite applies. Then, Madrid manages
9 hours and 18 minutes on 21 December,
whereas Stockholm can only reach 6 hours
and 6 minutes.
Influence of latitude and season
On its annual journey around the sun, the
Earth is tilted along its north-south axis by
23.5° against the axis of its orbit. From March
until September, the northern hemisphere
is more oriented towards the sun, between
Fig. A.1.1–3
The Earth's journey around the sun
The inclination of the Earth's axis on
its orbit around the sun causes the
Earth's axis
difference in duration of insolation and
brings about the seasons on Earth.
Tro cer
Tro icorn
21 December
13 000 km
Fig. A.1.1–2
Solar constant
21 June
Fig. A.1.1–4
Length of days
The length of days depends on the
season and the latitude.
60° ° nort
Lat titude
ckh d
21 December
Within Germany, too, there are different
angles of incidence for solar radiation.
21 June
Lowest level on the 21 December:
Hs = 90° – Latitude – 23.5°
Stockholm (59.3°): Hs = 90°–59.3°–23.5° = 7.2°
Würzburg (49.7°): Hs = 90°–49.7°–23.5° = 16.8°
Madrid (40.4°):
Hs = 90°–40.4°–23.5° = 26.1°
Würzburg lies at latitude 49.7° north. Given the angle
of axis of 23.5, this means a midday zenith of 63.8°
on the 21 June. At midday on the 21 December, this
angle is only 16.8°. The further south you go in the
northern hemisphere, the higher the midday sun is in
the sky, i.e. the angle of incidence increases with
reducing latitude. The sun only reaches a high level
of 90° towards the horizon (sun at zenith) within
The angle of incidence of the
the tropics.
midday sun varies by 47°
during the course of the year.
Fig. A.1.1–5
Sun's trajectory
The highest or lowest level of the midday sun
relative to latitude can be calculated using the
following formulae:
Highest level on the 21 June:
Hs = 90° – Latitude + 23.5°
Stockholm (59.3°): Hs = 90°–59.3°+23.5° = 54.2°
Würzburg (49.7°): Hs = 90°–49.7°+23.5° = 63.8°
Madrid (40.4°):
Hs = 90°–40.3°+23.5° = 73.1°
Sunrise in
8:14 h
6:24 h
5:11 h
A.1 Potential benefits of utilising insolation thermally
Fig. A.1.2–1
Atmospheric influence
The atmosphere reduces the
radiation level of the sun.
A part of its radiation is absorbed
and reflected. A further part
reaches the Earth's surface as
diffused and direct radiation.
Solar constant 1367 W/m2
Reflection from clouds
by the atmosphere
by the atmosphere
Diffused insolation
Ground reflection
Direct insolation
A.1.2 Available radiation on Earth
Global radiation
The influence of the atmosphere reduces
the absolute radiation level (solar constant)
of 1 367 W/m2 to approx. 1 000 W/m2 at the
Earth's surface. The atmosphere exerts a
varying degree of influence on the overall
radiation spectrum. Cloud cover reflects some
of the radiation, another part is absorbed
by the atmosphere (Latin: absorbere =
to swallow). Other radiation components
are scattered by more dense layers of
the atmosphere or clouds, turning it into
diffused radiation. Some radiation hits the
Earth directly.
That part of the radiation that hits the Earth
is either reflected or absorbed by the Earth's
surface. Absorption heats up the Earth's
surface, whilst reflection also generates
diffused radiation.
The total amount of radiation, both diffused
and direct is referred to as global radiation.
The proportion of diffused radiation as a
percentage of global radiation in Germany is,
as an annual average, approx. 50 percent –
less in summer, more in winter.
The difference between direct and diffused
radiation is, for solar applications, particularly
relevant for concentrated systems (parabolic
or elongated hollow reflectors) as these
systems utilise only direct radiation
(see chapter C.2.6).
Air mass
The irradiance on the Earth's surface is also
determined by the length of the path the
radiation travels through the atmosphere. This
reducing effect is described as air mass (AM)
and is the result of the angle of incidence
of insolation.
The shortest route occurs when the radiation
hits the Earth from the vertical (= 90°) and
this is described as AM 1. The longer the
path radiation takes to reach the Earth's
surface, the greater the reducing effect of
the atmosphere.
Fig. A.1.2–2
Insolation (Germany)
Proportion diffused insolation
Proportion direct insolation
The output limits between diffused and direct radiation fluctuate. Even light that appears to be weak, with a
high proportion of diffused radiation offers useful irradiance.
significantly. It ranges from severely overcast
conditions with approx. 50 W/m2 to
1 000 W/m2 when the sky is clear.
To be able to calculate the amount of
insolation that is actually converted into solar
thermal energy, the duration of insolation
must also be taken into account. Energy is the
"output" during a defined period, for which the
unit of measure is watt-hour (Wh). The energy
of global radiation is stated in amounts per
day, month or year.
The maximum daily total in Germany in
summer is approx. 8 kWh/m2. However,
even on a sunny winter's day up to 3 kWh/m2
irradiance is available.
Fig. A.1.2–3
Insolation (Germany)
Global radiation [Wh/(m2·d)]
The level of radiation on a defined area is
referred to as irradiance. From a physical
viewpoint, irradiance is therefore an output
per area expressed in watts per square
metre (W/m2). The solar irradiance fluctuates
4 000
Direct insolation
2 000
Diffused insolation
The average daily totals of global radiation during the course of a year vary by a factor of almost 10.
The proportion of diffused radiation reaches an annual average of approx. 50%.
A.1 Potential benefits of utilising insolation thermally
Fig. A.1.2–4
Global radiation in Germany
Averaged over many years, Germany achieves
annual average totals of global radiation of
950 kWh/(m2 /p.a.) in the north-German low
lands and 1 200 kWh/(m2 /p.a.) around Freiburg
or in the alpine region. Worldwide, these
values lie between 800 kW/h (m2 /p.a.) in
Scandinavia, for example, and 2 200 kWh
(m2 /p.a.) e.g. in the Sahara.
Individual monthly totals of global radiation
energy can deviate from the average taken
over many years by up to 50 percent,
individual annual totals by up to 30 percent.
Global radiation
Fig. A.1.2–5
Hours of sunshine in Germany
Over many years, the annual average hours
of sunshine in Germany lies between
1 400 hours in southern Lower Saxony and
in excess of 1 800 hours in north-eastern
Germany. The distribution of hours of
sunshine in Germany deviates from the
distribution of total global radiation. The
coastal regions can expect more frequent
sunshine than the interior.
Deviations from these average values can
be quite substantial. Apart from the different
annual values, there are also regional and
even local deviations.
Hours of sun p.a.
Fig. A.1.2–6
Inclination, orientation and insolation
Subject to the angle and orientation
of a surface, the level of insolation –
relative to a horizontal area –
– 15 %
+ 10%
+ 10 %
+ 5%
– 40 %
– 15%
+ 5%
– 20%
– 20 %
– 25%
reduces or increases.
– 40%
– 25 %
Deviation from global radiation
Inclination of the receiver surface
The values of global radiation energy
relate to the horizontal level. These values
are influenced by the inclination of the
receiver surface.
If the receiver surface is angled, the angle of
incidence changes, as does the irradiance,
and consequently the amount of energy. The
annual total global radiation energy relative
to surface is also subject to the angle of
these surfaces.
The amount of energy is greatest when the
radiation hits the receiver surface at right
angles. In our latitudes, this case never arises
relative to the horizontal. Consequently the
inclination of the receiver surface can "help
this along". In Germany, a receiver surface
angled at 35° receives on average 12% more
energy when oriented towards the south,
compared to a horizontal position.
Orientation of the receiver surface
An additional factor for calculating the
amount of energy that can be expected is
the orientation of the receiver surface. In the
northern hemisphere, an orientation towards
the south is ideal. Deviations from the south
of the receiver surface are described as the
"angle of azimuth". A surface oriented towards
the south has an angle of azimuth of 0°.
Contrary to a compass, the angles in solar
technology are stated as south = 0°,
west = + 90°, east = – 90° etc.
Figure A.1.2–6 demonstrates the interaction
of orientation and angle. Relative to the
horizontal, greater or lesser yields result. A
range can be defined between south-east
and south-west and at angles between 25
and 70°, where the yields achieved by a solar
thermal system are ideal. Greater deviations,
for example, for systems on a wall, can be
compensated for by a correspondingly larger
collector area.
A.2 Fundamental parameters of collector systems
Fundamental parameters of collector systems
Collectors are heat sources that, in many ways, are different from conventional
heat sources. The most obvious difference is the source of primary energy
used to generate heat, i.e. the "fuel" used is insolation.
On the one hand, this free source of energy
will be – as far as is humanly possible to
predict – available for ever; on the other hand
it can hardly be calculated for the relevant
demand and its actual availability is limited.
Particularly during the heating season, when
most heat is required, the least amount
of solar energy is available and vice versa.
In addition, the sun cannot be started and
stopped subject to demand. These general
conditions require a fundamentally different
approach than when designing energy
systems that have an output that is available
on demand. With a few exceptions, therefore,
systems that utilise solar radiation energy are
supplemented by a second heat source, in
other words they are engineered and operated
as dual-mode systems.
The above illustration shows a simple dualmode system. Here, the boiler delivers a
specified amount of hot water at any time
required. The collector system is integrated
into the system so that as much energy as
possible is yielded from insolation, and that
as little fuel as possible is consumed by
the boiler.
Even this simplified example shows that
the successful operation of a solar thermal
system is not only subject to the collector but
equally to the sensible interaction between
all components used. To successfully plan
the effect of a collector as part of an overall
system, the next few chapters explain its
fundamental properties and evaluation criteria.
A.2.1 Collector efficiency
When collectors heat up as a result of
insolation, they transfer some of that heat to
the ambience through thermal conduction of
the collector material, thermal radiation and
convection (air movement). These losses can
be calculated with the heat loss correction
values k1 and k 2 and the temperature
The efficiency of a collector describes
the proportion of the insolation "striking"
the aperture area of the collector that is
converted into available heating energy.
The effective surface of a collector in solar
effective terms is described as the aperture
area (see chapter B.1.3). The efficiency is,
amongst other things, subject to the operating
conditions of the collector; the calculation
method is the same for all collector types.
differential ΔT (i.e. Delta T) between the
absorber and the ambience. (Further details
relating to the absorber in chapter B.1.2.)
The temperature differential is stated in
K (= Kelvin).
Some of the insolation striking the collectors
is lost through reflection and absorption at the
glass pane and through absorber reflection.
The ratio between the insolation striking the
collector and the radiation that is converted
into heat on the absorber is used to calculate
the optical efficiency. It is described as H0
(i.e. approx. zero).
Fig. A.2.1–1
Energy flow pattern inside the collector
The insolation striking the collector
is reduced by the optical losses.
The remaining radiation heats up
the absorber. That proportion of
the heat which is transferred by
the collector to the ambience is
described as "thermal losses".
Insolation on to collector
Optical losses
Reflections off the glass pane
Absorption in the glass pane
Reflection off the absorber
Absorber heated by insolation level
Thermal losses
Collector casing
with thermal insulation
Thermal conduction of the
collector material
G Absorber heat radiation
A.2 Fundamental parameters of collector systems
Fig. A.2.1–2
Characteristic parameters for different collector types
The heat loss correction values
and the optical efficiency are
essential parameters for the
collector performance.
Flat-plate collector
Flat-plate collector
with antireflection glass
tube collector
The greater the
temperature differential
between collector and
outside air, the greater
the thermal losses – the
efficiency therefore
falls with the rising
operating temperature
of the collector or with
the drop in the outside
air temperature.
Optical efficiency
Heat loss correction value k1
Heat loss correction value k 2
W/(m2·K 2)
k1 · ΔT
k 2 · ΔT2
The optical efficiency and the loss correction
values are determined in accordance with the
procedure described in the European Standard
EN 12975. These represent the important
parameters of a collector. These must be
stated in the appliance datasheets (see
www.viessmann.de > Produkte >
H = H0 –
These three values plus the irradiance Eg are
sufficient to illustrate the collector efficiency
and its curve.
Maximum efficiency is achieved, when the
differential between the collector and the
ambient temperature is zero and the collector
loses no energy to the environment.
Fig. A.2.1–3
Collector efficiency
Optical efficiency
Heat loss correction values in W/(m2 · K)
Heat loss correction values in W/(m2 · K 2)
Temperature differential in K
Irradiance in W/m2
Characteristic efficiency curves
tube collector
Flat-plate collector
With increasing temperature
differentials towards the ambience,
vacuum tube collectors offer
advantages in terms of efficiency.
Temperature differential (K)
A.2.2 Idle temperature
A.2.3 Collector output
The collector heats up to the idle temperature
if no heat is drawn from the collector (no heat
Maximum output
The maximum collector output is defined as
the product of the optical efficiency H0 and
the assumed maximum insolation that can be
absorbed of 1 000 W/m2.
transfer medium circulation; the pump is at
a standstill). In this state the thermal losses
are of the same magnitude as the radiation
absorbed, in other words the collector output
is zero.
In Germany, commercially available flat-plate
collectors reach idle temperatures in excess
of 200 °C in summer, vacuum tube collectors
reach temperatures of approx. 300 °C.
At an assumed optical efficiency of
80 percent, the maximum output of one
square metre collector surface is 0.8 kW.
However, in normal operation this value is
rarely achieved; the maximum output is only
relevant for sizing the safety equipment.
Design output
For that reason, a design output is determined
when designing a solar thermal system. It is
required for engineering the installation and
particularly for sizing the heat exchanger.
For this, VDI 6002 part 1 defines a specific
collector output of 500 W/m2 as the lower
limit. To be on the safe side, we would
recommend a slightly higher value of
600 W/ m2 when applying low temperatures,
in other words when operating with collectors
that are expected to be efficient. All system
components and solar packs from Viessmann
are based on calculations with this value.
Installed output
The appropriate technical literature refers to
an additional output magnitude that is only
used for statistical purposes when comparing
different energy generators. For surveys
regarding all installed collector systems in
a region, amongst the other details, the
installed output is quoted in m2. In this
case, it is 700 W/m2 absorber area (average
performance at maximum insolation). This is
irrelevant for system design and engineering.
A.2 Fundamental parameters of collector systems
A.2.4 Collector yield
For sizing a solar thermal system and for
sizing the system components, the collector
output is less relevant than the expected
system yield.
For example, for systems used to provide
solar central heating backup, a more steeply
raked angle of inclination is more sensible
for the overall yield and the operating
characteristics, since the optimum yield is
decisive for spring, autumn and winter. In
summer, when only DHW is backed up by
solar heating, the "poorer" angle of inclination
results in small excesses. During spring
and autumn the "better" angle of inclination
achieves a higher yield of available energy.
This balances out the provision of energy,
looking at the year as a whole, and the system
yield is higher than if the system was oriented
towards the maximum insolation.
The collector yield results from the product
of the expected average output (kW) and
an appropriate unit of time (h). The resulting
value in kWh is related to a square metre
collector or aperture area (see chapter B.1.3)
and is stated in kWh/m2. Relative to a day, this
value is important to enable the solar cylinder
to be sized. The specific collector yield relative
to a whole year is stated as kWh/(m2 /p.a.)
and is a vital variable for sizing and operating
the system.
An orientation towards
the optimum insolation
at a specific location
is only sensible when
the amount of radiation
striking the collector can
be utilised effectively.
Fig. A.2.4–1
Yield and inclination
Relative collector yield (%)
The higher this value, the more energy is
supplied to the system by the collectors.
Operating conditions, at which the collector
could still supply energy, but the cylinder
would, for example, already be fully heated
up, also affect the annual assessment. In such
cases, there would be no yield. The collector
yield is the essential variable used to assess
the efficiency of a solar thermal system. It
will be particularly high if the collector area
is ideally oriented in accordance with the
utilisation focus and is free from shading.
The optimum insolation does not necessarily
equate to the optimum yield.
Collector inclination
The monthly distribution of yields for a surface oriented due south is subject to the angle of inclination.
A.2.5 Solar coverage
This aspect that sets the solar yield in
relation to the amount of heat used, takes
into account the cylinder losses and has
established itself as the conventionally used
data for solar coverage. However there
is the option of setting the solar yield in
relation to the amount of energy consumed
for reheating. The coverage calculated by
that route is higher. When comparing solar
thermal systems, attention should be paid to
the relationship on which the solar coverage
is based.
The higher the solar coverage, the greater the
savings in conventional energy. This makes
it clear why those interested in solar thermal
systems are frequently looking for systems
with a high coverage. However, designing a
sensible solar thermal system always entails
finding a good compromise between yield and
solar coverage.
One general rule applies. The higher the solar
coverage, the lower the specific yield per
square metre collector area – this is due to the
unavoidable excesses in summer and the low
collector efficiency.
It is customary in Germany, to design for a
solar coverage of 50 to 60 percent for DHW
heating in detached houses; in apartment
buildings 30 to 40 percent are more commonly
assumed. Solar central heating backup can
hardly provide standard values, as the solar
coverage is largely dependent on the energetic
quality of the building (insulation, air-tightness,
etc.) (see chapter C.2.2).
Fig. A.2.5–1
Solar coverage for DHW heating
Solar coverage in %
Apart from the yield, the solar coverage
is the second essential variable required
for designing a solar thermal system. The
coverage states the percentage of the energy
required for the intended use that can be
supplied by the solar thermal system.
A good compromise between yield and
solar coverage is generally also a good
compromise between investment outlay
for the solar thermal system and savings in
conventional energy.
Small solar
thermal system
Large solar
thermal system
Energy (yield) in kWh/(m2 / p . a.)
A good compromise between solar coverage and solar yield
must be found for every solar thermal system.
To recap, the efficiency drops with a rising
temperature differential between collector
and ambient temperature.
A.3 Fundamental differences between operating modes
Fundamental differences between operating modes
The most common variety of solar thermal systems in Germany comprises collectors,
a control unit with pump and a well insulated DHW cylinder.
Inside the collector, solar energy strikes
a coated panel (absorber). Copper pipes
through which the heat transfer medium
circulates are affixed to the bottom of the
absorber. The absorber is heated by solar
radiation and transfers that heat to the heat
transfer medium inside the absorber pipes. A
controller and a pump ensure that the heat is
removed via pipelines. The heat is transferred
inside the cylinder to the domestic hot water
(DHW) by means of an internal indirect coil.
Generally, all pumped solar thermal systems
are designed this way. However, the operation
can differ fundamentally. These differences
are explained below.
For details regarding non-pumped gravity-fed
systems (thermosiphon systems), see chapter
A.3.1 Pressurised system
with antifreeze
A.3.2 Pressurised system
with thermal frost protection
These systems utilise a heat transfer medium
A system with thermal frost protection is
similarly constructed to the system with
antifreeze described here. The difference lies
in that the heat transfer medium comprises
pure water without additional antifreeze. To
prevent the water from freezing in winter, the
conventionally generated heat is transported
from the cylinder to the collector. To reach
an energy assessment for these systems,
the energy consumption required to heat
the collector must be deducted from the
energy yield in summer. The energy use in
winter is severely affected by the respective
temperatures; generally it rarely lies below ten
percent of the solar yield.
that is generally composed of a mixture
of water and antifreeze (glycol). The heat
transfer medium is forced – by a pump –
through the absorber tubes, absorbs the
thermal energy from the absorber to transfer
it afterwards inside the indirect coil to the
cylinder contents.
In winter, the glycol protects the system
against freezing up – the heat transfer
medium however remains sealed inside the
system. In addition, this system offers the
highest possible corrosion protection, as
commercially available heat transfer media are
mixed with corrosion inhibitors.
Sealed unvented systems always require
an expansion vessel to accommodate the
expansion of the heat transfer medium as
well as any vapour that might arise inside
the collector.
Where these systems are connected to the
heating circuit without system separation,
other rules must be observed during
engineering and installation (handling of
heating water, test pressure, etc.) than when
dealing with separate solar circuits.
This system is the most popular in central
Europe, accounting for a 95 percent
market share.
Fig. A.3.1
System with antifreeze
Fig. A.3.2
System with thermal frost protection
Solar thermal systems
from Viessmann are
pressurised systems
with antifreeze.
These systems
Ensure a reliable frost
protection in winter
Consume no
generated heating
energy to protect the
collector from frost
Enable the solar
circuit to be
easily piped
Offer the highest
possible corrosion
protection for all
system components
A.3 Fundamental differences between operating modes
A.3.3 Drainback system
For drainback systems it is significant that
the heat transfer medium drains from the
collector when the system is not in use. This
only works with collectors connected from
below, where the absorber can be drained by
gravity. All pipework leading away from the
collector must have a natural fall. The heat
transfer medium collects in a container.
A drainback system is a self-draining system
that is generally operated with pure water.
For that reason, all system components that
are at risk from frost must be able to drain
completely. A drainback system must never
be enabled in winter when low temperatures
prevail, even if the collector itself is heated
by insolation. This system type must have a
consistent fall in the entire installation which
is very hard to achieve, particularly in existing
Increasingly, drainback systems are operated
with a water and glycol mixture. This would
make the complete draining of the pipework
in winter superfluous to protect the system
against freezing up. These systems were
developed mainly to reduce the load on heat
transfer media in systems where long idle
periods must be expected.
Fig. A.3.3
Drainback system
The use of auxiliary energy (power for the
pump) is always higher in drainback systems
than in pressurised systems, since the system
is always filled again upon start-up.
B Components
Reliable information regarding construction and function of the essential components
of solar thermal systems is a vital prerequisite for the efficient engineering and
installation of systems.
This chapter deals with the individual
components of solar thermal systems.
Technical details as well as the essential
functional connections will be explained.
This will illustrate what makes a good
collector, what characteristics a suitable
cylinder should offer and what must be
taken into account during engineering
and installation of the components in the
The information contained herein
demonstrates that solar thermal systems with
powerful components are able to be operated
over the long term with reliability and a high
degree of efficiency.
primary circuit.
Construction and function
Area designations
Quality and certification
Selecting a suitable collector type
Aspects concerning collector fixing
Collectors as design features
Why store energy?
Principles of cylinder technology
Cylinder types
Cylinder heating
Heat exchangers/indirect coils
Primary circuit
Collector circuit
Heat transfer medium
Stagnation and safety equipment
B.1 Collectors
Industrial production of solar thermal collectors began in the mid 1970s as a
response to the oil crisis. Since then, a worldwide standard has developed for these
appliances, focussing on central Europe. Solar collectors are high grade products
with a useful service life in excess of 20 years.
Fig. B.1-1 Acredal collector –
The technical development of collectors has
more or less reached a level of maturity; a
fundamental change of concept for these
appliances is not expected in the coming
years. Optimisation potential is still latent
in the detail, such as the materials used.
Therefore, the current priority for research and
development is system integration and new
forms of utilising solar thermal energy.
Viessmann can call on more
than 30 years of experience in
collector technology.
This section covers the fundamentals of
collector technology. We are going to consider
the differences between the flat-plate and
vacuum tube collector types as well as their
function under different operating conditions.
Typical differences between collectors are the
construction of absorbers and the insulation
of the collector against the environment.
However, the physical process – conversion
of light into useful heat – is the same for all
collectors. Light energy is converted on the
absorber into thermal energy.
The specific application of concentrating
systems for generating power from solar
energy is described in chapter C.2.6.
B.1.1 Construction and function
Flat-plate collectors
In Germany, flat-plate collectors enjoy a
market share of approx. 90 percent, relative to
the total area covered. For flat-plate collectors,
the absorber is generally permanently
protected from the elements by a casing
made from coated sheet steel, aluminium or
stainless steel and a front cover made from
low-ferrous solar safety glass. An anti-reflex
(AR) coating on the glass further reduces
reflections. Thermal insulation minimises
heat losses.
Flat-plate collectors are easily and safely
installed in and on domestic roofs.
Increasingly, collectors are also mounted
on walls or as freestanding units. Flat-plate
collectors are more affordable than tube
collectors. They are used for DHW heating
systems, swimming pool water heating and
for central heating backup.
For standard flat-plate collectors, a gross
collector area (external dimensions) of approx.
2–2.5 m2 has been established.
Fig. B.1.1-1 Flat-plate collector
Vitosol 200-F
The casing of Viessmann flat-plate collectors
comprises an all encasing folded aluminium
frame without mitre cuts or sharp edges.
Together with the seamless weather and
UV-resistant pane seal and the puncture proof
back panel, these features ensure a long
service life and permanently high efficiency.
Vacuum tube collectors
The conversion of light into heat at the
absorber is generally identical for flat-plate
and tube collectors. However, significant
differences apply to the thermal insulation.
In tube collectors, the absorber is similar to a
Thermos flask in that it is set into a glass tube
that is under vacuum pressure (evacuated).
The vacuum features excellent thermal
insulative properties. Consequently, heat
losses are lower than for flat-plate collectors,
particularly at high temperatures – in other
words, under specifically those operating
conditions that can be expected for central
heating or air conditioning.
One prerequisite for reliability and a long
service life of vacuum tube collectors is the
maintenance of the vacuum in the long term
by providing reliable seals. This is ensured
with Viessmann collectors. The minimum
amounts of gas (primarily hydrogen) that enter
the tubes are bound by the thin film made
from barium ("getter") that has been steamed
onto the tube.
Fig. B.1.1-2 Vacuum tube collector
Vitosol 300-T
B.1 Collectors
Fig. B.1.1–3
Vacuum tube collector Vitosol 200-T
In vacuum tube collectors, a distinction is
drawn between designs with direct flow and
those that incorporate the heat pipe principle.
In vacuum tube collectors with direct flow,
the heat transfer medium circulates directly
inside the absorber pipes that are arranged
inside the tube. These offer a particularly wide
choice of installation location.
Direct flow vacuum tube collector with coaxial pipe at the absorber.
Fig. B.1.1-4
With heat pipes, a medium, generally water,
is evaporated inside a sealed absorber pipe.
The steam condenses in the aptly named
condenser at the upper end of the tube – this
is where the energy is passed to the heat
transfer medium. This process requires a
specific angle of installation for the collector
to ensure that the heat from the tube can be
transported to the condenser.
Vacuum tube collector Vitosol 300-T
Vacuum tube collector with heat transfer based on the heat pipe principle.
B.1.2 Absorbers
The absorber is at the core of each collector.
This is where the insolation is converted into
heat. The coated absorber panel transfers
the heat to a liquid heat transfer medium via
soldered-on and pressed-on pipes. Generally,
absorbers are made from sheet copper,
sheet aluminium or glass. The coating is
highly selectively applied, i.e. it ensures that
the insolation is as fully converted into heat
as possible (high absorption, A = alpha) and
that only a small amount of heat is lost again
through radiation from the hot absorber (low
emissions, E = epsilon).
The panels are coated in a galvanic process
(black-chrome absorber) or the absorber
layer is steamed onto the substrate (known
as "blue layers"). Both processes offer
highly selective coating. Both coatings are
differentiated according to their durability
regarding environmental influences in
specific application areas (e.g. air containing
chloride near the sea) and in their absorption
or emission characteristics at different
temperatures. However, the latter has little
bearing on the operational characteristics of
a solar thermal system and can therefore be
ignored in engineering terms.
Fig. B.1.2–1
Absorber coatings
Those parts of the absorber pipework that
are exposed to sunlight can be painted matt
black; this process is no longer used in
absorbers. Advanced absorbers are no longer
black but, subject to the viewing angle, blue
or green in hue.
Black paint
Galvanic application
"Blue layers"
Black paint
"Blue layer"
factor A
factor E
The surfaces are identical in the
conversion of radiation into heat;
their radiation properties highlight
the differences between systems.
Flat-plate collector absorbers
With flat-plate collectors, the absorber is
made of strips or solid panels (finned or fullarea absorber). Finned absorbers are made
from absorber strips, each of which is fitted
with a straight absorber pipe. These are
arranged in a harp shape (Fig. B.1.2-2). With
full-area absorbers, the tube can also be
arranged in a meandering shape across the
entire absorber surface (Fig. B.1.2-3).
Fig. B.1.2–2
Harp-shaped absorber pipe
Collectors with harp-shaped absorbers
are characterised, under normal operating
conditions, by a comparatively low pressure
drop. They do, however, harbour the risk of
an irregular flow pattern. Meander absorbers
ensure a highly reliable heat transfer as the
medium is only routed through a single pipe.
In smaller systems, this difference is
irrelevant from an engineering viewpoint. In
larger, more complex collector arrays, these
differences in flow technology must be taken
into account (see chapter C.1).
Flat-plate collectors with harp
absorbers offer benefits because of
their low pressure drop.
B.1 Collectors
Fig. B.1.2–3
Meander absorber pipe
The number of connections to the absorber
must be taken into consideration. Where
a collector is only equipped with two
connections, the collectors can only be
connected in series if there is no additional
external pipework. Collectors with four
connections offer a substantially greater
flexibility where hydraulics are concerned –
they make engineering easier and significantly
improve the operational reliability, particularly
in larger collector arrays.
Collectors with meander absorbers
offer the benefit of an even and
reliable heat transfer.
Absorbers in vacuum tube collectors
Fig. B.1.2–4 Absorber pipes heat pipe system / direct flow
Heat pipe
Direct flow
Finned absorbers
With these types of collector, the absorber
consists of a flat fin with a welded-on
absorber pipe. Tubes with direct flow employ
a coaxial pipe. A heat transfer medium is
routed from the return into the inner pipe;
the medium returns through the outer pipe
welded to the absorber. In the process, the
medium is heated up. In heat pipes, a single
pipe is used that is sealed at the bottom.
Each vacuum tube in Viessmann tube
collectors can be rotated along its longitudinal
axis – enabling the absorber to be perfectly
aligned with the sun in unfavourable
installation locations.
To transfer heat from the absorber,
either gravity (heat pipe) or the
direct flow principle is used.
Fig. B.1.2-5 The heat pipe
condenser is dry-connected to the
solar circuit.
Circular glass absorbers
With this type of collector, two glass tubes –
one inside the other – are welded together
and evacuated. The absorber is steamed
onto the inner glass tube. The solar energy is
transferred to the heat transfer medium via
heat conducting plates and absorber pipes set
into these plates. To enable the utilisation of
the radiation on the reverse side of this type
of absorber, a mirror is required. Due to their
design, the optical efficiency – relative to the
aperture area – of this collector type is approx.
20 percent (absolute) below the value of
collectors with flat absorbers.
B.1.3 Area designations
For collectors, three different area
designations are used as the reference
variable for output and yield details. However,
technical literature does not always make
it clear which area reference is used. All
values are clearly stated in the datasheets for
Viessmann collectors.
Gross collector area
The gross collector area describes the
external dimensions of a collector and
results from length x width of the external
dimensions. As regards the output of such
appliances or their assessment, the gross
collector area is irrelevant. However it is
important when planning the installation and
when calculating the roof area required. When
applying for subsidies, the gross collector area
is frequently crucial.
Absorber area
The absorber area refers exclusively to the
absorber. In fin absorbers, overlaps of the
individual strips are excluded, as the covered
areas are not part of the active area. For
circular absorbers, the total area is counted,
even if certain absorber areas are never
exposed to direct insolation. Consequently,
the absorber area for circular absorbers can
be larger than the gross collector area.
Aperture area
Apertures are the "lenses" – put simply – the
openings of optical appliances. Transferred
to a collector, the aperture area is the biggest
projecting surface through which insolation
can enter.
In the case of flat-plate collectors, the
aperture area is the visible part of the glass
pane, in other words that area inside the
collector frame through which light can enter
the appliance.
In vacuum tube collectors – both with flat
as with circular absorbers without reflector
areas – the aperture area is defined as the
total of the longitudinal cross-sections of all
glass tubes. Since the tubes contain smaller
areas at the top and bottom with no absorber
panels, the aperture area of these collectors is
slightly larger than the absorber area.
The aperture area is
increasingly gaining
recognition as the
decisive standard
variable for sizing
collector systems.
However, in some
cases the absorber
area is also used. It
is therefore crucial to
differentiate between
the individual values.
For tube collectors with back-mounted
reflectors, the projections of these mirror
areas are defined as the aperture area.
Fig. B.1.3–1 Area designations for flat-plate collectors and vacuum tube collectors
The size of collectors is stated
in square metres. For this it is
important to know the reference
area to which the stated size refers.
A Absorber area
B Aperture area
C Gross area
B.1 Collectors
B.1.4 Quality and certification
Collectors are constantly exposed to the
weather and to high temperature fluctuations.
They must therefore be made from materials
that can withstand these conditions.
Viessmann collectors are made from highgrade materials, such as stainless steel,
aluminium, copper and stabilised special solar
glass. The resulting high stability as well as
the output details relating to the collectors are
tested by certified institutes.
Collector test to EN 12975
This test includes examinations to determine
the collector output as well as tests
regarding the stability towards environmental
influences, such as rain, snow or hail.
Solar Keymark
The certification in accordance with the solar
keymark is also based on the collector test
EN 12975. However, the test samples are
randomly drawn from the manufacturing
process by an independent test institute.
Viessmann collectors are tested compliant
with the solar keymark.
Fig. B.1.4–1 The high efficiency
and long service life of Viessmann
collectors are the result of
intensive development work
CE designation
With a CE designation (here in accordance
with the Pressure Equipment Directive),
manufacturers guarantee compliance with the
appropriate standards. External tests are not
"Blue Angel" (RAL-UZ 73)
The "Blue Angel" can be acquired as an
additional label. This symbol carries no weight
where building regulations or approvals are
concerned. Until 2007, it was required for
obtaining subsidies in accordance with the
German market stimulation programme.
To obtain this symbol, not only the output
details of collectors, but also their ability to
be recycled and the materials used relative
to their cumulative energy expenditure (KEA)
were investigated.
Further symbols from associations or
joint quality initiatives
In addition to the standard-related tests, there
are further labels or quality seals, although
their additional benefit is hardly discernible
for users, manufacturers or installers. In line
with most suppliers, Viessmann refrains from
applying these identifications.
B.1.5 Selecting a suitable
collector type
What is decisive for selecting the appropriate
collector type – apart from the available space
and the installation conditions described in
the following – is the expected temperature
differential ΔT between the average
collector temperature and the outside air
(see chapter A.2.1).
flat-plate collectors are more affordable than
vacuum tube collectors and deliver good
yields in relation to their price, particularly
when used to heat DHW.
The average collector temperature results
from the average between the flow and return
temperatures and essentially influences the
collector efficiency, in other words, its output.
The solar thermal system yield is crucial
for the collector selection. It is therefore
necessary for an assessment to take place
to determine the operating range of the
collector over its entire operating period –
in most applications this is generally one
year. This would then result in the expected
temperature differential.
Fig. B.1.5–1 shows that the average
temperature differential ΔT, for example in
systems for DHW heating with lower solar
coverage, is significantly smaller than for
systems used for central heating backup.
However, the price/performance ratio is also
important when selecting collectors. The
curve alone would always result in a decision
in favour of a vacuum tube collector. However,
Fig. B.1.5–1 Collector efficiency curves
The greater the differential
between the collector and ambient
temperature, the greater the benefit
of vacuum tube collectors.
DHW heating
Low coverage
DHW heating
High coverage,
central heating backup
tube collector
Air conditioning
Process heat
Flat-plate collector
Temperature differential (K)
B.1 Collectors
Fig. B.1.6–1 Fixing options
Vacuum tubes with direct flow should
be installed horizontally with sloping
connections, if long stagnation periods
are expected.
Heat pipes require a certain minimum
angle of installation, in other words they
cannot be installed level or horizontally.
Large area collectors designed for
roof integration cannot be installed
freestanding on flat roofs or on
the ground.
Flat-plate collectors cannot be installed
horizontally level.
Pitched roof
Flat roof
Wall/balcony rail/balustrade
Freestanding installation
B.1.6 Aspects concerning collector fixing
Solar thermal collectors are heat sources
that remain functional, subject to correct
installation and operation, for substantially
longer than for 20 years. As they, unlike
almost all other heating components, are
constantly subject to weather influences,
they pose some special challenges regarding
their fixing. Fixings must remain permanently
corrosion-resistant and statically safe,
lightning protection is very important and, due
to their exposed position, the architectural
design of a collector system plays an
important role.
In response to the strong market development
over the past few years, pre-fitted solutions
are now available for almost all types of
roof and installation situations. For this,
collector and fixings form a single static
unit. Viessmann offers statically fully tested
systems for all conventional roof types and
suitable for all Vitosol collectors as part of
its standard product range – this translates
into enhanced security for designer and
installer alike.
The following sections explain the basics
of the different fixing options – detailed
information regarding installation and all
associated components including extensive
graphics and diagrams can be found in the
Vitosol technical guide.
B.1.6.1 Fixing options
On account of their diverse designs, solar
collectors can be installed in almost any
building concept, in new build as well as in
modernisation projects, either on the building
or close by. As required, they can be installed
on pitched roofs, flat roofs and on walls as
well as freestanding on the ground.
a. Pitched roof
Fig. B.1.6-2 Principle of pitched roof installation
With collectors on pitched roofs, a
distinction is made between rooftop
In detached houses, the most common
method of fixing is the installation parallel to
a pitched roof. Collectors may be installed
above the roof (rooftop installation) or may be
integrated into the roof (roof integration).
installation and roof integration.
To assess which method of installation on
a pitched roof is the most practicable, the
required area for a collector system is roughly
estimated. For this it is essential to allow
sufficient space around the collector to ensure
a safe installation and, if required, to be able
to accommodate the roof cover frame.
The question of shading must also be
given serious consideration. Looking at the
installation from a collector facing south, the
area between south-east and south-west
must be free of shade at an angle towards
the horizon not exceeding 20°. It should be
remembered that the system is to operate for
longer than for 20 years, and that during this
period trees would grow substantially.
During the first roof inspection, the type of
roof cover will be noted, so that all necessary
components can be taken into consideration
when preparing an offer. The calculation
of the time taken to install also depends
on the roof type. There are significant
differences between simple pitched roofs
(e.g. conventional roof tiles), difficult pitched
Fig. B.1.6-3 Shading (top view)
roofs (e.g. S-tiles that are fixed with mortar)
and roofs where the assistance of a roofing
contractor is recommended (e.g. slate). The
collector installation must have no detrimental
effect on the protective function of the roof.
After the installation, the roof must therefore
be water-tight at all fixing and outlet points,
in other words water must be able to run off
freely everywhere.
Fig. B.1.6-4 Shading (side view)
Area without shade
Area without shade
When selecting the installation surface, bear in mind that
The expected level of shading over 20 years in use must also
shading can only be tolerated in the morning and evening.
be taken into account.
B.1 Collectors
Fig. B.1.6–5
Rooftop installation (cross-section)
When selecting the fixing system,
also take the static requirements
into consideration. Standard fixings
only provide safety in standard
cases under normal load conditions.
Fig. B.1.6–6
Rooftop installation with roof ties or roof hooks
Rooftop installation
In rooftop systems, collectors and the roof
frame are always connected to ensure a
statically safe installation. At each fixing
point, one component (roof hook, roof tie)
penetrates the waterproof level below the
collector. This requires a completely rainproof and safe anchorage, as the fixing points
and therefore also any possible defects
will no longer be visible post installation.
The selection of fixing material depends
on the expected wind and snow load
(see chapter B.1.6.3).
Both methods of fixing (roof tie and roof hook)
offer a reliable connection with the rafters.
Fixing to the existing battens is unsuitable,
as quality and stability either cannot, or can
only be determined with difficulty. Besides,
an overall static can hardly be determined for
fixing on commercially used battens. Vitosol
collectors must be fixed with the appropriate
installation material.
Subject to the static requirements, roof ties or hooks are used as fixing materials for rooftop installations.
Roof integration
With roof integration, the flat-plate collector
is simply integrated into the roof in place of
the roof cover. Consequently, the collector
lies statically safe on the overall substrate
comprising battens and rafters.
Fig. B.1.6–7
One benefit of roof integration is
its design aspect. Here, collectors
are integrated into the roof and are
visually part of the roof.
Roof integration (cross-section)
Different installation solutions are available
where water routing is concerned. Either
the glass cover of the collector forms the
layer in contact with water (in other words, in
principle it replaces the "hard roof cover" to
DIN 4102-7) or an additional sealing layer is
fitted below the collector. Viessmann prefers
the second version, as it prevents the ingress
of water should the glass break or if other
collector faults should arise. Although such
damage occurs rarely (e.g. extreme hail storm
or vandalism), resulting water damage could
be quite severe.
Although sarking membranes can reduce the
consequences of leaks and may therefore
be sensible for physical building reasons,
they can never replace a permanently
watertight layer.
The minimum roof angle approved for roof
integration also provides security against the
ingress of water and snow (see technical
documents). If, on the other hand, the
collector were to lie flat, it would not provide
a positive slope in its capacity as a "large roof
tile" in the areas where it joins the roof cover.
b. Flat roof
In larger projects in multi-storey buildings
or in commercial applications, collectors are
frequently mounted on angled supports. The
benefit lies in that the system can generally
be aligned with due south and be installed at
an optimum angle.
Here, too, the first steps towards engineering
are the feasibility study and a rough
estimate of the installation area, giving due
consideration to the required clearances from
the edges and safe installation.
Fig. B.1.6–8 Roof integration
A collector system can be installed on any
solid substructure or be freestanding. For
freestanding installations, the collector
system is secured against slippage and
lift-off by weights (ballast). "Slippage" is
the movement of the collectors on the roof
surface due to wind, because of insufficient
friction between the roof surface and the
collector fixing system. Securing against
slippage can be effected by guy ropes or
fixing to other roof components. This always
requires a separate connection.
In flat roof installations the collector
slope can be matched to the system
operating mode. A steep or shallow
angle is used depending on the
utilisation focus.
Fig. B.1.6–9
Installation on a substructure
Fig. B.1.6–10
Freestanding installation
B.1 Collectors
Fig. B.1.6–11
Flat roof installation (row clearance)
The calculation procedure is specified
in VDI 6002 part 1. Although phases of
shading in the morning and evening cannot
be avoided, the loss in yield can be regarded
as irrelevant.
The resulting clearance between rows is
calculated as follows:
Collector row distance
Collector angle of inclination
Collector height
Angle of the sun
The clearance between collector
rows must be sufficient to
sin(180° – (A+B))
z = Collector row clearance
h = Collector height
A = Collector angle of inclination
B = Angle of the sun
prevent shading.
Clearance between collector rows
When installing several rows of collectors in
series behind each other, suitable clearance
to prevent shading must be maintained.
Determining this clearance requires the
angle of the sun at midday on the 21.12, the
shortest day of the year. In Germany this
angle (subject to latitude) lies between 11.5°
(Flensburg) and 19.5° (Konstanz).
Using Würzburg as an example with a collector of
1.2 m height, angled at 45°:
1.2 m
sin(180° – (45°+16.5°))
= 3.72 m
The centre dimension z of the collector rows must
be at least 3.72 m in this case.
Würzburg is approximately located on latitude 50°
north. In the northern hemisphere, this value is
deducted from a fixed angle (90° – 23.5° = 66.5°)
The Viessmann technical guides contain the
corresponding clearances between rows of all
collector types at different angles of the sun.
(see chapter A.1.1).
In Würzburg the sun at midday on the 21.12
therefore stands at 16.5° (66.5° – 50° = 16.5°).
Fig. B.1.6-13
Fig. B.1.6–12
Flat roof installation (horizontal)
Flat roof installation (horizontal)
Flat roof, horizontal
Vacuum tube collectors with direct flow can
also be installed horizontally on flat roofs. The
yield per m2 collector is a little reduced in this
case (see chapter A.1.2). On the other hand,
the amount of time required for installation
may, subject to circumstances, be much
reduced. If the collector tubes are oriented
east-west, the yield can be slightly improved
by rotating the individual tubes by up to 25°.
Flat-plate collectors cannot be mounted
horizontally, as the glass cover cannot be kept
clean simply through rain, and the venting of
the collector would be more difficult.
c. Wall
Generally speaking, all collector types can be
mounted on walls.
However, it should be noted that this type
of installation is subject to certain legal
requirements. For the rules regarding the
implementation of collector systems, see the
list of Building Regulations (LTB) [Germany]
or local regulations. This combines the
technical rules of all Federal States [Germany]
Fig. B.1.6–15
Installation on walls (vertical)
> 4m
If vacuum tube collectors are installed vertically on walls, the
absorber inclination can be adjusted accordingly. The catch
tray is a security device.
Fig. B.1.6-16
Installation on walls (sloping)
regarding the use of "linear supported glazing"
(TRLV) issued by the Deutschen Instituts
für Bautechnik (DIBT) [German Institute
of Building Technology], which includes
collectors. The TRLV is primarily concerned
with the protection of pedestrian and traffic
areas against falling glass.
Glazing installed at an angle of inclination in
excess of 10° towards the vertical is referred
to as overhead glazing. Glazing with an angle
of inclination of less than 10° is known as
vertical glazing.
Fig. B.1.6–14 Installation on walls,
vertical surfaces.
The TRLV applies to all overhead glazing and
vertical glazing, the top edge of which lies
four metres or higher above traffic areas.
For these cases, only standard safety glass
compliant with DIN 1249 is permissible.
Collector glazing does not meet this standard,
otherwise its optical properties would be too
severely impaired. Therefore, collectors above
traffic areas must be secured using suitable
measures, such as netting mounted below or
trays that would catch any falling glass.
In installations parallel to a wall (facing south),
on an annual average, approximately 30
percent less radiation hits the collector than
in installations on 45° supports. If the main
period of use falls into spring and autumn
or winter (solar central heating backup),
higher yields may still be achieved from the
collectors, subject to the prevailing conditions
(see chapter A.2.4).
If collectors are not installed parallel to the
wall, the yield would be comparable to that
of flat roof or pitched roof systems. Where
several rows of collectors are arranged above
each other, certain clearances must also be
observed to prevent shading. Contrary to
supported systems on flat roofs it is the zenith
of the sun in the height of summer, not winter,
that must be taken into consideration here.
Fig. B.1.6–17 Installation on walls,
sloping surfaces.
In the case of sloping installation on walls, the collector
inclination can be adjusted.
B.1 Collectors
B.1.6.2 Corrosion resistance
Viessmann solar collectors and fixing systems
are made from robustly weather-resistant
materials that promise a long service life – this
must be taken into account when installing
collectors. This applies in particular to the
selection of fixing materials regarding their
corrosion resistance and to their handling by
the fitters.
Small fasteners made
from zinc-plated ferrous
material are not safe
from corrosion in
aluminium or stainless
steel designs. Rusty
screws – and such
screws will rust – are
not only unsightly but
will also, in the long
term, endanger the
static safety of the
entire superstructure.
The use of zinc spray
does not alleviate
this problem.
Fig. B.1.6–18 When selecting
materials, ensure that corrosion
is reliably prevented.
Right: Fig. B.1.6-19
fixing elements
The safest solution is offered by stainless
steel and/or aluminium. Both materials are
inherently, as well as in combination with
each other, resistant to corrosion. When
used near the coast, aluminium components
need to be anodised or otherwise additionally
protected. Viessmann fixing materials are
exclusively made from stainless steel or
aluminium, including all associated bolts, nuts
and ancillary fixing elements. If, due to special
structural circumstances, a collector retainer
is individually designed and manufactured,
the corrosion protection of such components
must also meet these high quality standards.
If in larger (flat roof) systems, a design made
from galvanized supports is used for reasons
of cost or static demands, such a design must
be manufactured in line with conventional
procedures for fixings in the roof area. No
holes should be drilled after fitting galvanized
components to the roof. The actual collector
fixings are mounted with support clips. We
cannot recommend holes being drilled into the
supports prior to galvanizing, as later on they
rarely fit with any degree of accuracy on site.
The rafter anchors or roof hook fixings
must also be made from corrosion-resistant
material. Although there is no direct contact
with rain water, frequently humidity in the
air condenses on metallic components
immediately underneath the roof membrane.
B.1.6.3 Wind and snow loads
Every collector fixing must be designed
so that it can absorb the locally occurring
maximum wind and snow loads and that,
as a consequence, appliances and building
structures are protected from damage. The
appropriate rules to be observed here are
described in DIN 1055 or EN 1991.
Snow represents an additional weight bearing
down on the construction. Therefore, when
engineering a solar thermal system, take
the snow load zone into account where the
system is to be installed.
Wind has a pressure or suction effect on
the construction. For this, the height of the
building is a vital factor. DIN 1055 specifies
wind zones and terrain conditions that,
together with the building height result in the
respective specific load assumptions.
With Viessmann collectors, all fixing elements
and accessories are tested to EN 12975.
Their stability is verified – including in their
interaction with all components. This applies
to standard fixings as much as to special
versions under extraordinary conditions, such
as for snow load zone 3 (less than 1 percent
of the territory of Germany).
The verified stability to EN 12975 is an
essential prerequisite for the stability of
overall constructions but is, on its own,
inadequate for the engineering of a system.
To ensure the safety of the overall
construction, the following questions need
Fig. B.1.6–20
to be addressed in the run-up to completing
the engineering:
1. Will the existing or intended roof structure
bear the weight of the collector, its
substructure and the additional load
through snow, wind pressure or wind
2. Are the fixing points or – in the case
of freestanding systems – the ballast
correctly sized in relation to the building
height to ensure the system stability?
Question 1 can only be answered if
adequate knowledge about the building
or its condition is available and certain
parameters from the response to question
2 are known. To facilitate the latter in an
easy and practical manner, Viessmann
offers a suitable calculating program. After
entering certain data (collector type, angle of
incidence, building height, location, etc.), the
relevant load assumptions for fixings can be
quickly determined.
Special conditions
prevail in the edge
areas (unpredictable
turbulences) that
only allow building
under special load
assumptions. Mounting
collectors in these areas
is risky and should
therefore be avoided.
Certain parts of the roof are subject to special
t Corner area – limited on two sides by the
end of the roof
t Edge area – limited on one side by the end
of the roof (excluding eaves)
The width of strips in corner and edge areas
must be calculated relative to the building and
location in accordance with DIN 1055 part 4.
It may not be less than 1 m. This calculation is
part of the program.
The corner and edges of
Corner and edge area
a roof are not suitable for
installing collectors.
Corner area
Edge area
B.1 Collectors
Fig. B.1.6–21 Separating clearance
B.1.6.4 Lightning protection
The installation of a lightning protection
system is voluntary, subject to there being
no legal requirements in the country of
installation. Subject to the location of the
building, its height and type of use, authorities
may determine a level of risk from which
the respective lightning protection class
is derived. This is required to determine
the necessity and type of lightning
protection system.
The same rules apply to collectors, their
fixings and components as for all other
parts of the building and its installations
that are at risk of a lightning strike. For that
reason, observe all recognised technical
rules appertaining to lightning protection
when installing solar thermal systems. This
concerns defence against the risk of direct
lightning strikes (external lightning protection)
and that of induced over voltage (internal
lightning protection).
a. External lightning protection
If a lightning protection system is already
installed, generally integrate collectors and
their fixings into that system. This makes
it necessary to raise the entire lightning
protection system to the current technical
standard. Older lightning protection systems
that are technically outdated or no longer
meet current standards, will offer some
protection to the existing building. That will,
however, be ineffective as soon as system
changes are made.
0.5 m
When fitting collectors, maintain a safety clearance towards
the lightning protection system.
Lightning protection on pitched roofs
Integrate a solar thermal system on a pitched
roof into the lightning protection system so
that the collectors are also protected against
a direct lightning strike. For that to be the
case, the entire collector surface area must
lie within the mesh of the lightning protection
system. For this, maintain an all-round safety
clearance of approx. 0.5 m between the
collector array and the conducting parts of the
lightning protection system. For the precise
calculation of this separating clearance see
DIN EN 62305 part 3.
Lightning protection on flat roofs
If collectors are supported on the flat roof of
a building with a lightning protection system,
the air terminal rods of the lightning protection
system must protrude far enough over the top
edge of the collectors.
Fig. B.1.6–22 Lightning ball procedure
Lightning ball
20 m
30 m
45 m
60 m
1 Lightning ball (radius according to lightning protection category)
The air terminal rods must be high enough to avoid the lightning ball touching the collectors.
Air terminal rod
Fig. B.1.6–23 Lightning protection overview
The installation of collectors on
roofs raises questions regarding
Lightning protection system installed?
lightning protection. Should a
lightning protection system be
created or modified, is additional
appropriate expertise required, for
Are all system components in the safety zone?
(observe separating clearance)
Are system components particularly exposed?
(possibility of direct lightning strikes)
Establish external lightning protection; carry out risk analysis if required.
Additional measures:
– Connecting solar circuit line to equipotential bonding
– Surge protection, sensor and controller
Generally no additional external lightning protection required.
Recommended measures:
– Connecting solar circuit line to equipotential bonding
– Surge protection, sensor and controller
As a test, the "lightning ball procedure" may
be used: an imaginary ball is "rolled" over
the system to be protected. For this, the ball
surface must only touch the air terminal rods.
The ball radius is determined by the lightning
protection category.
b. Internal lightning protection
The internal lightning protection prevents
damage through flashover in the building
installations when the building is hit by a
direct lightning strike.
Buildings without a lightning
protection system
The risk of a direct lightning strike is not
increased by installing a collector array on a
pitched roof.
In buildings and collectors without external
lightning conductors, integrate the flow and
return lines of the primary circuit in the same
way as all other installation lines into the main
earthing system.
It is a widely held
misconception that
omitting the earthing
system results in a
lower risk of lightning
strikes into a nonearthed collector array.
However, circumstances are different when
installing collectors on a flat roof. Here, the
collectors are frequently exposed points and
therefore potential striking points. The solar
thermal system would therefore require
protective measures to be taken.
If the collector system is installed on a
building with external lightning protection
and there is an adequate clearance between
the collector components and the lightning
protection system, proceed likewise.
Earthing the metal components via an
externally routed earth cable offers adequate
protection (subject to observing the separating
clearance towards other metal components).
This earth conductor would be connected with
an earthing bolt in the foundations or another
suitable earthing facility.
DIN EN 62305 part 2 offers various
procedures or means for assessing the
lightning damage risk. For a quick check as
to whether and which measures may be
necessary, see the overview in Fig. B.1.6–23.
If the collector array is earthed separately (flat
roof without lightning protection system), we
would recommend the integration of the solar
circuit by copper cable of at least 16 mm2 into
the main earthing system.
The internal lightning protection is also
important when system components are at risk
from strikes near the system. It reduces the
risk of overvoltage through electromagnetic
lightning pulses inside the building, and
protects the system components. To deflect
these risks, so-called lightning boxes are used
as overvoltage protection.
B.1 Collectors
B.1.7 Collectors as design features
Flat-plate and tube collectors offer many
opportunities to act as aesthetic design
features in the building architecture. Coupled
with a high degree of functionality, these
systems offer imaginative opportunities for
modern architecture.
Viessmann tube collectors are not simply
matched to the design of a building, instead
they are used as an element of design in
In the "City of tomorrow" in Malmö, Sweden,
ideas about an ecological blueprint town have
already taken impressive shape. 500 living
units cover their entire energy demand
exclusively from renewable resources. The
Viessmann tube collectors give this estate a
quiet avant-garde visual appeal that perfectly
demonstrates the innovative integration of
technology into architecture.
Fig. B.1.7–1 Collector wall in the "City of tomorrow".
Viessmann collectors are also very effective
as wide projections or as freestanding
structures: whilst the collectors absorb solar
energy, they are also effective at providing
shade, as demonstrated by the example of a
school in Albstedt.
Fig. B.1.7–2 Collector providing shade.
Frame and cover panels for Viessmann
collectors are available in all RAL colours,
as are the connection chambers. Thus they
provide a harmonious transition between
collector array and roof.
Fig. B.1.7–3 Collector as colour-matched roofing element.
There are many examples of collectors
used as design elements. These show that
solar thermal systems are more than "just"
collectors. They are multi-functional and
in addition a clearly visible aesthetically
appealing contribution to protecting natural
resources and the climate. This should always
be part of the motivation in favour of investing
in a solar thermal system.
Fig. B.1.7–4 Collectors as a design element of the Heliotropes in
Freiburg, Germany.
B.2 Cylinders
The cylinder in a solar thermal system must balance the fluctuations
in timing between the availability of insolation and the demand for heat.
The previous chapters have described the
available insolation and collector technology.
This has made it clear that the energy demand
and the energy generated by solar thermal
systems must not only be considered in
terms of amount. Of primary concern is its
availability over time. This is quite unlike
systems with a heat generator where the
selected output is always available. For that
reason, the cylinder technology is of particular
relevance to solar thermal systems.
Fig. B.2–1 Vitocell 100-U with
integral Solar-Divicon.
This chapter explains the principles of
cylinder technology, plus the different types
of cylinders and methods of heating them.
For information regarding the applicationdependent sizing, see chapter C.2.
B.2.1 Why store energy?
Two characteristics are typical for the
operation of a solar thermal system; these
determine the storage requirements.
One square metre of collector area has a
maximum output that can be calculated. The
collector yield likely to be achieved, relative to
a given period, can also be calculated (in kWh
per period). For this, the following applies:
the longer the period considered, the more
accurate the yield forecast and vice versa
– the shorter the period, the less accurate
the prognosis.
Firstly, particularly on sunny days there are
relatively long periods of "heat generation",
i.e. the collector produces heat for longer
periods of time. To obtain the required amount
of energy, the output of the collector system
must, consequently, be smaller than, for
example, that of a boiler system, which would
make the required amount of energy available
with significantly shorter burner runtimes, but
with a higher output.
This allows the annual insolation and the
resulting annual yield to be forecast with
relatively small fluctuations. However, it
is impossible to make such forecasts for
individual days or hours. This represents
a significant difference between the solar
collector as heat source and a boiler,
for example.
Secondly the period of heat generation
and the period of heat consumption are
rarely the same. Heat generation in a
conventional system is regulated by demand;
heat generation by means of a collector
system is exclusively dependent on the
available insolation.
These particular features highlight that for
the successful operation of solar thermal
systems, an adequately sized cylinder to store
the heat yielded from solar energy is a must.
Fig. B.2.1–1
Draw-off profile
The daily sequence of the draw-off
profile in an apartment building
indicates the demand for available
heat. However, the heat generation
Solar heat generation
is not governed by this demand, but
Flow rate (at 45 °C) in l/min
by the available insolation.
Cylinder demand
Daily draw-off profile
B.2 Cylinders
B.2.2 Principles of cylinder technology
B.2.2.1 Cylinder medium
Generally, water is used to store heating
energy. It is affordable, always available and
technically easy to control (storage, heating,
drawing). In addition, water has a high thermal
capacity of cw = 4.187 kJ/(kg·K).
In the heating sector, the indication
cw = 1.163 Wh/(kg·K) is more common. It is
irrelevant to storing heat whether the medium
is heating water or potable water.
In case of central
heating or air
conditioning with solar
backup, the building
too acts as an energy
store – its capacity
must be taken into
consideration when
sizing the system.
Apart from the short-term storage of solar
energy in conventional floorstanding cylinders,
it can also be stored for longer periods.
Long-term or seasonal cylinders mostly
operate with water as their storage medium
and have a volume of several thousands
of litres (large floorstanding cylinders) or
even several thousands of cubic metres
(e.g. concrete basin).
Research into ways of storing heat by a
physical-chemical method, referred to as
"latent heat stores", is currently ongoing. This
method uses, for example, the phase change
(solid to liquid and vice versa) of materials,
such as paraffin and salt to store heat.
Fig. B.2.2–1
B.2.2.2 Energy content
When sizing a cylinder, its energy content
is more important than its volume. The
energy content of a cylinder depends on the
temperature spread. The wider the spread,
the higher the content of available energy per
volume unit of the cylinder.
To determine the cylinder volume, the
temperature spread on the heat consumer
side is taken into account. The maximum
cylinder temperature is dictated by the
medium, i.e. by the water. The minimum
possible cylinder temperature is therefore the
decisive variable for determining the required
cylinder volume.
To size DHW cylinders, the average cold
water temperature (e.g. 10 °C) is taken as
minimum temperature. In the case of buffer
cylinders used to heat DHW (e.g. via a
freshwater station), the minimum cylinder
temperature is determined by the cold water
temperature and the temperature differential
between the DHW heat exchanger inlet
and outlet (temperature differential). For
solar central heating backup, the minimum
temperature during the heating season is
defaulted by the heating return.
energy content
The energy content of a cylinder
is largely subject to the lowest
temperature that may occur
DHW cylinder
Buffer cylinder
for DHW heating
Buffer cylinder
for central heating backup
90 °C
90 °C
90 °C
inside the cylinder.
80 K
46.4 kWh
Cold water
10 °C
75 K
43.5 kWh
15 °C
60 K
34.8 kWh
30 °C
Maximum energy content with a cylinder volume of 500 l each
B.2.2.3 Temperature stratification
Detached house with 4 occupants, 28 l DHW
demand per person (60 °C), representing 112 l
per day.
At a cold water temperature of 10 °C, this
represents an energy amount of 6.5 kWh, plus heat
demand for cylinder losses (1.5 kWh) and DHW
circulation (1.5 kWh).
As a result, the total energy amount for DHW
heating is 9.5 kWh.
Independent of their volume, solar cylinders
are generally made in the shape of slim
floorstanding cylinders – consequently,
the different density of hot and cold water
allows the formation of good temperature
stratification or layering. This results in the
lighter warm water "floating" on top of the
heavier cold water. Subject to the water not
being turbulated through internal flows, this
stratification remains very stable.
To achieve a high solar coverage, twice the amount
of energy is to be stored, that is 19 kWh.
Fig. B.2.2–4
Temperature stratification
The cylinder volume is calculated as follows:
cw · ΔT
Cylinder capacity (cylinder volume)
Energy amount
Thermal capacity of water
Temperature differential
Solar cylinder
With a cold water temperature of 10 °C, the volume
required to store 19 kWh at a maximum cylinder
temperature of
60 °C:
19 000 Wh/(1.16 Wh/(kg·K) · 50 K)
328 l
80 °C:
19 000 Wh/(1.16 Wh/(kg·K) · 70 K)
234 l
90 °C:
19 000 Wh/(1.16 Wh/(kg·K) · 80 K)
205 l
Except for the border layer which is only a few centimetres deep,
the temperature layers will not mix and remain very stable.
The energy content of a cylinder (see Fig.
B.2.2-1) can be calculated by changing
the formula.
Q = m · cw · ΔT
A lower layer inside the cylinder that is
reasonably cold ensures that the solar circuit
can operate with low return temperatures,
enabling the solar thermal system to operate
highly efficiently. The temperature layers
inside the cylinder must be protected
against turbulence.
Inside the DHW
cylinder, the DHW
circulation in particular
can result in a
severe mixing of the
cylinder content.
Apart from the flow rate
and the runtime of the
DHW circulation pump,
the connection of the
DHW circulation return
must also be taken into
consideration. It must
not be connected to
the cold water inlet of
the cylinder, otherwise
the complete cylinder
volume would be
circulated by the DHW
circulation pump.
B.2 Cylinders
B.2.2.4 Heat losses
When determining the cylinder volume for a
solar thermal system, the heat losses of the
cylinder must also be taken into consideration.
A larger cylinder volume can store more
energy, but will equally suffer higher heat
losses (and they are more costly). Although,
with increasing cylinder size the specific heat
losses fall, but the absolute losses rise.
In principle, the following applies: one large
cylinder is more advantageous than several
small cylinders. The heat losses of one large
cylinder will always be, on account of the
better surface/volume ratio compared to
several small cylinders, significantly lower.
However, when making a cylinder selection,
the project-specific limits must also be
considered, such as the width of doors and
the room height (size when tilted!). In addition,
the number of cylinders is determined by the
selected system connection.
approx 3 kWh/d – subject to the optimum
installation of the cylinder and its connections.
Significantly higher losses may occur if the
thermal insulation is inadequate.
For example, heat losses can occur if heat can
rise from the cylinder through the pipework.
These convection losses can be prevented,
for example through the connection of the line
concerned via a thermosiphon loop or through
gravity brakes.
Poorly insulated cylinder connections are critical
because such losses can easily double the
stated value. For example, the losses from a
300 l cylinder can amount to 4 kWh/d · 365 d =
1 460 kWh. Taking 50% as unavoidable losses,
just the avoidable loss at a solar coverage
of 50 percent would represent an additional
demand of 1 m2 collector area and an additional
consumption of 50 litres fuel oil or equivalent
other form of fuel.
Cylinder losses are differentiated according
to standby losses (in kWh/d, see DIN
4753-8) and the heat loss rate (in W/K, see
DIN V ENV 12977-3).
Subject to size, a high grade floorstanding
cylinder for solar DHW heating in a detached
house has a heat loss of between 1.5 and
Fig. B.2.2–5
Convection losses
The difference in density of
the cylinder water can result
DHW mixer
in undesirable heat losses
through convection flow.
This leads to continuous heat
losses from the cylinder via
connections and pipework.
Heat losses
Heat losses
Solar cylinder
Pipe connection
Convection losses through
gravity circulation in the pipework
Convection losses through
circulation inside the pipe
B.2.2.5 Cylinder material
DHW cylinders for the storage of domestic
hot water are made from stainless steel or
enamelled steel. As faults in the enamel
coating cannot be totally prevented, even in
the most careful production, an additional
corrosion protection is required for these
cylinders. This function is fulfilled by
impressed current anodes, sacrificial or
magnesium anodes.
DHW cylinders made from stainless steel
generally require no additional corrosion
protection. In addition they are also lighter.
On the other hand, buffer cylinders are not
filled with oxygen-rich potable water but are
filled with heating water. Therefore, steel
cylinders can be used without any corrosion
protection. As they are also operated at low
pressure (heating circuit instead of the mains
water network), they offer a price advantage
over DHW cylinders.
To complete the round, buffer cylinders
made from plastic should also be mentioned.
Although this material is light and economical,
it can only be used with low maximum
temperatures. Furthermore, such cylinders
can only be operated at zero pressure, making
an additional heat exchanger essential.
Fuel oil tanks that are no longer required are
sometimes seen as an affordable solution
for storing heat, as they offer a large cylinder
volume of several thousands of litres.
t The highly unfavourable surface/volume
ratio and the difficulties in applying thermal
insulation result in high heat losses.
Therefore in summer, the tank would turn
into an undesirable heat source.
t The installation of heating and draw-off
technology is very extensive.
t Modifications (cutting, grinding, welding)
are only possible with a nitrogen filling.
t Such cylinders can only be operated at
zero pressure.
B.2 Cylinders
B.2.3 Cylinder types
Fig. B.2.3–1
Viessmann DHW cylinders
B.2.3.1 Cylinders with internal
indirect coils
The entire water content is heated only by an
indirect coil drawn right to the cylinder floor.
a. Potable water as storage medium
Dual-mode cylinders
In new build or when installing a completely
new heating system, a dual-mode DHW
cylinder is the preferred option for DHW
heating in smaller systems.
for utilising solar energy are
characterised by their slimline
cylindrical shape. They are made
from stainless steel or enamelled
steel; they are corrosion protected
and feature highly effective all-round
CFC-free thermal insulation.
When potable water is used as the storage
medium, only DHW heating can be provided
by a solar thermal system. Drawing on the
stored energy inside the cylinder for other
purposes, such as central heating backup
is not recommended. DHW cylinders
must generally be able to withstand 10 bar
Mono-mode cylinders
If an existing heating system is being
extended by a solar thermal system and the
existing DHW cylinder continues to be used, a
second mono-mode DHW cylinder is installed
upstream. Larger (even newly installed) DHW
heating systems can feature two mono-mode
Fig. B.2.3–2
Mono-mode cylinder
Mono-mode DHW cylinder Vitocell 100-V
A dual-mode DHW cylinder is fitted with two
indirect coils – a lower coil for connection to
the collector circuit for solar heating the DHW
and an upper coil for connection to a boiler to
provide reheating.
Generally, when using potable water as the
storage medium, ensure that the cylinder
areas exclusively heated by solar energy or
a pre-cylinder are pasteurised in accordance
with the appropriate standards concerning the
hygiene level of potable water.
Fig. B.2.3–3
Dual-mode cylinder
Dual-mode DHW cylinder Vitocell 100-B
b. Heating water as storage medium
If heating water is selected as the storage
medium, buffer or combi cylinders are
used. These are predominantly used in
systems where the yielded solar energy is
to be used for DHW heating and for central
heating backup.
In large solar thermal systems for DHW
heating, heating water is used as the storage
medium. In such cases the storage medium
does not require pasteurisation.
Buffer cylinder
With heating water buffer cylinders, the
solar energy can be directly used to heat
the heating circuit or can be utilised via a
freshwater station for DHW heating.
Additional heat sources, such as solid fuel
boilers, can also be connected to the buffer
cylinder. This way, the energy flow in dual and
multi-mode systems can be ideally "managed"
in the buffer cylinder.
The cylinders are designed in accordance with
the pressure stages in the heating circuit.
Corrosion protection is superfluous as these
are sealed unvented cylinder circuits.
Fig. B.2.3–4
Buffer cylinder
Fig. B.2.3–5
Buffer cylinder concept
DHW heating
The buffer cylinder as "energy
manager" enables the integration
of different heat sources
and consumers.
Heating water buffer cylinder with internal indirect solar coil
Vitocell 140-E
B.2 Cylinders
Combi cylinders
The combi cylinder is a combination of heating
water buffer cylinder and DHW cylinder. It
is also suitable for several heating sources.
Heat is transferred for DHW heating via an
indirect coil (for the Viessmann heating water
buffer cylinder Vitocell 340-M and 360-M
via corrugated stainless steel pipes), through
which the cold supply water is routed and
thereby heated.
Fig. B.2.3–6
Combi cylinder
B.2.3.2 Cylinders for external heating
Size is not the only crucial element when
selecting cylinders with external heating, as
the heat input is determined by the selection
of the plate-type heat exchanger (see chapter
B.2.5.2). The application options, as well as
the corrosion protection requirements and
pressure resistance, correspond to those of
cylinders with internal indirect coils.
Fig. B.2.3–7
DHW cylinder
Primary cylinder Vitocell 100-L
Multi-mode heating water buffer cylinder with integral DHW
heating Vitocell 340-M
Fig. B.2.3-8
Buffer cylinder
Cylinder for storing heating water Vitocell 100-E
B.2.4 Cylinder heating
B.2.4.1 Stratification heating
The stratification principle sees the layering
of the water heated by solar energy at a
level that has the same temperature. There
is then no mixing with cooler levels. Internal
and external indirect coils are suitable for
stratification heating.
Stratification concept
When heating a cylinder by means of an
internal indirect coil without stratification,
the complete cylinder content is heated
evenly. Until the useful temperature in
the standby part of the cylinder has been
reached, the collector array must deliver heat
over a comparatively long period. If useful
heat is required before this temperature has
been reached, a booster heater delivers the
required temperature level.
Fig. B.2.4–1
The stratification principle can reduce the
demand for reheating by stratifying the water
heated by solar energy in an unmixed state,
where possible at the useful temperature
level, into the standby section of the cylinder.
This way the useful heat from the collector
array can be made available earlier – under
optimum conditions, prior to the booster
heater starting.
Stratification concept
The benefit of stratification
is the more rapid achievement of
the target temperature.
With a standard dual-mode cylinder,
the collector continuously heats the
entire cylinder volume. The entire
content has been heated when the
target temperature is reached.
With stratification, the target
temperature is reached earlier in the
upper cylinder section. However, the
total cylinder content reaches target
temperature at the same time [i.e.
14.00 h in the example shown], as in
a standard cylinder.
9.00 10.00 11.00 12.00 13.00 14.00
9.00 10.00 11.00 12.00 13.00 14.00
B.2 Cylinders
Fig. B.2.4–2 Matched flow operation
Subject to the insolation level, the
collector circuit operates with a
high or low flow rate. This enables
the cylinder to be heated to the
target temperature level.
The cylinder is heated to a lower
temperature level if the insolation
is insufficient.
65 °C
45 °C
For stratification, the collector circuit is
generally run with a wider temperature
spread, i.e. the flow rate is reduced compared
to conventional cylinder heating. The result
is a higher average collector temperature and
consequently a lower collector efficiency.
Due to their low heat losses, vacuum tube
collectors are more suitable for stratification
than flat-plate collectors – this is particularly
pertinent for systems used for central
heating backup.
Fig. B.2.4–3
Stratification with internal indirect coil
25 °C
25 °C
The flow rate of the collector circuit is
regulated for stratification so that, at the
collector outlet (solar flow), the target
temperature, i.e. the useful temperature plus
the temperature differential of the indirect
coil, is being maintained. Should the solar
energy be insufficient to achieve that, layering
will be effected lower down in another
cylinder to maintain stratification. This results,
subject to insolation and the temperature
levels already achieved, in different volume
flows in the solar circuit, known as the
"matched flow operation".
Technical realisation
Cylinders with internal indirect coils contain
an up-current insert through which the water
heated to the useful temperature level can
rise into the standby section of the cylinder
with the least possible mixing. If the target
temperature is no longer achieved at the
collector outlet, the water heated to below
the required level flows out of the up-current
pipe into the layer with the same temperature
(= same density).
During stratification with an up-current "chimney", the water
heated by solar energy rises inside the "chimney" to the level
which has the same temperature.
Fig. B.2.4–4 Stratification with external indirect coil
Either the pump speed follows the
insolation, resulting in the cylinder
being heated to target temperature,
or the cylinder heating is regulated
via valves at a constant flow rate.
In case of the external heat transfer, the
cylinder is heated from above for as long
as the collector circuit can make the target
temperature available. When that temperature
can no longer be achieved, the solar heat is
either layered into deeper colder zones via
valves, or the pumps stop.
Favourable conditions for stratification on
the heat transfer side are offered by systems
with a high temperature spread, such as is
the case with solar thermal DHW heating.
However, the potential benefit of stratification
(saving reheat energy) will only manifest itself
in systems with a high solar coverage (> 50%)
if a demand exists before midday, even in the
summer months.
In the case of consumption profiles with
peaks in the morning and evening, the solar
thermal system has sufficient time during
the summer to heat the cylinder adequately
even without stratification. Stratification with
this consumption profile is therefore only
beneficial in spring and autumn.
In both cases stratification is only beneficial if
reheating is regulated precisely in accordance
with demand.
Large solar thermal systems for DHW heating
that are sized in accordance with VDI 6002
part 1 for high yields and therefore low solar
coverage, hardly achieve the level of useful
temperatures. Although the large cylinders
used for this purpose are generally equipped
with an external indirect coil, the system
operation is not regulated to the DHW
temperature as target temperature, making a
case in favour of stratification difficult.
In the case of solar thermal systems that also
back up central heating, heating circuits that
operate with a wide spread (radiators) are
particularly suitable for stratification.
B.2 Cylinders
B.2.4.2 Heating with gravity –
thermosiphon systems
Gravity principle
In the case of thermosiphon systems, the
transfer between cylinder and collector is
governed by gravity, also referred to as the
"thermosiphon principle". Instead of a pump,
the pressure differential between the hot
and cold heat transfer medium is utilised as
propulsion energy. For this, the collector (heat
generator) must be located below the cylinder
(heat consumer).
Fig. B.2.4-5 Viessmann
thermosiphon systems are not used
in central Europe.
The heat transfer medium is heated by
insolation inside the collector. The hot liquid
in the collector below is lighter than the cold
liquid in the cylinder above the collector. As
soon as the lighter hot liquid rises, gravity
circulation commences.
Inside the cylinder, the heated liquid passes
its heat to the stored DHW and then falls
back to the lowest point in the collector
circuit. In other words, it causes the liquid to
circulate. This circulation is interrupted when
the temperature/density differential between
the collector and cylinder is so small that it is
inadequate for overcoming the pressure drop
inside the collector circuit.
Pure water which is free from bubbles and at
20 °C has a specific gravity of 0.998 kg/l, and
at 50 °C has a specific gravity of 0.988 kg/l –
accordingly, the difference amounts to
approx. 10 grams per litre (= 1 percent). The
driving force in this circuit is therefore very
low, compared to pumped systems.
During operation, typical characteristics
therefore result for a thermosiphon system:
t Low flow rate
t No turbulent flow inside the absorber
t The collector circuit must have a low
pressure drop (short length, large crosssection)
t The cylinder must be prevented from
cooling down at night through reverse
gravity circulation.
Fig. B.2.4–6 Thermosiphon systems
Single circuit systems are only used
in regions free from the risk of frost.
For two circuit systems,
cylinders with twin walls as heat
exchanger are used.
Single circuit system
Two-circuit system
Single circuit and two circuit systems
Thermosiphon systems are generally classed
as single circuit or two circuit systems. In
single circuit systems, the DHW is heated
immediately at the collector. In two circuit
systems, the heat transfer medium inside
the collector circuit and the DHW inside the
cylinder are separated by a heat exchanger/
indirect coil.
Single circuit systems are exclusively used
in regions free from the risk of frost, as the
collectors would freeze-up and be destroyed
even after the slightest frost. In addition, all
components must be corrosion-resistant,
as with the DHW, oxygen is permanently
introduced into the system. The advantage of
this system lies in the easy compact layout
and a correspondingly favourable price.
In regions where frost cannot be excluded,
two line systems are used. The collector
circuit in these systems is filled with a heat
transfer medium that is safe from the risk
of frost. In most cases, cylinders with twinwalls are used for the heat transfer. The heat
transfer medium heated inside the collector
transfers its heat to the DHW between the
internal and external skin of the cylinder.
Fig. B.2.4–7
Apart from the collector circuit, the DHW
side is also subject to the risk of possible
frost. Therefore, the cylinder is installed in
a room above the collector that is free from
the risk of frost or permanently protected
against freezing up via a booster heater. The
connected pipework (cold and hot water)
must also be included in the frost protection
measures. As an alternative, the cylinder and
all supply lines can be drained when there is a
risk of frost.
The risk of overheating must also be taken
into account for single circuit and two
circuit systems. If there is no opportunity
for a thermostatic control, heat continues
to be transported into the cylinder until the
stagnation temperature has been reached. In
single circuit systems, the water inside the
cylinder then reaches boiling point. In two
circuit systems it is the heat transfer medium
between the cylinder walls which does this.
Any required reheating will be effected either
immediately inside the cylinder via an electric
immersion heater. Alternatively the DHW is
reheated by a regulated instantaneous water
heater installed downstream. From an energy
viewpoint the latter option is preferable.
Reheating options
In most cases, electric immersion
Electric immersion heater
Instantaneous water heater
heaters are used for reheating. An
instantaneous water heater is more
expensive, but, from an energy
aspect, makes more sense.
B.2 Cylinders
B.2.5 Heat exchangers/indirect coils
B.2.5.1 Internal indirect coils
Heat exchangers in solar thermal systems
must transfer relatively small loads with the
lowest possible temperature loss. Always
observe this when selecting heat exchangers.
Errors made at this point can substantially
reduce the system yield. Each design
should transfer the medium at its coldest
temperature possible back to the collector.
For internal indirect coils, a temperature
differential between the solar circuit and the
surrounding cylinder water of 10 K to 15 K
is common.
Subject to the design of the indirect coil, a
ratio between collector area and exchanger
surface of between 10:1 and 15:1 results.
In other words, 10 to 15 square metres
of collector area can be connected to
each square metre of exchanger surface.
Connecting a larger collector area can lead to
the temperature spread exceeding 15 K.
Independent of type, 600 watts per square
metre collector area is the basis applied as
the sizing output for the calculation of heat
exchangers/indirect coils.
In solar thermal systems
it is particularly relevant
to cool the collector as
efficiently as possible,
i.e. to extract as much
heat from the collector
as possible.
The temperature differential inside the heat
exchanger/indirect coil between the outlet
on the primary side (to the collector) and
the inlet on the secondary side (from the
cylinder) – in the case of an internal indirect
coil, the surrounding cylinder water – should
be as small as possible. The smaller it is, the
more solar heat can be transferred to the
cylinder content.
Fig. B.2.5–1
For the Viessmann cylinder range, the values
in Fig. B.2.5–1 apply.
Internal indirect coils
The transfer rates of the internal
indirect coils depend on the
temperature differential between
Vitocell 100-B / 300 l
1.5 m2 exchanger
of larger collector arrays can also
be transferred.
Temperature differential Tcoll – Tcyl (K)
the feedwater (Tcyl) and the solar
circuit flow (Tcoll). The heat output
Vitocell 100-B / 500 l
1.9 m2 exchanger
Vitocell 340-M / 750 l
1.8 m2 exchanger
Vitocell 340-M / 1000 l
2.1 m2 exchanger
Vitosoll 200-F
Flow rate 25 l/(h·m2)
Vitosoll 200-T
Flow rate 40 l/(h·m2)
Collector area (m2)
B.2.5.2 External heat exchangers
For plate-type heat exchangers, 5 K between
solar circuit return and cylinder return are
thought to be ideal. To make that possible,
heat exchangers should preferably be
selected that ensure that the medium has to
travel the longest possible distance inside the
exchanger (great thermal length).
Independent of the collector type, for
conventional applications (= conventional
temperatures), a design output of 600 W/ m2
is assumed. When operating with higher
temperatures (process heat), this design
output can be reduced to 500 W/m2.
Pressure drop
It has proven useful to limit the pressure drop
on both sides to a maximum of 100 mbar.
The sizing of a plate-type heat exchanger
in a solar thermal system is subject to the
selected cylinder type and cylinder medium –
the engineering principles are always the
same. It is irrelevant which program is
selected for sizing. The input parameters are
always determined in the same way.
When designing systems it may be sensible
to make a comparison calculation. The first
calculation is based on a max. 100 mbar
and the second on max. 150 mbar. Should
this result in a significantly more affordable
heat exchanger then the exchanger may
be selected regarding the overall pressure
drop on the relevant side (generally the
primary circuit).
Flow rate, primary side
The flow rate in the primary circuit (collector
side) results from the collector type used and
the selected specific collector throughput, for
flat-plate collectors for example 25 l/(h·m2).
The parameters described above are adequate
for sizing a plate-type heat exchanger. For
further sizing information, see chapter C.2.1.2.
Flow rate, secondary side
Conventionally, a plate-type heat exchanger
is designed for the same heat flow on both
sides. To achieve this, a flow rate reduced
by 15% must be assumed for the secondary
side (cylinder side) compared to the primary
side. This balances the slightly lower thermal
capacity of glycol:water mixtures.
In central Europe, propylene-glycol with a
concentration of 40 percent is generally used
in the primary circuit, and water is used in the
secondary circuit.
The installation in solar thermal systems is
subject to the standard installation conditions
for plate-type heat exchangers. Shut-off
valves and flushing connections should be
standard for any plate-type heat exchanger.
For information regarding heat exchangers
used for heating swimming pool water, see
chapter C.2.4.
Plate-type heat exchangers must
be free from the risk of frost that
The inlet temperature of the secondary circuit
can be assumed to be 20 °C; the cylinder
water will not reach lower temperatures
during operation. If the system is sized
with the recommended 5 K, then an outlet
temperature in the primary circuit of 25 °C
is assumed. The calculation results in the
temperature values for both the secondary
circuit outlet and primary circuit inlet.
may result from cooled-down heat
transfer media from the solar circuit.
Fig. B.2.5–2 External heat exchangers (plate-type heat exchangers)
heat exchanger
Secondary circuit
Primary circuit
Temperature sensor
Frost stat
Three-way valve (frost protection)
B.3 Primary circuit
Primary circuit
All components and pipework that connect the collector with the cylinder are
described as the primary circuit of a solar thermal system.
This chapter describes typical operating
conditions for a solar thermal system and the
resulting consequences for engineering such
systems. For this, the individual components
of the primary circuit are considered in detail
and are illustrated in their interactive state.
B.3.1 Collector circuit
B.3.1.1 Determining the flow rate
Collector systems can be operated with
different specific flow rates. The applicable
unit for this is the flow rate in litres/
(h · m2), the reference variable being the
Direct flow vacuum tube collectors with
individual tubes linked in parallel within the
collector, require a specific flow rate of at
least 40 l/(h · m2). A matched flow operation
absorber area.
is not recommended for this type of collector
as this would put at risk the even internal flow
through the collector as a whole.
At the same collector output, a higher flow
rate means a lower temperature spread in the
collector circuit; a lower flow rate means a
higher temperature spread.
Attempting to significantly raise these values
in favour of a slightly higher efficiency is not
sensible, as the associated higher pump rate
required cannot be compensated for.
With a high temperature spread (= low flow
rate), the average collector temperature
increases and the collector efficiency
decreases. However, lower flow rates mean
less auxiliary energy is expended for operating
the pump, and smaller connection lines
are possible.
In complex collector array hydraulics with
several collector assemblies linked in parallel,
matched flow operation requires highly
accurate engineering (see chapter C.1.2).
A system with seven flat-plate collectors at 2.3 m2,
Engineers differentiate between
t Low flow operation = operation with
flow rates up to approx. 30 l/(h · m2)
t High flow operation = operation with
flow rates above 30 l/(h · m2)
t Matched flow operation = operation
with variable flow rates
in other words with 16.1 m2 absorber area, and a
required specific flow rate of 25 l/(h · m2) has a
throughput of 402.5 l/h or 6.7 l/min.
This value must be reached at maximum pump rate
(= 100%).
An adjustment can be made at the output stage
of the pump.
The operating modes low flow and high flow
are not defined to a set value by a standard
and are used in various forms in literature.
Which operating mode is the right one?
The following applies for safe engineering:
The specific flow rate must be high enough
to ensure a reliable and even flow through
the entire array. For flat-plate collector
systems and vacuum tube collectors with
heat pipes, this value is 25 l/(h · m2) at 100
percent pump rate. The optimum flow rate
(relative to the actual cylinder temperatures
and the current insolation level) in systems
with a Vitosolic solar control unit will adjust
itself automatically in matched flow operation.
Single array systems that include both of
the collector types mentioned above can
be operated without problems down to
approx. 50% of the specific flow rate. For
a precise setting, see the solar control unit
operating instructions.
Select the first pump stage above the
required value.
B.3 Primary circuit
Fig. B.3.1–1
Flow velocity
Subject to the flow rate and
pipe dimension, different
flow velocities result. The
recommended range lies between
Flow rate
Flow velocity in m/s
(total collector area)
Pipe dimension
in m3/h
in l/min
DN 10
DN 13
DN 16
DN 20
DN 25
DN 32
a good compromise between
pressure drop and ventilation.
0.4 and 0.7 m/s and represents
DN 40
Recommended pipe dimension
Contrary to the
heating circuit, venting
the solar circuit is
also made more
difficult by oversized
pipes. The air has to
move downwards,
not upwards.
Sizing the solar circuit line
The flow velocity is decisive for sizing the
solar circuit lines and is achieved at the
calculated overall flow rate.
In Figure B.3.1–1 you can check the flow
velocities with different pipe dimensions at
the respectively different flow rates.
To minimise the pressure drop, the flow
velocity in the pipe should not exceed 1 m/s.
A flow velocity between 0.4 and 0.7 m/s
is recommended. A higher flow velocity
increases the pressure drop; a substantially
lower velocity will make venting harder (see
chapter B.3.3).
B.3.1.2 Principles of calculating the
pressure drop
Pressure drop of the solar thermal system
For solar thermal systems, calculating the
pressure drop is one of the prerequisites for a
trouble free and energy efficient (regarding the
pump current) operation of the overall system.
Generally, the same rules apply here as for any
other hydraulic system.
For the example with seven collectors (throughput
402.5 l/h or 6.7 l/min), the following values result:
t For copper pipe 15x1 (DN 13)
a flow velocity of 0.84 m/s
t For copper pipe 18x1 (DN 16)
a flow velocity of 0.55 m/s
t For copper pipe 22x1 (DN 20)
a flow velocity of 0.35 m/s
Copper pipe 18x1 is therefore selected.
The overall pressure drop of the primary
circuit of the solar thermal system
(glycol circuit) results from adding the
following resistances:
t Collector pressure drop
t Pipework pressure drop
t Individual pressure drop of the fittings
t Pressure drop of the internal indirect coil
inside the cylinder or of the primary side of
the external plate-type heat exchanger.
At low temperatures around freezing, the
high viscosity of the heat transfer medium
may result in a pump rate some 50 percent
higher than for pure water. From a medium
temperature of approx. 50 °C, in other
words, for the controlled operation of solar
thermal systems, the difference in viscosity is
only minor.
Fig. B.3.1–2
Pressure drop and viscosity
Viscosity [mm2/s]
Information regarding the
heat transfer medium
When calculating the pressure drop, take
into account that the heat transfer medium
has a different viscosity to pure water. The
hydraulic characteristics of the media become
more similar as the temperature of the
media increases.
-30 -20 -10
The viscosity differences between water and glycol/water
mixtures shrink with increasing temperatures.
Method of calculation
1. The specific flow rate for the collectors is
determined by the type of collector used
and the intended method of operation of
the collector array (see above: Determining
the flow rate). Depending on the way the
collectors are linked, the pressure drop of
the collector array results.
Fig. B.3.1–3
Calculating the pressure drop
2. The overall flow rate for the system
results from multiplying the specific flow
rate by the absorber area. Assuming the
required flow velocity of between 0.4
and 0.7 m/s, the pipework dimension is
then determined.
3. If the pipework dimension has been
determined, the pressure drop of the
pipework can be calculated (in mbar/m).
4. External heat exchangers must be
calculated as well and should not exceed
a pressure drop of 100 mbar. For smooth
tube internal indirect coils, the pressure
drop is much lower and can be ignored in
smaller systems (< 20 m2).
5. The pressure drops of further solar
circuit components can be seen from the
technical documentation and are included
in the overall calculation.
Temperature [°C]
1 Flow rate and pressure drop
of the collector array
Sizing the pipework
Pipework pressure drop
Heat exchanger pressure drop
Valves, etc. pressure drop
100 110 120
B.3 Primary circuit
Collector pressure drop
Collectors are subject to the same rules as all
other hydraulic components:
t When linked in series, the total pressure
drop equals the total of all individual
pressure drop values.
t When linked in parallel, the total pressure
drop equals the individual pressure drop.
(Assumption: all individual pressure drop
values are equal).
For pressure drop diagrams for the Vitosol
collectors, see the technical documentation or
The pressure drop diagrams refer respectively
to the complete collector. If collectors are
linked in parallel, the pressure drop of the
total collector array equals that of one single
collector. If collectors are linked in series, the
pressure drop increases due to the higher
flow rate per collector and, in addition, the
individual pressure drop values of all collectors
are added up.
A system with two flat-plate collectors at 2.3 m2, in
other words with 4.6 m2 absorber area, and a
required specific flow rate of 25 l/(h · m2) has a
throughput of 115 l/h.
If the collectors are linked in parallel, then the
throughput per collector is approx. 1 l/min (57.5 l/h).
The individual pressure drop of a single collector is
approx. 70 mbar. The pressure drop values are not
added up; in other words the total pressure drop of a
complete collector array is therefore also 70 mbar.
57.5 l/h
57.5 l/h
Flow rate 25 l / (h · m2), i.e. 115 l / h
If the collectors are linked in series, then the
throughput per collector is approx. 2 l/min (115 l/h).
The individual pressure drop of a single collector is
approx. 200 mbar. The pressure drop values are
added up; in other words the total pressure drop of
the complete collector array is therefore 400 mbar.
115 l/h
115 l/h
Flow rate 25 l / (h · m2), i.e. 115 l / h
Fig. B.3.1–4
Pressure drop Vitosol 200-F
Within the range of the
In both cases the following applies to the total
recommended specific flow rate of
collector array: the average collector temperature is
25 l/(h · m2), the pressure drop of the
identical; the efficiency is almost equal.
collector is approx. 70 mbar.
Pressure drop in mbar
Flow rate in l/min
Pipe pressure drop
Conventionally, pipework is calculated with
the aid of a sizing program – this is a must
for large systems with complex hydraulics.
For simple systems with copper pipes,
approximate values can be applied given the
following assumptions:
t Operating temperature: 60 °C
t Medium: Water:glycol (60 : 40)
t 1 bend (not elbow!) per 2 m copper pipe
t Necessary ball valves and tees (e.g. for fill
& drain valves)
The values in fig. B.3.1–5 correspond to these
approximate values.
For the example with seven collectors (throughput
402.5 l/h) the table shows for the selected 18x1
copper pipe a pressure drop of approx. 5.6 mbar/m
including all fittings.
Fig. B.3.1–5
Flow rate
Pressure drop and pipe diameter
Pressure drop per metre pipework (incl. fittings) in mbar/m
Pipe dimension
in m3/h
DN 10
DN 13
DN 16
DN 20
DN 25
The intended solar circuit should have a length of
18 m. Therefore, a total pressure drop of approx.
100 mbar results.
When using prefabricated Viessmann solar
lines (corrugated stainless steel pipe, DN 16)
the pressure drop values from Fig. B.3.1-6 can
be applied.
Fig. B.3.1–6 Pressure drop, corrugated stainless steel pipe DN16
Range between 0.4 and 0.7 m/s
Pressure drop in mbar/m
Additional components of the
primary circuit
The primary circuit components intended
for a system must flow into the pressure
drop calculation in accordance with
manufacturer's details.
5 6
Flow rate in l/min
30 40
The individual pressure drop values of
components that are assembled into the
Viessmann Solar-Divicon are taken into
account in the calculations for pump sizing
in the following chapter.
B.3 Primary circuit
B.3.1.3 Solar circuit pump
Ensure that the pump
and any additional
electrical components
for speed control are
actually suitable for that
purpose. An additional
relay may be required
in connection with the
Vitosolic solar control
unit, depending on the
current drawn by the
pump. In that case,
disable the variable
speed control of
the pump.
Type selection
In sealed solar circuits, commercially available
centrifugal pumps are used. Subject to the
pump being reliably protected against over
temperatures on site, no other requirements
regarding temperature resistance must
be taken into consideration. The operation
with water:glycol mixtures is generally
unproblematic; if in doubt, refer to the
pump manufacturer.
Some solar thermal systems are offered for
which other pump types are recommended,
such as gear pumps. These pump types are
necessary because components with very
high pressure drop values are used in these
systems. All system schemes in this manual
and the Viessmann components used in these
schemes are designed for operation with
commercially available centrifugal pumps.
As solar thermal systems have become more
popular, special solar circuit pumps with
matching curve have established themselves.
These are characterised by being very
efficient in the typical solar thermal systems
operating ranges (comparatively low flow
rates with high pressure drop). Increasingly,
these solar circuit pumps are also offered
as high efficiency pumps with low power
consumption, which improves the overall
efficiency of the solar thermal system.
Fig. B.3.1–7 Pre-assembled solar circuit assembly
Pump sizing
The selection of the pump is made according
to the conventional process with reference
to the curve, subject to the flow rate
and pressure drop of the entire system
being known.
If controllers for variable flow rates are
used (matched flow operation), this has no
influence on the pump selection – these must
be designed for maximum output. When there
is low insolation, the speed controllers can
reduce the power consumption of the pump
and thereby reduce the speed (not increase!).
Fig. B.3.1–8
Solar-Divicon scheme
Option: pre-cooling
vessel/DEV connection
Safety assembly
and DEV connection
Shut-off valve
Non-return valve
Solar circuit pump
Flow meter
The Solar-Divicon comprises not only a solar circuit
pump, but also all components required for operating the
primary circuit.
Fig. B.3.1–9
Solar-Divicon scheme, 2-line system
The Solar-Divicon can be extended
by an additional solar pump line for
systems with a second pump circuit
or a bypass circuit.
A pump is already part of the pre-assembled
Viessmann solar circuit assemblies (SolarDivicon). It is suitable for operation with the
heat transfer medium from Viessmann.
Solar pump line
Safety assembly
Expansion vessel (DEV)
Drip container
types (Fig. B.3.1–10) enable the hydraulic
engineering of the system to be completed.
For conventional applications in detached
houses, the Solar-Divicon PS 10 is generally
adequate; it is also part of the pre-assembled
Viessmann solar packs.
The Solar-Divicon comprises all components
that are essential for the system operation and
is offered in two output sizes (PS10 and PS20).
The Solar-Divicon and
the solar pump line are
unsuitable for direct
contact with swimming
pool water.
Systems with a second pump circuit or with
a bypass circuit do not require an additional
complete Solar-Divicon, only an additional
solar pump line. This is also available in two
output sizes (P10 and P20).
Always install the SolarDivicon at a lower point
than the collectors to
prevent steam from
entering the expansion
vessel in the event
of stagnation.
The curves for the respective Solar-Divicon
Fig. B.3.1–10 Curves of the Solar-Divicon types
Type PS 10 or P 10
Type PS 20 or Type P 20
Head in m
Head in m
Pump rate in m3/h
Pump rate in m3/h
Pump rate in l/min
Pump rate in l/min
A Pressure drop curve of Solar-Divicon or solar pump line
B Residual head
I - III Pump output stages
The Solar-Divicon as well as the
solar pump line are offered in two
rating categories.
B.3 Primary circuit
B.3.1.4 Flow meter
The flow meter indicates the flow rate and
serves – together with two thermometers –
to control the system function. Both are
integrated into the Solar-Divicon.
In single array systems, a flow meter is built
into the system return. In the past, this flow
meter was frequently combined with an
adjusting valve with which the system flow
rate could be adjusted. This is no longer
routinely done, since reducing the flow rate
via a hydraulic butterfly valve would consume
a disproportional amount of auxiliary energy
(pump current).
Fig. B.3.1–11
Flow meter
Any marginal exceeding or shortfall of the
flow rate has hardly any influence on the
yields in single array systems. It is enough
if the required flow rate in the system is
achieved approximately via the stage settings
of the pump. This achieves a higher total
energy statement for the system.
Conventional flow meters are equipped with
a transparent glass or plastic pipe with a scale
on which a spring-loaded ring or something
similar indicates the current throughput.
This component as an inline version is
comparatively resistant to temperature, which
Fig. B.3.1–12
Reading a flow meter
The flow meter as inline version
(left) is part of the Solar-Divicon.
The bypass version is used near
the collector to balance several
array sections.
is why it is installed in the system return
where it is safe from steam. If this part is
destroyed by excessively high temperatures,
heat transfer medium will escape.
In systems comprising several array
sections, flow meters are installed near the
collector, in other words in areas where high
temperatures must be expected. Under
such circumstances, bypass versions are
used. If array sections must be balanced,
combinations with butterfly valves are
recommended for these layouts.
B.3.1.5 Non-return valve
B.3.2 Pipework
Particularly at night, the collector may become
colder than the water in the cylinder. There
is a risk that the solar cylinder could be
drained of heat through incorrect [reverse]
circulation. The greater the temperature
differential between the hot cylinder and the
cold collector, the higher the up-draught that
would lead to such undesirable circulation. A
reverse circulation can be recognised by the
heating of the collector without insolation.
Like all other components, pipework too must
be made from temperature-resistant materials
that are suitable for media containing glycol.
For most system types, plastic is unsuitable
where low temperature ranges cannot be
reliably assured. Zinc-plated/galvanised pipes
are also unsuitable as the zinc layer reacts
chemically with the heat transfer medium,
making the medium unusable.
To prevent such reverse circulation, a nonreturn valve (gravity brake) is installed into the
solar circuit return. The differential pressure
for opening the valve is adjusted so that on
one side the thermal up-draught is insufficient
to open this valve and on the other side the
lowest possible use of auxiliary energy (pump
current) is required.
The valve is always installed in the flow
direction downstream of the pump and
upstream of the branch to the diaphragm
expansion vessel and the safety valve. The
non-return valve is already integrated into the
In case of unfavourable pipework runs – i.e.
long vertical sections without offsets – it may
still happen that the thermal up-draught opens
the valve. In this case, the installation of a
two-way valve is recommended. This valve
would be controlled in parallel with the solar
circuit pump and will only open if the pump
is running.
To prevent circulation inside the pipe at the
DHW connection of the cylinder, a sloping
pipe run or a thermosiphon loop in the
pipework near the cylinder connection is
normally adequate (see chapter B.2.2.4).
Considering a reasonable price/performance
ratio for the entire installed pipework, practical
installations have shown that copper pipe
up to DN 40 is the most favourable material.
Steel pipes are used for anything larger.
Relative to the operation and the system
yields, both materials are of equal value,
subject to the pipes being correctly insulated
and that longitudinal expansion is adequately
compensated for.
Pipe connections
Generally, copper lines in solar circuits are
hard soldered or joined by pressfittings.
Soft solder could be weakened, particularly
near the collectors, due to the maximum
temperatures that may occur there.
Graphitized gaskets are unsuitable for use
with glycol.
In case of connections with hemp seal, use a
pressure and temperature-resistant sealant.
Hemp connections should be used as little as
possible due to their comparatively high air
permeability and should never be used in the
immediate vicinity of collectors.
Metal connections or joints with double
O-rings, as used by Viessmann, are the
most suitable.
When using
pressfittings, ensure the
seal rings are suitable
(resistant to glycol and
temperature). Use only
seal rings approved by
the manufacturer.
Pipework fixings
The same rules apply to the design and
installation of the solar circuit lines as for other
pipework in the heating sector.
t Pipes must never be affixed to other lines
nor be used as support for other lines
or loads.
t Fixings must safeguard noise protection.
t The thermal expansion of pipework must
be taken into consideration.
Fig. B.3.2–1
The expansion coefficient of
Longitudinal expansion (mm)
copper pipe is 30% higher than
that of steel pipes.
Longitudinal expansion (copper pipe)
Length of pipe: 5 m
The final point deviates from the empirical
values established in the heating sector.
The wide maximum temperature spread in
the primary circuit of a solar thermal system
(– 25 °C to in excess of + 175 °C = > 200 K)
results in substantially greater longitudinal
expansion. One metre of copper pipe
expands – independent of the pipe diameter –
by approx. 1.7 mm at a temperature rise
of 100 K, i.e. at a solar circuit line allow for
at least twice that longitudinal expansion
(approx. 3.5 mm per metre).
In conventional heating installations, this
longitudinal expansion is significantly
less. Conventional measures for fixings,
expansion bends and compensators cannot
accommodate the significantly higher
temperature differentials and frequent load
changes in solar circuits. If conventional
experiences are transferred to the solar circuit,
tensions would result that would inevitably
lead to cracks in pipes, fittings or joints
resulting in leaks.
Temperature differential (K)
Fig. B.3.2–2 Compensation of longitudinal expansion
Compensation measures are
required for the longitudinal
expansion to be expected in
the pipework due to the wide
temperature differentials in the
primary circuit.
Fixed point
Fig. B.3.2–3 Damage through
longitudinal expansion.
Floating point
To calculate the compensation measures
required for pipe sections that may be subject
to steam loads, a maximum temperature of
200 °C is assumed; for all other pipe sections,
120 °C. If, for example, flexible pipes are used
to connect the collectors (corrugated stainless
steel pipes), then the expansion forces exert
no damaging influence on the connection
fittings. Also never ignore the load limits of
compensators or expansion joints. System
designers must inform the installer clearly
regarding these particular features.
Generally take the same measures for
compensation as in all other pipework. To
prevent damage, the collector connection
must be made either in the immediate
vicinity of a fixed point or with flexible
pipework material.
To minimise heat losses from the primary
circuit pipework, the pipes must be 100
percent insulated in accordance with the
requirements of the EnEV [Germany – or
local regulations] similar to heating circuit and
DHW lines. If insulating material is used that
deviates in its thermal conductivity from the
value stated in the EnEV, adapt the minimum
thickness of the thermal layer accordingly.
Generally, the thermal insulation material
must withstand the operating temperatures
to be expected and must be permanently
protected against the influence of moisture.
Otherwise the insulative properties would
deteriorate. Some insulation material that
can be subjected to high thermal loads,
such as mineral fibre, cannot provide
reliable protection against moisture through
condensation due to the frequent load
changes resulting in correspondingly wide
temperature differentials in the primary circuit.
Use of UV-resistant insulation materials would
only offer a partial solution as it would not
prevent bites by small animals. On the other
hand a cover (e.g. metal sheath) protecting
the insulation against being bitten by small
animals generally also provides an adequate
UV protection, making the selection of a UVresistant insulation material superfluous.
Fig. B.3.2–4
Insulation with brittle zone
Minor brittleness on the inside of
a closed-cell insulation hose with
suitability for high temperature can
be tolerated.
The conventionally used high temperature
versions of close-cell insulating hoses, on
the other hand, offer adequate protection
against moisture, but cannot be loaded with
temperatures higher than approx. 170 °C.
However, the connections at the collector
can be subjected to temperatures up to
200 °C (flat-plate collector); for vacuum
tube collectors these temperatures can be
substantially higher.
At temperatures in excess of 170 °C the
insulating material changes its structure
and becomes brittle, thereby changing its
insulative effect. However, the brittle zone is
limited to a few millimetres immediately at
the pipe, in other words most of the insulative
cross-section remains unaffected. This risk
of reduced insulation near the collector
connections is acceptable, as the overloads
occur only for short periods, and the possible
damage to the insulation represents no
further risk to other components.
It is particularly important to protect the
insulation of the primary circuit that is routed
in the open against pecking damage and
bites from small animals, as well as against
UV radiation. These problems are frequently
underestimated – with the consequence that
the pipe insulation in this area cannot achieve
anything like the 20 year service life intended.
Copper pipe
Brittle zone
Fig. B.3.2–5 Damage through small
animal bites.
Fig. B.3.2–6 Protection against
small animal bites and UV radiation.
B.3 Primary circuit
Fig. B.3.3–1 Air vent valve versions
Subject to installation location and requirements, there are
suitable components that ensure the correct ventilation of
the primary circuit.
Manual air vent valve
Quick-acting air vent valve
Air separator
B.3.3 Ventilation
Correct ventilation of the collector circuit is
a prerequisite for trouble-free and efficient
operation of the solar thermal system.
Air in the collector circuit generates noise
inside the solar circuit and puts the reliable
flow through the collectors or through
individual array sections at risk. In addition it
can lead to accelerated oxidisation of organic
heat transfer media, such as the commercially
available mixtures of water and glycol.
Install automatic air
vent valves together
with a shut-off valve,
if steam cannot be
reliably prevented in the
relevant pipe section.
Subject to the
maximum temperature
reached by the heat
transfer medium, the
deaeration may take up
to 6 months (e.g. the
winter months).
Air vent valves are used to vent air from the
collector circuit – namely these are those that
are manually opened and closed, or automatic
air vent valves. The latter are offered as
automatic quick-acting air vent valves or as
air separators. Heat transfer media must
be vented for longer than pure water, and
automatic air vent facilities are preferred in
solar thermal systems.
Every time a heating system is filled, air is
introduced into the collector circuit. This
is largely replaced when filling the circuit
with heat transfer medium. However, a
part of this is admixed into the liquid flow
in the form of small bubbles which are only
vented gradually over time. Another part is
dissolved in the heat transfer medium and
will only be released at higher temperatures.
This air collects in the collector circuit at the
highest point or forms air pockets in horizontal
sections of the pipework.
Larger amounts of air inside the collector
circuit can stop the transport of heat transfer
medium. Air collecting in the pump can cause
a risk of the pump bearings running hot,
causing the pump to be damaged.
To make venting the system easier, air vent
valves can be fitted, namely at the highest
point of the collector circuit and in positions
where air pockets might form.
In case of stagnation, the heat transfer
medium evaporates inside the collector, and
the steam bubble expands into a part of the
pipework. Therefore, the air vent valves at
the high points of the system – particularly
at the collectors – must be closed with a
shut-off valve after the filling procedure has
been completed.
Air vent valves in the roof area are not
required for straight-line pipe runs without
pronounced offsets. For operational venting,
a central air vent valve is installed into the
flow line in the heating room, i.e. in the flow
direction upstream of the heat exchanger (see
Fig. B.3.3–2). The installation location must be
free from steam.
Air vent valves must be carefully selected
and sized. Air takes longer to vent from
water:glycol mixtures than from pure water.
In summer, when the medium is very hot,
additional air will be expelled from the heat
transfer medium – this process is well-known
from heating systems in winter.
It is important to check with the
manufacturers of air separators whether the
separation capacity specified in the technical
documentation refers to mixtures of water
and glycol.
Fig. B.3.3–2
Systems with a greater static ceiling,
particularly systems with several array
sections, are particularly at risk from air locks.
In these cases, the utilisation of vacuum
deaerator systems is recommended. The
air is reliably removed from all system parts
through the saturation deficit of the medium.
Caution – system
high points at risk
from steam or roof
installations: only use
air stops with manual
air vent valves in
these locations.
To enable the air vent valve to be able to fulfil
its purpose inside the heating room – in other
words below the collector – the air bubbles
must be routed downwards together with
the heat transfer medium against gravity.
Consequently the pipework is sized so that
the flow velocity is at least 0.4 m/s. If the
medium flows more slowly, the air bubbles
are no longer reliably transported with
the medium.
In systems with a static pressure above
2.5 bar (building height > 25 m) it is hardly
possible inside the heating room to separate
the air bubbles released in the collector. To
make venting easier, install an air separator or
an air stop at a higher point. However, an air
stop requires regular manual venting following
the filling.
Central air vent valve in the flow
Air separator
2 Automatic air vent valve
B.3.4 Heat transfer medium
The heat transfer medium transports the
heat from the collector to the cylinder.
The medium is heated up in the absorber
pipework; it transfers the energy to the water
inside the cylinder via the indirect coils inside
the cylinder.
Water is the basis of most heat transfer
media – with a few exceptions in high
temperature applications. It is particularly
suitable for this purpose because of its high
thermal capacity.
To prevent the heat transfer medium from
freezing and to prevent it from causing
damage inside the collector or external
pipework, the water is mixed with antifreeze
(conventionally propylene glycol) – in central
Europe in a concentration of approx. 40%
by volume.
1.2-propylene glycol is a hardly flammable
liquid that is non-poisonous and biologically
degradable. Subject to EU criteria it is neither
subject to compulsory marking nor to special
transport regulations. Its boiling point is
188 °C; its specific gravity is 1.04 g/cm3.
B.3 Primary circuit
The heat transfer medium used by Viessmann
offers additional corrosion-protection which
has a beneficial effect on the service life of
the entire system.
Glycol is an organic product with conventional
wearing characteristics. The heat transfer
medium is therefore supplemented with an
ageing protection: an alkaline buffer ensures
that the pH value of the medium remains
stable within the alkaline range (> 7.0)
over the long term. This safeguards the
corrosion protection.
Heat transfer media that are only exposed to
low thermal loads can easily last for approx.
ten years. However, they must be tested
regularly for their glycol density and pH value
(see chapter E.1.4).
The heat transfer medium is subjected
to higher loads if the system stagnates
frequently. The glycol molecules "crack" at
temperatures from approx. 170 °C. They
can then link up with other molecules,
accelerating the formation of acids (risk
of corrosion).
At high temperatures, glycol becomes prone
to oxidation. If there is oxygen in the system,
the heat transfer medium will be damaged
and solid deposits may form. Scientific
research has clearly shown that leaky
systems with a permanent addition of oxygen
are significantly more troublesome than high
temperatures as a result of stagnation.
Fig. B.3.4–1 Severely degraded heat transfer medium
High temperatures and oxygen will
damage the heat transfer medium
and solid deposits will form.
For systems with expected long stagnation
times (e.g. when providing central heating
backup by solar energy), an annual check
of the heat transfer medium including the
reporting of the results is recommended (see
chapter E.1.4). Always identify these aspects
fully and precisely when writing invitations to
tender for maintenance work.
To achieve optimum operational reliability and
high overall efficiency, Viessmann systems
are designed for the use of propylene glycol
as a heat transfer medium.
Alternative heat transfer media, such
as thermal oil or liquid sodium, are still
in research or are unsuitable for the
temperatures that commonly occur in DHW
heating or central heating backup.
Fig. B.3.4–2 Viessmann heat transfer media
Tyfocor HTL
Tyfocor G-LS
Tyfocor LS
up to 2001
05/2003 to 2008
up to 04/2003; from 2008
may be mixed with
Tyfocor HTL
Tyfocor G-LS
Tyfocor LS
Permissible mixing
When topping up, make sure the heat transfer media can be mixed.
The Tyfocor heat transfer medium used by
Viessmann is available in different versions.
The differences are not concerned with the
base material propylene glycol, but with
the different additives used (inhibitors) for
protection against corrosion and ageing. The
colour identifies the respective type. When
topping-up an existing system, observe that
these media are not able to be mixed with
one another.
B.3.5 Stagnation and safety equipment
B.3.5.1 Stagnation in solar thermal
A solar collector always generates heat when
light strikes the absorber – independent
of any current demand. If heat cannot be
transferred any longer or is not sensible, the
system switches off and goes into stagnation.
When there is insolation, this results in a
rise in temperature inside the collector up to
the maximum temperature, where energy
yield and loss are in balance. Temperatures
inside the collectors then reach levels that
generally exceed the boiling point of the heat
transfer medium.
For example, it is important for the regulated
operation of a system for central heating
backup with solar energy to include the
expected stagnation phases in calculations.
The simulation program enables the
determination of the expected timing and
length of stagnation.
Inherent safety means the following:
t The system must not be damaged by
t The system must not represent any risk
during stagnation.
t Following stagnation, the system must
return to operation automatically.
t Collectors and connecting lines must be
designed for the temperatures expected
during stagnation.
During stagnation, higher temperatures and
pressures are reached in the solar thermal
system. Consequently, pressure maintaining
systems and safety equipment are sized in
accordance with this operating state.
However, faults and power failure can also
lead to a system stagnating, when no heat
is drawn from the collector. Such operating
conditions must always be taken into
consideration when designing a system, e.g.
the inherent safety of the system must always
be guaranteed.
you can check the times at which
stagnation must be expected.
Daily maximum temperatures in the collector (°C)
Fig. B.3.5–1
From the results of this simulation
Stagnation in solar thermal systems
Period of stagnation to be expected
B.3 Primary circuit
Collector characteristics during stagnation
Fig. B.3.5–2 Stagnation phases
The stagnation characteristics of solar thermal
systems has been the subject of intensive
research over the past few years. The
processes taking place inside the collector
during stagnation are now well-known and
split into five phases.
Phase 1
Stagnation begins when the solar
circuit pump is shut down.
Phase 1: Liquid expansion
During insolation, the medium no longer
circulates because the solar circuit pump has
been switched off. The heat transfer medium
volume expands and the system pressure
increases by approx. 1 bar, until the boiling
temperature has been reached.
Phase 2: Evaporation of the heat
transfer medium
At the boiling point, steam forms inside the
collector; the system pressure rises further by
approx. 1 bar. The medium temperature will
be approx. 140 °C.
Phase 3: Collector boils dry
For as long as there is heat transfer medium
inside the collector, steam will be produced.
During this process, the glycol:water mixture
increases in concentration, and the boiling
point rises. The system pressure continues to
rise and reaches its maximum; the medium is
heated to a temperature of up to 180 °C.
Phase 4: Superheating
The medium concentration results in
progressively less water being able to be
evaporated. Consequently, the boiling point
rises and consequently the temperature
inside the collector. As a result, the collector
output falls and the amount of steam in the
system drops off. The pressure drops and
the temperature in the collector reaches
the stagnation temperature. This condition
continues until the insolation is inadequate
for holding the collector at stagnation
Phase 5: Refilling the collector
When insolation reduces, the collector
temperature and system pressure fall. The
steam condenses and the heat transfer
medium is pushed into the collector. If liquid
meets overheated collector parts, minor
steam hammer can still occur.
T1 125 °C
90 °C
3.5 bar
Phase 2
After approx. 10 minutes the
collector reaches the boiling point
and produces steam
T1 140 °C
90 °C
4.5 bar
Phase 3
After a further 30 minutes the
steam has largely expanded.
T1 180 °C
90 °C
5.0 bar
Phase 4
The collector remains at the
stagnation temperature until the
insolation subsides.
T1 200 °C
80 °C
4.5 bar
Phase 5
As the insolation subsides, the
temperature drops and the steam
T1 130 °C
50 °C
3.5 bar
Favourable stagnation characteristics are
shown by collector arrays, if liquid pockets
are prevented that would have to evaporate in
phase 3. It is always the layout of the entire
array which is crucial rather than that of the
individual collector.
To be able to describe and design around the
processes in the collector during stagnation,
new terminology was introduced and defined
for solar thermal systems:
t The maximum steam volume (Vd)
denotes the liquid volume that can be
accepted by the diaphragm expansion
vessel during evaporation.
t The steam range (DR) denotes the
length of the pipe run that is affected by
steam during stagnation. The maximum
DR is dependent on the heat loss
characteristics of the pipe run, in other
words fundamentally from its insulation.
Conventional details refer to 100 percent
insulation strength.
t The steam production output (DPL)
is the output of the collector array that,
during stagnation, is transferred into
the pipework in the form of steam. The
maximum DPL is influenced by the
draining characteristics of the collectors
and the array.
Vitosol collectors can be allocated to
maximum steam output levels, subject
to their installation position and type of
connection. These are important for sizing the
pre-cooling vessel (VSG) and the diaphragm
expansion vessel (MAG).
Meander type absorbers, compared to
harp-shaped absorbers, have shown more
favourable characteristics, since the steam
generated in the upper collector range can
fully empty the meander pipe by pressure.
With flat-plate collectors that fully drain,
the influence of the angle of inclination on
the stagnation characteristics can hardly be
measured. However, the stagnation properties
of vacuum tube collectors can be significantly
improved by a favourable arrangement.
Characteristics of different collector arrays
A lower system pressure has proven to be
beneficial where stagnation characteristics are
concerned. It is therefore important to adjust
the system pressure to an optimum level:
1 bar positive pressure (during filling with a
heat transfer medium temperature of approx.
20 °C) at the collector is perfectly adequate.
The steam load of the entire system can be
reduced if stagnation phase 3 is as short
as possible or is avoided altogether. This is
always the case when in phase 2 the liquid
medium is fully pushed out of the collector, in
other words if it does not have to boil dry.
Fig. B.3.5–3
For vacuum tube
collectors Vitosol 300-T
(heat pipe), a DPL
of 100 W/m2 can
be expected,
independent of the
installation position.
Steam output levels of collectors or collector arrays
Flat-plate collector
Flat-plate collector
Tube collector, connecting
Tube collector,
without liquid pocket
with liquid pocket
chamber at the side
connecting chamber
at the top
Maximum 60 W/m2
Maximum 100 W/m2
Maximum 100 W/m2
Maximum 200 W/m2
Subject to collector type and
hydraulic connection, different
steam output levels can occur.
B.3 Primary circuit
B.3.5.2 Maintaining the pressure and
cooling line
Correct engineering, correct implementation
and maintaining the correct pressure are of
crucial relevance to the operational reliability
of a solar thermal system (see chapter E.1.1).
Experience over many years has shown that
the cause of one of the most frequent faults
lies in these areas.
The diaphragm expansion vessel has three
important functions:
t It holds the amount of liquid that is
required to make up the volume reduction
caused by the very low temperatures and
deaeration during operation.
t It accommodates the expansion of the
heat transfer medium caused by the rising
temperature in normal operation.
t It accommodates the volume expansion
caused by steam being generated during
the stagnation phases.
The first two functions of the diaphragm
expansion vessel are the same as those
in conventional heating systems and are
calculated in similar fashion. The third function
represents the actual engineering challenge
for solar thermal systems. During stagnation,
steam not only forms in the collector, but also
parts of the connection line are filled with
steam. The volume of steam that needs to
be taken into consideration when sizing the
diaphragm expansion vessel is also subject to
the installation position and the collector type.
Until now, this steam formation was included
in the sizing of the diaphragm expansion
vessel by adding lump-sums. This method
of calculation continues to be permissible
and existing systems will not need to be
converted or newly calculated.
However, the position-dependent steam
output levels have been extensively
researched. Building on that research, the
much more accurate method of calculation
is now introduced here. A more affordable
alternative to the current sizing of diaphragm
expansion vessels can result from this
particularly with larger systems. When
engineering the pressure maintaining facility,
it must first be established whether steam
can get into the diaphragm expansion
vessel or other fittings that are sensitive to
temperature during stagnation. Where that
is the case, a heat sink must be allowed for.
Only after this stipulation has been made
can the volume of the diaphragm expansion
vessel be determined.
Determining the reach of steam
The volume of steam occurring during
stagnation generates the largest expansion
volume. This comprises the contents of
the collector fully drained by steam (the
assumption is that there is no residual
liquid) and the steam volume that is found
in the pipework during stagnation phase 3
(see chapter B.3.5.1).
The length of pipework that holds steam
during stagnation is calculated from the
balance between the steam output level of
the collector array and the heat losses of
this pipework.
The steam output level of the overall array
is the product of the aperture area and
the specific steam output level in W/m2
(see Fig. B.3.5–3).
The losses from a solar circuit pipe made from
copper 100% insulated with commercially
available material are assumed on the basis of
the following practical values:
Size 12x1, 15x1 and 18x1:
Size 22x1 and 28x1.5:
25 W/m
30 W/m
The maximum steam range (DR) in metres is
calculated as follows:
DRmax =
DPLmax · Akoll
DRmax Maximum steam range in m
DPLmax Maximum steam output level in W/m²
Aperture area in m2
dissipation of the pipework
in W/m
For a sample system with two flat-plate collectors
and a solar circuit line made from 15x1 copper pipe,
the following applies:
DPLmax = 60 W/m2
A koll
= 4.66 m2
= 25 W/m
DRmax =
60 W/m2 · 4.66 m2
25 W/m
In other words, the steam is pushed up to
11.18 metres into the collector connection line.
Fig. B.3.5–4 Cooling line
If the steam range is shorter than the actual
pipework length in the solar circuit (VL and
RL) between the collector and the diaphragm
expansion vessel, then the steam cannot
reach the diaphragm expansion vessel during
stagnation. If the steam range is greater, allow
for a cooling line to protect the diaphragm
of the expansion vessel against thermal
overload. Steam condenses again in this
cooling line and reduces the liquefied heat
transfer medium to a temperature < 70 °C.
To protect the diaphragm expansion vessel against excess
Check valve
Cooling line
temperature, the heat transfer medium is cooled down in the
cooling line upstream of the diaphragm expansion vessel.
Where the
installation position
and consequently
also the stagnation
characteristics of
collectors are unknown,
maximum values for the
DPL (100 or 200 W/m2)
are assumed.
B.3 Primary circuit
Fig. B.3.5–5 Steam propagation
Left: the steam can propagate in
the flow and return; the diaphragm
expansion vessel is installed in the
return, together with the cooling line.
Right: the steam can propagate
only in the flow; the diaphragm
expansion vessel is installed in the
Check valve
Cooling line
flow, together with the cooling line.
DEV and cooling line in the return
DEV and cooling line in the flow
Determining the cooling line
expected, to locate the diaphragm expansion
vessel and cooling line in the flow. In that
case, the return will not be subject to steam
loads. On the other hand it will then also no
longer be available for dissipating energy.
Subject to where temperature-sensitive
components are installed, such as pumps, it
may be sensible, if frequent stagnations are
Fig. B.3.5–6 Sizing the cooling line
1' "('#(')
#!'! '%&
'&&#'"! (+ #
Subject to the DPL of the collector array and the heat dissipation of the pipework, a residual cooling capacity may result that must be delivered by the cooling line. Systems with
connection lines ≥ DN 20 can be calculated with an Excel worksheet (see information on page 95).
Determining the residual cooling capacity
Determining the heat sink
The difference between the steam output
level of the collector array and the heat
dissipation of the pipework to the point
where the diaphragm expansion vessel is
connected results in the required residual
cooling capacity. For this, the position of
the diaphragm expansion vessel and that of
the cooling line (heat sink) must be taken
into consideration, as the pipe run length
effectively available for heat dissipation
depends on this.
After the required cooling capacity has been
established, the type of heat sink required is
determined. In smaller systems, pre-cooling
vessels (VSG) are frequently used for this
purpose. For their cooling capacity up to
approx. 100 l capacity, see Fig. B.3.5-5.
ks =
(DPLmax · Akoll) – (
· Lrohr)
Cooling capacity of the cooling line
DPLmax Maximum steam output level in W/m2
Aperture area in m2
Heat dissipation of the pipework in W/m
Length of pipework
In addition to the pre-cooling vessel or in
their place, another type of heat sink may be
appropriate – in larger systems such a solution
may even be more economical.
Here, ribbed pipes or commercially available
radiators may be used as a heat sink. To
determine the output, the specified heating
output at flow and return of 75 °C / 65 °C
may be used, multiplied by a factor of
2 to take account of the significantly
higher temperature.
In some circumstances,
a contact protection may
need to be provided,
as the steam may
enter the cooling line
with temperatures up
to 140 °C when the
collector array stagnates.
The DPL of a flat-plate collector system of 10 m2 is
Fig. B.3.5–7 Cooling capacity of the VSG
600 W. The system is connected with 30 m copper
pipe DN 20. Accordingly, the steam has a range of
20 m (600 W / 30 W per m). Protection measures
are therefore not required.
the steam range too will double to 40 m; in other
words the steam can reach the diaphragm
expansion vessel. The required cooling capacity is
calculated as follows:
pre-cooling vessel is subject to
the volume.
Assuming the collector area doubles (20 m2), then
DPLmax = 60 W/m2
A koll
! " # $ = 20 m2
= 30 W/m
= 30 m
= (60 W · 20 m2) – (30 W/m · 30 m)
The cooling capacity
is 300 W.
For systems with a connection line up to
DN 20 (in other words a heat dissipation of
the solar circuit line of 25 W/m), Fig. B.3.5–6
enables a quick determination of the residual
cooling capacity.
The cooling capacity of the
Calculating the diaphragm
expansion vessel
When calculating the diaphragm expansion
vessel, add the contents of the heat sink V kk
to the liquid content of the system Va and the
pipework contents Vrohr.
To determine the steam volume in the
pipework Vdrohr, the contents of the pipework
between the collector and the heat sink (only
VL or VL + RL, subject to installation position)
and the contents of the heat sink are added.
B.3 Primary circuit
After determining the steam range, taking
account of the possibility of needing a heat
sink, the diaphragm expansion vessel can
then be calculated accurately. The required
volume is determined by the expansion of the
heat transfer medium in its liquid state, the
liquid stock and the expected steam volume
taking account of the static ceiling of the
system and the pre-charge pressure.
As a first step, the liquid content of the
system Va is calculated. It is the result of the
sum of the contents of all components in the
primary circuit.
Va = Vrohr + Vwt + Vkoll + Vfv
System volume in litres
Vrohr Pipework volume in litres (including
valves and fittings)
Vwt Heat exchanger volume in litres
Vkoll Collector volume in litres
Vfv Liquid stock in the diaphragm expansion
vessel in litres
The liquid stock amounts to 4% of the system
volume, but not less than 3 l.
Systems with 2 Vitosol 200-F (type SV) flat-plate
collectors, dual-mode DHW cylinder Vitocell 100-B
As a second step, the expansion volume Ve,
which is the result of the thermal expansion of
the heat transfer medium in its liquid state, is
Ve = n · ( t1 – t0) · Va
Expansion volume in litres
Expansion factor in 1/K
Upper mixture temperature in °C
Lower mixture temperature in °C
System volume in litres
As the lowest temperature, – 20 °C is
assumed, as the highest (in conventional
applications) 130 °C – this value is
simultaneously set at the controller as Tmax as
collector temperature. Should the temperature
increase beyond that, then the system shuts
down and enters stagnation.
At a temperature differential of 150 K the
expansion factor for Viessmann heat transfer
medium is B = 0.13.
Ve = B · Va
Ve Expansion volume in litres
B Expansion factor
Va System volume in litres
(300 l), 30 m solar circuit line made from 15x1
copper pipe:
The sample system results in the following:
Vrohr = 4 l
V wt = 10 l
Va = 20.66 l
V koll = 3.66 l
B = 0.13
V fv = 3 l (minimum)
Ve = 0.13 · 20.66 l
Va = 4 l + 10 l + 3.66 l + 3 l
The expansion volume is 2.69 l.
In other words, the system volume Va is 20.66 l.
Fig. B.3.5–8
Pipework contents
To determine the steam volume
in the pipework, the contents
per metre of pipe must be
taken into consideration.
Copper pipe
12x1 DN10 15x1 DN13 18x1 DN16 22x1 DN20 28x1.5 DN25 35x1.5 DN32 42x1.5 DN40
Contents l/m pipe
Stainless steel pipe DN 16
Contents l/m pipe
After determining the liquid stock Vfv and the
expansion volume Ve the total steam volume
Vd is calculated. It comprises the collector
content Vkoll and the contents of the pipework
subject to steam loads Vdrohr.
To determine the steam volume in the
pipework Vdrohr the length of the pipework
subject to steam loads is multiplied with the
contents per metre of pipework (see
Fig. B.3.5–8).
For the diaphragm expansion vessel, a
pressure factor is added that is determined
as follows:
Vdrohr = Content of pipework per metre · Ldrohr
Vdrohr Steam volume in the pipework in litres
Ldrohr Length of the pipework subject to
steam loads
pe + 1
Df =
pe – po
Pressure factor
Maximum system pressure at the safety
valve in bar, in other words 90% of the
response pressure of the safety valve
System pre-charge pressure in bar, in
other words 0.1 bar per 1 m static ceiling
plus 1 bar essential positive pressure at
the collector
For the sample system with a 6 bar safety valve the
static pressure should be 1.5 bar (15 metres static
ceiling); consequently the system pre-charge
For the sample system with 15x1 copper pipe the
pressure should be 2.5 bar.
following applies:
pe = 5.4 bar
= 0.133 l/m
= 11.18 m
Vdrohr = 0.133 l/m · 11.18 m
In other words, the steam volume Vdrohr is 1.487 l.
This enables the total steam volume Vd to
be determined.
Vd = Vkoll + Vdrohr (+ Vkk)
Total steam volume
Vkoll Collector volume
Vdrohr Steam volume in the pipework
in litres
Heat sink volume in litres
For the sample system, this means:
V koll
= 3.66 l
Vdrohr = 1.487 l
Vd = 3.66 l + 1.487 l (+ poss. V kk)
In other words, the total steam volume Vd is 5.147 l.
po = 2.5 bar
Df =
5.4 bar + 1
5.4 bar - 2.5 bar
The pressure factor Df is therefore 2.21.
For sizing the diaphragm expansion vessel,
the calculated total displaced volume plus
liquid stock is then multiplied with the
pressure factor:
Vmag = (Vd + Ve + Vfv) · Df
For the sample system, this means:
= 5.147 l
= 2.69 l
V fv
= 2.21
Vmag = (5.147 l + 2.69 l + 3 l) · 2.21
The minimum volume Vmag of the diaphragm
expansion vessel is 23.9 l.
As a pressure maintaining station that
automatically maintains the pressure on the
gas side, Df = 1 is assumed.
The entire calculation
process for sizing the
diaphragm expansion
vessel and any residual
cooling capacity that
may be required is
available as an Excel
worksheet at
B.3 Primary circuit
For advanced high
performance collectors
it is not sensible to
prevent the evaporation
of the heat transfer
medium by introducing a
higher pressure stage.
Viessmann solar thermal
systems are as standard
operated with a 6 bar
safety valve. Such a
valve is already part
of the pre-assembled
Viessmann SolarDivicon. It is approved
for operation in glycol
circuits and up to a
temperature of 120 °C.
B.3.5.3 Safety valve
B.3.5.4 Collecting container
The safety valve in the solar circuit must drain
heat transfer medium from the system when
the selected maximum system pressure
is exceeded. This maximum pressure is
determined by the component with the
lowest pressure stage.
The heat transfer media used by Viessmann
are non-toxic and biologically degradable.
Nevertheless, a collecting container should be
placed at the safety valve blow-off line. The
collecting container must be sized so that the
heat transfer medium in the entire system can
be collected.
Size the safety valve in accordance with
EN 12976 and 12977, in other words it must
be appropriate for the heat output of the
collector or collector assembly and must be
able to transfer their maximum output (optical
output H0 · 1 000 W/m2) (see Fig. B.3.5-9).
Use only safety valves sized for max. 6 bar
and 120 °C, which bear the marking "S"
(solar) as part of the product identification.
These safety valves cannot be used directly
at the heat source (the collector) but must
be installed in the solar thermal system
return in a flow direction downstream of the
check valve. Ensure that at this point, no
temperatures > 120 °C will occur.
Generally, a collecting container is provided
on site for larger solar thermal systems. It
is designed to store the medium without
pressure; the preferred material is stainless
sheet steel. Ferrous sheet steel would
corrode, making it unsuitable for collecting
and storing heat transfer media containing
glycol. The same applies to zinc-plated/
galvanised sheet steel.
Fig. B.3.5–9
Safety valve
The size of the safety valve is
determined by the size of the
collector array to be protected.
In small systems, the flask in which the readymixed medium is supplied, frequently acts as
the collecting container. It should be observed
that any expelled heat transfer medium can
reach temperatures that can match or even
exceed the melting point of conventional
PP containers (approx. 130 °C). From 70 °C
the container noticeably begins to lose
stability. The pressure drop can also mean
the medium is expelled as steam. To protect
the container, it should contain an amount of
liquid amounting to at least 10 percent of the
system volume. Although with this solution
it cannot be guaranteed that the cylinder is
not destroyed and medium can escape, it is
acceptable because of its low level of risk.
Aperture area
Valve size
(size of the inlet
up to 40
up to 80
up to 160
Fig. B.3.5–10 Collecting container
In larger solar thermal systems, stainless steel containers
with a cover are used. We recommend labelling the
container accordingly.
The container is covered to prevent
contamination (splashing) when the safety
valve opens.
A valve should be provided near the bottom
of the container to enable it to be flushed
and filled easily. The valve must be of
adequate size; a simple fill & drain valve
would be inadequate.
C System selection and sizing
The selection of a suitable system forms the basis for planning solar thermal systems.
Apart from the customer-specific heat demand, structural characteristics of the
building are also taken into account into the process.
This chapter initially highlights the main
options available when designing a
collector array. It will detail the different
hydraulic requirements and show how the
installation work can be reduced through
optimum planning.
opportunities for solar thermal systems and
combinations with renewables are introduced.
Finally, the basic characteristics of
the engineering software ESOP are
explained and the main steps for system
simulation illustrated.
For sizing the additional components, different
systems are introduced and explained,
together with their specific requirements.
On that basis, the major engineering steps are
highlighted and illustrated with sample system
schemes. In addition, alternative application
100 C.1
Designing/engineering the collector array
101 C.1.1 Layout of single array systems
102 C.1.2 Layout of multi-array systems
105 C.1.3 Collector arrays with different orientation
106 C.2
Sizing a system for solar DHW heating
Sizing a system for solar central heating backup
Utilisation profiles in commercial applications
Swimming pool water heating
Cooling with solar backup
High temperature applications
136 C.3
Combination with renewables
137 C.3.1 Solar thermal systems in combination with biomass boilers
138 C.3.2 Solar thermal systems in combination with heat pumps
140 C.4
System simulation with ESOP
C.1 Designing/engineering the collector array
Designing the collector array
High output can be provided in a relatively small space by boiler systems and heat
pumps. That is not possible with solar thermal systems. Solar thermal systems offer
a comparatively low output density; increasing the output therefore always means a
corresponding increase in the size of the collector area.
Fig. C.1–1
Area terms
A Single collector
B Collector assembly/
partial array
C Collector array
If the output is to be doubled, also double
the collector area. Collectors cannot be built
in any size, since the installation options,
installation area and static set natural limits.
Consequently, large solar thermal systems are
composed of many individual collectors linked
together. This requires careful planning of the
collector hydraulics.
The advanced connection technology of
Viessmann collectors enables a flexible
response to the most diverse requirements
made of a collector array, resulting from the
required size and the preconditions on the roof.
C.1.1 Layout of single array systems
Fig. C.1.1–1
Single array system
In single array systems, the collector
assembly is connected directly with one
return and one flow, respectively.
Within the collector assembly there are
various options for linking the collectors.
Vitosol flat-plate collectors can be assembled
into a single array comprised of up to 12
individual collectors. These can be connected
on alternate sides or on one side only.
The Vitosol 200-T vacuum tube collectors
can be combined into arrays comprised of up
to 15 m2. These can also be connected on
alternate sides or on one side only. The top
pipe inside the collector is empty and is not
connected with the tubes. It is used for the
single-sided connection (see Fig. C.1.1–3).
Single array system: collector assembly = collector array
Fig. C.1.1–2
Connection versions in the collector array (flat-plate collector)
The Vitosol 300-T vacuum tube collectors
can be combined into arrays comprised of up
to 15 m2. This type of collector can only be
connected on one side.
The flow rate in litres/(h·m2) described in
chapter B.3.1 must be maintained for all
collector types.
With single-sided connection, the
Vitosol 300-T vacuum tube collectors
achieve a pressure drop of 220 mbar in an
array of 15 m2.
Fig. C.1.1–3
Connection versions in the collector array (Vitosol 200-T)
≤ 15 m2
Connection on alternate sides
≤ 15 m2
Single-sided connection to the left
(preferred option)
Fig. C.1.1–4
Connection versions in the collector array (Vitosol 300-T)
≤ 15 m2 – connection to the left
≤ 15 m2 – connection to the right
C.1 Designing/engineering the collector array
Fig. C.1.2–1 Multi-array systems (identical partial arrays)
If the partial arrays of a multi-array system are of the same
size, balancing valves are not necessary when implementing
the pipework in accordance with the Tichelmann principle.
2 levels of partial arrays linked in parallel
C.1.2 Layout of multi-array systems
In pipework according
to the Tichelmann
principle, the pipework
between the collector
array and the cylinder
system is routed so that
the total of the flow and
return pipe lengths is
roughly the same for
each collector.
The collector assemblies described in C.1.1
can, as partial arrays, be assembled into multiarray systems.
This is best achieved when all partial arrays
(collector assemblies) are of the same size,
are linked in the same way and consequently
have the same pressure drop. In other words,
when no balancing valves need to be used.
The partial arrays are linked in parallel; the
connection pipework follows the Tichelmann
principle. The number of collectors must
always be taken into consideration during
engineering to ensure this safe layout. If sizing
the systems should result in 17 collectors, for
example, then the number is reduced to 16
to achieve two partial arrays of the same size
with eight collectors each.
If these partial arrays have to be split again,
due to the connection situation, perhaps
because of areas that are physically far apart,
then two levels of parallel linkage will be
created. The pressure drop should be approx.
100 mbar to safeguard the safe flow through
all partial arrays. If the partial arrays have an
identical pressure drop of this magnitude,
balancing valves will not be required when the
connection follows the Tichelmann principle.
Multi-array systems with unequal partial
arrays (regarding size, shading or pressure
drop) must be balanced. These valves
are installed close to each other, where
possible immediately at the tee. This makes
balancing easier as they can all be observed
Arranging the balancing
valves one behind the
other in flow direction
has not proven to be
Fig. C.1.2–2
Multi-array system (unequal partial arrays)
Balancing unequal partial arrays
Partial flow 1
Balancing valves are used to balance out partial arrays of
Partial flow 2
unequal size to safeguard an even flow through all collectors.
Fig. C.1.2–3
Connection of partial arrays
Even if, in multi-array systems with different
partial fields, the upper array for example
is the same size as the total of both lower
arrays, the pressure drop would still be
different. The partial arrays indicate different
characteristics and must therefore be
balanced (see Fig. C.1.2–2 and Fig. C.1.2–3).
Check all available options to optimise
the collector array hydraulics. Sometimes
favourable circuits can be developed that
would allow the balancing to be omitted.
For multi-array systems with different partial
fields (see Fig. C.1.2–3) there is an alternative,
that guarantees a reliable flow through the
array without balancing valves: both lower
arrays are combined and linked in parallel with
the upper partial array (see Fig. C.1.2–4).
Installation care
Apart from careful planning, the quality of
installation is also highly important. Hydraulic
circuits in large collector arrays are sensitive.
The careless use of tees, elbows or bends
in collector array pipework can put at risk
the clean flow through partial arrays linked
according to the Tichelmann principle.
Even little differences in pressure drop can
lead to an uneven flow through the collector
assemblies or partial arrays.
Balancing unequal partial arrays
Partial flow 1
Partial flow 2
Partial flow 3
For uneven partial arrays, the flow must be balanced in every partial array.
Fig. C.1.2–4
Connection of partial arrays (versions)
Partial arrays linked in parallel
Balancing valves are not required when the hydraulic system has been optimised.
Fig. C.1.2–5
Hydraulic detail
Minor variances in connection
pipework can result in an uneven
flow through partial arrays. The
consequences are loss of output
and an increased risk of stagnation.
C.1 Designing/engineering the collector array
Fig. C.1.2–6
Flow velocity in the partial arrays
Sizing the connection piepwork
To safeguard the required flow
velocity, the internal diameters
of the connecting pipework must
be sized to the specific flow rate
through the partial arrays.
Pipework and fittings between the
partial arrays
To safeguard reliable venting, the pipework
within the partial arrays is sized, just like the
main line, to a flow velocity of between 0.4
and 0.7 m/s.
Multi-array systems require one air vent
facility per partial array for commissioning.
This does not require an automatic air vent
valve (quick-acting air vent valve); a manual
valve is quite sufficient. For this, observe the
temperature resistance.
Looking only at the
main line of the system
(for example with
thermometers in the
heating room) enables
no conclusions to
be drawn regarding
the correct system
function as the flow
temperatures of the
partial arrays have
already become mixed
at this point. It is not
possible to recognise
whether the entire
partial array might be
receiving less flow.
For commissioning and maintenance work,
the partial arrays must be able to be isolated.
If the collector array, or parts thereof, can be
fully isolated through shut-off valves and can
therefore be separated from safety equipment
(safety valve and diaphragm expansion
vessel), valves must then be protected against
incorrect operation (removable or sealable
valves). The ability to isolate partial arrays
also requires a facility to drain the relevant
partial array.
Testing/adjusting the system during
commissioning and the regular inspection
of the collector array will be made easier if
a sensor well is fitted into the flow of each
partial array. For Viessmann collectors,
this is available as an accessory for the
collector connections.
The sensor well enables the capture of the
medium temperature in the flow of each
partial array whilst in operation. As the return
temperature is the same for all partial arrays,
the possibly varying flow temperatures
allow conclusions to be drawn regarding the
flow through the partial arrays. VDI 6002
part 1 recommends that a deviation of up
to 10 percent between the arrays should be
permitted. The results of adjustments and
maintenance must be recorded.
For permanent monitoring, the individual
partial arrays can also be equipped with
permanently fitted sensors.
Fig. C.1.3–1
Yield and orientation
The influence of different orientations
Influence of the alignment on the daily insolation progress (surface pitched at 45°)
of partial arrays is so minor that in small
systems it is acceptable.
Insolation (kWh/m2 · h)
Facing south
Facing south-west
Facing west
8:00 10:00 11:00 12:00 14:00 16:00 18:00 20:00
C.1.3 Collector arrays with different
The building may dictate that collector arrays
are installed with different orientation. In
that case it must be decided whether the
system is operated as a whole or in separate
parts (with individual pumps or a completely
separate solar circuit). For a review, the
insolation processes are assessed on collector
surfaces with different orientation.
Fig. C.1.3–1 shows daily insolation progress in
hourly resolution on a surface pitched at 45°.
It can be seen that these processes are very
close together.
The lower the angle of inclination, the closer
the processes are (see chapter A.1).
Particularly in smaller systems it is
recommended, because of the higher
operational reliability and the lower installation
costs, to operate arrays that are not split up,
subject to them not varying by more than 90°
from each other. The low heat losses, due to
the collectors not receiving insolation even
though they receive a flow, are acceptable
compared to the advantages such systems
offer. When using vacuum tube collectors, the
losses can hardly be measured, permitting
deviations up to 180°. Use a radiation sensor
for the control; this should be located centrally
between both arrays.
Something similar applies to collector arrays
with different inclination. For example, if one
partial array is fitted to a wall and another
partial array is mounted on the roof, these
could also be operated together.
For arrays with different orientation and
different inclination, the yield progression
of both partial arrays must be calculated
separately with a simulation program. It can
only be determined on this basis how the
system should be operated. Viessmann would
be happy to assist in the planning.
C.2 Sizing
Security of supply
Optimised with solar
Fig. C.2–1 Solar thermal systems
operate in dual-mode. For this,
the conventional part is ideally
supported by solar technology.
If the principle function of the components of a solar thermal system are known, then
these components can be sized. The following sections explain the rules and practical
experiences that apply to this sizing.
The Viessmann
technical guides include
examples of complete
hydraulic schemes with
circuit diagrams for
these system types.
With every facility that is designed to provide
a service, the planning of a solar thermal
system initially also requires the definition
of design goals. Solar thermal systems are
almost always part of a dual-mode system.
As a result, the design goals essentially refer
to the intended solar coverage, in other words
to the required ratio between solar energy
and conventional energy relative to the total
energy demand.
The reference variables for the solar coverage
are always the heat amount (not the output)
that is provided by the respective heat
source during the period under consideration,
generally one year.
The following engineering/design information
refers exclusively to the sizing of solar
thermal system components. Under our
climatic conditions, a solar thermal system
cannot, on its own, provide security of
supply. Consequently, conventional system
components are sized independently of the
solar thermal system.
However, the interaction between the
individual heat sources is of elementary
relevance for the highest possible efficiency
of the overall system and therefore for the
best possible energy savings.
C.2.1 Sizing a system for solar
DHW heating
Calculating the DHW consumption
For calculating the demand and consumption,
we have to differentiate between the
maximum demand of a point of use and the
design consumption.
t The maximum demand of a point of use
forms the calculation base to safeguard
the supply. It is the engineering variable for
the DHW cylinder and for calculating the
reheat output of the boiler (to DIN 4708).
t The design consumption forms the
basis for the ideal utilisation of the solar
thermal system. The design consumption
describes the average expected
consumption during the summer months
and it is the engineering variable for sizing
the solar thermal system.
The maximum demand determined in
accordance with DIN 4708 is generally higher
by a factor of 2 than the actual demand.
Where possible, the consumption should
be measured over a longer period of time
to aid the system engineering. However, for
practical reasons this is not always possible.
If no accurate details can be determined for
the point of use, the consumption will be
estimated as follows.
In a detached house the average consumption
per head is higher than in apartment buildings.
The consumption is assumed to be 30 l per
person at 60 °C for the following design
considerations. For apartment buildings, the
recommended value according to VDI 6002
part 1 is 22 l per person at 60 °C.
C.2.1.1 Solar thermal systems for
DHW heating with high coverage
(detached houses and two-family
The design aim for DHW heating in a
detached house and in two-family homes
is generally approx. 60% solar coverage.
Mathematically speaking, this results in
full coverage during summer. Excess heat
levels that cannot be utilised stay within
acceptable limits; the user notices the solar
heat significantly and can manage for longer
periods without conventional reheating. For
reasons of system technology and economy,
a significantly higher coverage would not be
sensible in detached houses.
Subject to location, in Germany there are
on average 3 to 4 hours sunshine each day
during the summer months. If these hours
of insolation could be relied upon every day,
then consumption and generation could easily
be used to size the components. However,
for practical reasons this is not the case
in Germany.
To achieve a solar coverage of approx.
60 percent, practical experience has shown
that considering two days is appropriate –
twice the expected daily demand flows into
the solar cylinder. The collector system is
sized so that the total cylinder contents can be
heated on a single sunny day (approx. 5 hours
full sunshine) to at least 60 °C. This would
enable poor insolation the following day to be
bridged. This aspect also determines the ratio
between cylinder volume and collector area.
If solar energy is stored
in potable water,
then the cylinder or
cylinder areas are not
permanently heated by
the boiler system. This
makes pasteurisation
essential, as described
in the DVGW code of
practice W551. Observe
this when designing
a system.
C.2 Sizing
Fig. C.2.1–1
System with dual-mode cylinder (EFH)
Systems with DHW cylinder
These systems can be implemented with a
dual-mode cylinder (recommended for new
installations or full modernisation) or, as precylinder systems, with a mono-mode cylinder
for retrofitting.
The cylinder material is irrelevant to sizing.
For new installations, the use of a dual-mode DHW cylinder is recommended.
Fig. C.2.1–2
System with pre-cylinder (EFH)
DHW cylinder
In case of retrofitting, the solar cylinder can also be operated in mono-mode as pre-cylinder.
In central Europe, on a clear summer's day,
approx. 5 kWh insolation per m2 reference
area is available. To enable this amount of
energy to be stored, for flat-plate collectors
at least 50 l cylinder volume per m2 collector
area is allowed; for vacuum tube collectors at
least 70 l, subject to the system only heating
DHW. These details refer to the solar cylinder
or that part of the dual-mode cylinder that is
not heated by the second heat source. The
part connected to the second heat source
only becomes available for storing solar heat
if the collector system reaches a temperature
that is higher than the reheat temperature.
As a rule of thumb for dual-mode cylinders in
detached houses or two-family homes (high
coverage), per 100 l cylinder volume 1.5 m2
flat-plate collector or 1.0 m2 vacuum tube
collector can be assumed. Requirement: the
roof area intended for the installation has a
maximum deviation of 45° from due south and
an angle of inclination of between 25° and 55°.
Yield losses through unfavourable orientation
or inclination are compensated for by a slightly
increased collector area (see chapter B).
Further consumers
If a dishwasher is connected to the DHW
supply (generally not a problem; please
observe manufacturer's details), for modern
appliances this would mean an increase
in consumption of approx. 10 l (60 °C) per
cleaning cycle. If a dishwasher is connected to
the DHW supply via a pre-cooling vessel, on
average approx. 20 l (60 °C) per washing cycle
are assumed.
Fig. C.2.1–3
Sizing overview, DHW heating
These sizing details are based
Occupants DHW demand
60 °C in l
on the following assumptions:
Surface area
300 l
400 l
160 l
200 l
300 l
500 l
Influences on solar coverage
Consumption rates are allocated step by
step to the cylinder sizes and collector areas
specified in Fig. C.2.1–3; these steps are
a result of the component sizes. A solar
coverage of approx. 60 percent can therefore
only be a guide. The coverage is largely
dependent on the actual consumption – on
volume as well as on the draw-off profile. If
peak consumption occurs, for example, in the
afternoon, the same system would achieve
a higher coverage than if the peak draw-off
occurred in the early hours of the morning –
subject to reheating being appropriately
controlled in time.
Additional influences, such as location,
inclination and orientation of the collector area
do have an effect on the actual coverage and
simulation results for a small system, but not
on the selection of components.
500 l
consumption of 30 l per person
at 60 °C. If the consumption is
substantially higher, select in
2 x SV / 2 x SH
2 x SV / 2 x SH
1 x 3 m2
2 x SV / 2 x SH
1 x 3 m2
2 x SV / 2 x SH
2 x 2 m2
3 x SV / 3 x SH
2 x 2 m2
4 x SV / 4 x SH
2 x 3 m2
4 x SV / 4 x SH
2 x 3 m2
5 x SV / 5 x SH
4 x 2 m2
6 x SV / 6 x SH
3 x 3 m2
accordance with litres per day.
Reference system: Location Würzburg,
45° roof pitch with orientation due south,
61% solar coverage.
The following variations result from diverging
framework conditions:
Reference system
Collector inclination 30°
Collector inclination 60°
Orientation south-west
Solar coverage for DHW (%)
It can be seen that these effects are relatively minor.
Increasing or reducing the system would result in
incorrect sizing. 60 percent coverage is therefore a
guide, not a goal.
C.2 Sizing
Systems with buffer or combi cylinders
With solar thermal systems in detached
houses or two-family homes, combi
or heating water buffer cylinders are
conventionally only used in conjunction with
systems that are used for central heating
backup. Accordingly, these appliances are
designed and constructed for this kind of
operation (size, connection). However, the
option is also open to use these cylinders
exclusively for solar DHW heating.
Combi and heating water buffer cylinders
are only available from a certain size
upwards; they are therefore hardly suitable
for small systems designed to only provide
DHW heating.
Fig. C.2.1–4
System with heating water buffer cylinder and freshwater station (EFH)
System with heating water buffer cylinder and freshwater station
In principle, the same sizing rules apply
to buffer and combi cylinders as for DHW
cylinders. However, the use of buffer or combi
cylinders has some limits, since their draw-off
and reheating capacity is substantially lower
than that of DHW cylinders. The pressure
drop of internal indirect coils must also
be taken into consideration. It is therefore
impossible to allocate a system based on
the number of occupants. For this, a projectrelated check of the application options must
always be carried out. For further information
in this connection, see the technical
datasheets of combi cylinders and of the
freshwater stations.
Fig. C.2.1–5
System with combi cylinder
System with combi cylinder
When storing solar energy in heating water, the DHW can be
When storing solar energy in heating water, the DHW can be
heated externally, for example (freshwater station).
heated internally, for example (combi cylinder).
Fig. C.2.1–6
Consumption and generation (MFH)
Fig. C.2.1–7
Sizing nomograph (MFH)
Energy amount in kWh
Design consumption
Average monthly consumption
A Flat-plate
B Tube collector
Solar yield with correct sizing
100 90 80 70 60 50 40 30 20 10
1000 2000 3000 4000 5000 6000 7000
Collector area in m2
DHW consumption in l/d
A sizing nomograph can be used to
provide a first rough estimate of the
necessary collector area.
C.2.1.2 Solar thermal systems for
DHW heating with high collector
yield (apartment building)
In apartment buildings, systems are
frequently optimised towards maximum
yield – per m2 installed collector area, since
as much solar energy as possible should be
harvested. For this, the collector system must
be sized so that it will not stagnate, in other
words that it will not produce excess energy
that cannot be utilised.
The system is then designed to operate free
from excess for the design consumption
during the weak load phase in summer. It is
therefore sized so that the amount of energy
generated by insolation can be accepted by
the DHW system at all times.
Particularly with systems of this size,
consumption should be measured. Where
that is not possible, the consumption values
can be assumed in accordance with VDI 6002
part 1 as 22 l per person/day at 60 °C.
For the determined design demand, the
necessary amount of energy for heating
this amount of DHW from 10 °C to 60 °C
is calculated as well as the collector area
required to generate that amount of energy.
System with flat-plate collectors, 240 occupants,
measured consumption 25 l per person at 60 °C,
The value determined this way is described as
the utilisation level (daily consumption 60 °C,
in l/ m2 collector area).
i.e. 6 000 l per day.
For a system with high yields per m2, no value
lower than 60 l DHW consumption per m2
collector area should be applied. This is how
the collector area is determined.
determined on the basis of the collector efficiency.
For an average clear summer's day, the maximum
available solar energy per m2 collector area can be
This is for
flat-plate collectors approx. 3.4 kWh/(m2 · d),
and for vacuum tube collectors approx.
4.3 kWh/(m2 · d).
If the system is optimised towards its
utilisation level, the coverage to be achieved
will inevitably be curtailed – it is approx.
35 percent. An increase in the solar coverage
would result in excesses and reduce the
specific yield (for this, see chapter B.2).
This energy is sufficient with a m2 flat-plate collector
at an angle of inclination of 45° and orientation
towards due south to heat approx. 60 to 70 l water
to 60 °C (for vacuum tube collectors approx.
25 percent more). This results in 100 m2 collector
area for heating 6 000 l water.
C.2 Sizing
Fig. C.2.1–8
Adjusting the collector area
When sizing the collector area,
shape and size of the installation
area must also be taken into
Flat roof with 42 collectors
consideration. Observe limitations
Flat roof with 40 collectors
imposed by clearances to edges and
between rows (see chapter B.1).
A Observe edge and corner areas
B Observe clearance between rows
The mathematically ideal collector area must
now be adapted to the conditions on the roof.
Wherever possible, collector assemblies
of the same size should be designed
(see chapter C.1).
To achieve the mathematically ideal collector
area of 100 m2 shown in the example,
theoretically 42.9 Vitosol 200-F collectors
would need to be deployed. Therefore, initially
the collector area is adjusted appropriately.
Only the adjusted collector area is then adopted
into the sizing of the other components.
Systems with DHW cylinder
Dual-mode cylinders of the magnitude
required here are not available and would also
not be sensible. Generally there is a DHW
cylinder with reheating facility in the system.
A DHW cylinder with solar heating is installed
upstream of such a cylinder – in terms of
design it would be similar to that in a small
system (see Fig. C.2.1–2). As an alternative, in
larger systems this pre-cylinder can be heated
via an external heat exchanger.
Per m2 absorber area for a flat-plate collector,
The lower the solar coverage, the shorter the
harvested solar energy resides in the cylinder
system and the fewer the heat losses. A
typical consumption profile in apartment
buildings indicates a peak in the morning and
in the evening. With low coverage, the solar
yields at midday (maximum generation) would
only need to be stored for a few hours, as
they would be consumed in the evening or no
later than the following morning. This short
storage time also increases the available solar
yields as well as the advantageous utilisation
level of the collectors.
allow for a pre-cylinder volume of 50 l; for
vacuum tube collectors allow for 70 l.
Storing solar energy in DHW offers a simple
concept in larger systems too. Since the
contents of the pre-cylinder must be heated
to 60 °C daily, it must not contain any more
DHW than is actually consumed during
drawing in the evening and morning. It has
to be fully cooled down again in the morning
to be available to accept solar heat. An
appropriate time for pasteurisation is the
late afternoon. Advanced control units check
prior to heating whether the pre-cylinder has
already reached the required 60 °C during
the course of the day on account of the solar
thermal system. If that was the case then
the reheating by the boiler is suppressed.
Up to approx. 30 m2 collector area, DHW
cylinders offer slight price benefits compared
Fig. C.2.1–9
System with pre-cylinder (MFH)
The pre-cylinder as
DHW cylinder is not
permanently heated
by the boiler system.
Therefore there is a
need for pasteurisation.
Pre-cylinder with internal indirect coil
Pre-cylinder with external indirect coil
to systems with buffer cylinders introduced in
the following.
Sizing the plate-type heat exchanger in
the heating circuit
If the output of the internal indirect coil is
inadequate to pass the solar output to the
cylinder medium (see Fig. B.2.5–1), plate-type
heat exchangers are used to heat DHW or
heating water buffer cylinders externally.
higher pressure drop – this may result in
a smaller heat exchanger. The VDI 6002
recommends a pressure drop of up to
200 mbar. The design output of the collector
array is set at 600 W/m2 aperture area.
The plate-type heat exchanger is sized so
that the primary circuit return transfers
heat transfer medium to the collector that
is as cooled down as low as possible.
This temperature should be 5 K above
the temperature of the incoming cold
cylinder water.
For sizing the heat exchanger in the design
program, 20 °C from the heating water buffer
cylinder (secondary circuit return) and 25 °C
to the collector (primary circuit return) can be
applied. The respective material details must
be entered for the heat transfer medium on
the primary side; the secondary side only
holds pure water. If a maximum pressure
drop is to be entered, a value of 100 mbar is
recommended for the first calculation. The
values identified in Fig. C.2.1–10 with xx are
the result of the calculation. As a check, a
second calculation is made with a somewhat
Fig. C.2.1–10
Sizing the heat exchanger (heating)
Recommended input variables
when calculating plate-type
Heat exchanger
heating circuit
xx °C
Buffer cylinder
20 °C
heat exchangers.
xx °C
25 °C
C.2 Sizing
Fig. C.2.1–11
System with heating water buffer cylinder (MFH)
DHW cylinder
Heating water buffer cylinder
Heat from the collectors is
transferred to the heating water
buffer cylinder (3) via the plate-type
heat exchanger. The DHW inside
the pre-cylinder (2) is heated by
solar energy via a second platetype heat exchanger and brought
to target temperature by the boiler
Systems with buffer cylinder
inside the DHW cylinder (1).
From a collector area of approx. 30 m2 heating
water buffer cylinders are used to store the
solar heat. In this magnitude, systems with
a heating water buffer cylinder offer a price
benefit compared to systems with a DHW
cylinder. Although somewhat more complex
system components are used (external heat
exchanger, 2 additional pumps), heating
water buffer cylinders are significantly
more affordable because of their lower
pressure stage and the absence of a need for
corrosion protection.
Fig. C.2.1–12
All components shown in Fig. C.2.1–12
are described in chapter C.3 and are sized
accordingly. One special condition applies to
large systems: if the pipework of the primary
circuit on the roof is longer than that inside
the building, then the use of frost protection
measures for the external heat exchanger is
appropriate. Even at low outside temperatures
it may happen that the collector is hotter
than the cylinder on account of insolation,
but that the pipework still contains very cold
heat transfer medium. Should the system
start in this condition, frost damage may
occur at the heat exchanger. To prevent such
damage, a motorised valve and a thermostat
are fitted into the primary circuit, and the run
to the heat exchanger is only enabled at a
temperature > 5 °C.
Components in the heating circuit
The heat exchanger size is calculated
as described under "Systems with
DHW cylinders".
Frost protection
Motorised valve
To protect the plate-type heat exchanger against frost
damage on the secondary side through cooled-down heat
Secondary circuit
Primary circuit
transfer medium (primary side), the motorised valve will only
enable the run at a temperature > 5 °C.
Fig. C.2.1–13
Components in the discharge circuit
Heating water buffer cylinder
The solar energy from the heating water buffer cylinder is
transferred to the DHW in the pre-cylinder via a plate-type
heat exchanger. The mixing valve limits the temperature
inside the plate-type heat exchanger.
Heating water buffer cylinder
To keep losses as low as possible, the buffer
cylinder system should, where possible, be
centred on only a single cylinder. Where that
is impossible, several heating water buffer
cylinders are linked in series to safeguard the
reliable heating and discharge.
The pre-cylinder combined with the platetype heat exchanger transfers the solar heat
stored in the heating water buffer cylinder to
the DHW. The pre-cylinder should not be too
large as it must also be included in the daily
pasteurisation routine. In practical applications
a value between 10 and 20 percent of the
design consumption has proven to be a
good yardstick.
The plate-type heat exchanger for discharging
the heating water buffer cylinder to the precylinder is sized so that the return transfers
water to the heating water buffer cylinder
that is as cooled down as possible – the
temperature should be 5 K above the
temperature of the incoming cold water
of the pre-cylinder. The value identified
in Fig. C.2.1–14 with xx results from the
calculation. In any case, several comparison
calculations with different values for flow
rates are carried out, where the hourly amount
(peak during one hour) should not fall below
25 percent of the daily consumption.
To check the plausibility: the calculated output
lies approx. 50 percent above that of the heat
exchanger in the heating circuit, subject to
the collector area having been calculated in
accordance with the above rules (utilisation
level 60 l/m2 absorber area).
The calculated flow rates are adopted into the
sizing of the pumps in the discharge circuit.
Fig. C.2.1–14
Sizing the heat exchanger (discharge)
When sizing the plate-type heat
Heat exchanger
discharge circuit
xx °C
exchanger for heating the DHW,
the return temperature to the buffer
60 °C
cylinder should only be 5 K above
the cold water temperature of
the pre-cylinder.
15 °C
Buffer cylinder
20 °C
C.2 Sizing
Fig. C.2.1–15
The Viessmann
technical guides include
examples of complete
hydraulic schemes with
circuit diagrams for
these system types.
Selection table, heating and discharge circuit
Vitosol 200-F
Vitosol 200/300-T (3 m2)
Volume in l
Number of
Charge set
Number of
Charge set
Heating water
at 60 °C in l/d
buffer cylinder
1 250
1 375
1 500
1 200
1 625
1 500
1 750
1 500
1 875
1 500
2 000
1 800
2 125
1 800
2 250
1 800
2 375
1 800
2 500
1 800
2 750
2 400
3 000
3 000
3 250
3 000
3 500
3 000
3 750
3 000
4 000
3 900
4 250
3 900
4 500
3 900
4 750
3 900
5 000
3 900
5 625
5 000
6 250
5 000
6 875
6 000
7 500
6 000
8 125
6 000
1 000
8 750
8 000
1 000
9 375
8 000
1 000
10 000
9 000
1 000
10 625
9 000
1 000
11 250
9 000
1 500
11 875
11 000
1 500
12 500
11 000
1 500
* Calculated pipe dimension. For this there are no pre-assembled heating sets.
This table offers a quick overview for selecting a suitable heating and discharge assembly for larger collector arrays.
For systems with a collector area up to 50 m2,
Viessmann offers complete pre-assembled
packages for large systems. Heating and
discharge assemblies are available for even
larger systems. These are selected using the
table in Fig. C.2.1–15.
C.2.1.3 Further system aspects
Regulation of reheating
The above systems with DHW cylinders
include information regarding the possible
need for pasteurisation. These measures are
designed to kill off bacteria in potable water.
The appropriate regulations [for Germany] are
contained in the DVGW code of practice W 551.
In large systems, the outlet temperature
of the reheated DHW cylinder must be a
constant 60 °C; in other words, the reheating
must not be regulated down.
In the solar field, the information regarding
the preheating stage in large systems is
particularly relevant.
According to the DVGW code of practice
W 551, large systems are those that are
not installed in detached houses/twofamily homes, that hold a pipework volume
(excluding DHW circulation return) in excess
of 3 l and a cylinder capacity in excess of
400 l. This does not refer to the volume of
the preheat stage, but the content of the
entire DHW cylinder. These systems require a
consistent outlet temperature at the reheated
cylinder of 60 °C. The preheating stage must
be heated to that temperature, daily. The
pasteurisation must reach all cylinders linked
up in the system.
All other regulations also apply unchanged
to solar thermal systems – for example
the rule regarding DHW circulation or
deviating regulation in areas sensitive to
hygiene (hospitals).
The system installer is obliged to inform the
user with regard to the correct handling of
pasteurisation. This information should be
confirmed in writing and should form part of
the handover documentation.
In small systems – particularly with dualmode DHW cylinders in detached houses – a
demand-dependent control of reheating
can substantially increase the solar yield.
Reheating is set so that the boiler will not heat
up the cylinder during the day – when a solar
yield can be expected. In addition, reheating
suppression can also be applied. For this the
reheating temperature is set back at certain
times to be selected to enable the highest
possible solar yield. Viessmann solar control
units can be linked to the boiler control unit for
this purpose.
C.2 Sizing
Fig. C.2.1–16
Connection of DHW circulation and DHW mixer
DHW circulation return (summer)
!#! B
DHW circulation (winter)
#" C
DHW mixer inlet
DHW circulation return (incorrect)
DHW circulation pump
DHW mixer
Check valve
Connecting the DHW circulation
DHW mixer
To safeguard the troublefree function of
a solar thermal system it is particularly
important that cylinder areas with cold water
are available to receive the heat generated
by solar energy. In other words, these areas
must not under any circumstances be reached
by the DHW circulation return. There is a
fault if "out of habit" the DHW circulation
return is connected to the cold water inlet on
a dual-mode cylinder. To connect the DHW
circulation, use only the DHW circulation
connection of the cylinder (and not the cold
water inlet). Otherwise the whole cylinder
content will be brought to the temperature of
the DHW circulation return. This also applies
if the thermostatic control for the DHW
circulation pump is to be used.
Particularly in systems with high solar
coverage, temperatures > 60 °C can
occur in summer. As protection against
scalding we recommend the installation of
a thermostatically controlled mixing valve.
This is fitted between the DHW outlet and
the cold water supply of the cylinder. Install
a check valve into the cold water supply
line to the DHW mixer to prevent incorrect
DHW circulation.
Fig. C.2.1–17
High limit safety cut-out
To prevent steam from forming
in the DHW network, a high
limit safety cut-out is fitted
at the top of the cylinder.
High limit safety cut-out
Solar circuit pump
High limit safety cut-out
The solar controller limits the maximum
cylinder temperature and terminates the
heating process when this temperature
is reached by the solar thermal system.
A control unit fault can lead to the pump
continuing to run when there is a high level of
insolation, which would result in the cylinder
being overheated. This occurs when the
collector output is higher than the dissipation
capacity of the cylinder and the primary
circuit. This risk is particularly acute when a
significantly smaller cylinder volume than
50 l/m2 absorber area is available – in other
words when combining swimming pools and
DHW cylinders.
To prevent steam from forming in the DHW
network, a high limit safety cut-out is fitted
at the top of the cylinder. This interrupts the
power supply to the solar circuit pump when
95 °C is exceeded.
The intended savings in primary energy in
such projects can only be achieved by the
interaction between system technology and
architecture – buildings as a whole must
therefore be considered and planned. For
systems of this kind there are therefore no
"off the peg solutions" available. Viessmann is
happy to support engineers and the heating
trade with such projects.
In the following sections, solar central heating
backup in existing buildings and in new build
with short term storage in cylinders up to
2 000 l is explained.
Design principles
For solar DHW heating, the energy generation
that is seasonally very different, is balanced
with the demand so that, over the whole year,
it can be as balanced as possible.
For solar central heating backup, supply and
demand are opposites.
Experience has shown that it is frequently
difficult for prospective customers to
accurately assess the opportunities of a
system for central heating backup in existing
buildings. In consultations, such inaccurate
assessments should be corrected as early as
possible and realistic expectations of central
heating backup should be demonstrated.
Central heating demand for a
house (built after approx. 1984)
Central heating demand for a
low energy house
DHW demand for DHW heating
Solar energy yield with 5 m2
absorber area
(flat-plate collector)
Solar energy yield with 15 m2
absorber area
(flat-plate collector)
In new buildings, systems can be planned
right from the start, so that a solar thermal
system – storing energy all season or during
part of the season – covers most of their
heating energy demand. The prerequisite for
this is a building with very low consumption,
sufficient space for a cylinder of 10 000 l or
more, and a roof oriented towards the south.
Energy demand and solar yield
In Germany, far in excess of 50 percent of
all installed collector areas are used in solar
thermal systems that not only deliver DHW
heating, but also provide central heating
backup. The solar central heating backup is
already state of the art.
Fig. C.2.2–1
Energy demand (%)
C.2.2 Sizing a system for solar central
heating backup
One disadvantage of solar central
heating backup with short term storage
takes the form of the excess heat in
summer that cannot be utilised.
Fig. C.2.2–1 shows that:
t The solar thermal system does not replace
the conventional heat source, and that its
output should also not be reduced.
t The solar thermal system should therefore
be considered as a part of the overall
system where the highest efficiency is
relevant, including that of the conventional
heat source. The integration of renewables
improves the efficiency of the overall
system, but cannot replace it.
t Without seasonal storage, the opportunities
for solar central heating backup are limited.
If this figure was extended by additional
curves including the solar energy yield
for an absorber area of 30 m2 or 50 m2 it
would become clear that the additionally
yielded energy would largely add to the
summer excesses – the averages of
generation and demand would hardly
increase at all.
t Every system for solar central heating
backup stagnates over longer periods in
summer if no consumers for summer are
integrated into the system. The associated
steam formation requires very careful
system engineering and implementation.
C.2 Sizing
There are three practical approaches for sizing
a system for solar central heating backup.
1. Focus on solar coverage
Frequently, the reference variable "solar
coverage" arises from the customer's wish
or expectation. Consequently, it finds its way
into many advertising contexts. However,
for solar central heating backup, sizing to
a specific solar coverage without detailed
consideration of the building to be heated is
not professionally sound. The solar coverage
results from planning that is matched to the
building; as target variable it has little use for
existing buildings.
To prevent condensation
in colder rooms (e.g.
cellar) on hot days, a
temperature increase
by just a few Kelvin
would be sufficient. In
the average detached
house approx. 0.05 m2
collector area per m2
cellar area are adequate
at standard cellar
height. This already
takes into consideration
that the solar thermal
system delivers more
energy at that time
than is required for
DHW heating.
2. Focus on the living space to be heated
The second option is a sizing with regard
to the living space to be heated. However,
if you take into consideration the highly
variable heating energy demand of buildings,
it soon becomes clear that a blanket sizing
recommendation would need to be drawn
inside a rather wide frame. The step from
0.1 m2 to 0.2 m2 collector area per m2 heated
living space represents a factor 2 regarding
the system size – this effect makes a
verifiable determination of a specific system
size significantly more difficult. In addition,
the demand for DHW heating in summer
receives too little prominence in planning – no
fixed relationship between living space and
the number of occupants consuming DHW
is provided. A system that is purely sized in
accordance with the living space will have
different characteristics in a building occupied
by two people with 250 m2 living space than
a system in a small detached house occupied
by a family of 5.
3. Focus on the (gross) annual efficiency
As assessment variable, Viessmann uses
the (gross) annual efficiency of the entire
heating system. The German heating
industry has agreed on that variable.
Respective recommendations have been
included in the information sheets of the
Bundesindustrieverbandes Deutschland
Haus-, Energie- und Umwelttechnik e. V.
(BDH) which can be downloaded at
www.bdh-koeln.de. The BDH information
sheets describe the recognised state of
the art and so enable reliable planning and
implementation of the system.
The sizing of a solar central heating backup
system should always be based on the heat
demand in summer. It is a combination
of the heat demand for DHW heating and
other project-specific consumers that can
also be supplied by the system such as, for
example, a heating energy demand to prevent
condensation in cellars.
A suitable collector area is chosen for this
consumption in summer. The collector area
chosen by this route is then respectively
multiplied by a factor of 2 and 2.5 – the results
form the range within which the collector area
should be for solar central heating backup.
The precise determination is then made taking
into consideration the building conditions
and the planning of an operationally reliable
collector array. Should the calculations, for
instance, result in seven or eight collectors,
when the south-facing roof area is only
sufficient for seven collectors, then it would
not be sensible to fit an eighth collector onto
the garage roof.
For a detached house, 7 m2 collector area (flat-plate
collectors) is assessed for DHW heating; there is no
other demand in summer.
In other words, the collector area for solar central
heating backup should lie between 14 m2 and
17.5 m2. Seven flat-plate collectors of 2.33 m2
absorber area each, i.e. 16.3 m2 are chosen.
If a swimming pool is available that can accept
heat in summer, then it has no effect on the
system sizing if the otherwise unheated basin
is to be tempered a little by excess heat.
For combinations of solar central heating
backup with open air and internal swimming
pools, the temperature of which is to be
maintained at a certain temperature, see the
information in chapter C.2.4.
Fig. C.2.2–2
Sizing table, central heating backup (EFH)
Occupants DHW demand
60 °C in l
Buffer cylinder
capacity in l
It is generally unimportant for sizing the
cylinders whether the system is equipped
with a combi cylinder or with a heating
water buffer cylinder plus DHW cylinder.
The system can bridge several bad weather
days in summer, consequently for flat-plate
collectors, per m2 absorber area, a 50 l
cylinder capacity is the lower limit – the ideal
range lies between 50 l and 70 l. This range
lies between 70 l and 90 l per m2 absorber
area for vacuum tube collectors.
4 x SV / 4 x SH
2 x 3 m2
4 x SV / 4 x SH
2 x 3 m2
750 / 1 000
4 x SV / 4 x SH
2 x 3 m2
750 / 1 000
4 x SV / 4 x SH
4 x 2 m2
750 / 1 000
4 x SV / 4 x SH
4 x 2 m2
1 000
6 x SV / 6 x SH
3 x 3 m2
1 000
6 x SV / 6 x SH
3 x 3 m2
This table offers a quick overview
for selecting the components for
solar central heating backup.
System layout
When determining the layout of the overall
systems there are two options for storing the
harvested solar energy and making that heat
available to the heating circuit, i.e. heating a
buffer cylinder and return temperature raising.
In systems with buffer cylinders, the cylinder
contents are heated to the level of the flow
temperature either by the solar thermal
system or by the boiler. The heating circuit is
then supplied directly by the heating water
buffer cylinder.
Fig. C.2.2–3
In systems with a buffer, the heating
circuit is supplied by the buffer cylinder.
Surface area
If the opportunity exists to freely select the
collector inclination for systems providing
central heating backup (flat roof, freestanding),
select 60°. This slightly steeper pitch than in
DHW heating provides – apart from higher
yields during spring and autumn – smaller
excesses in summer, thereby relieving the
entire system.
If the system can only be installed parallel to
the roof at an inclination < 30°, solar central
heating backup with flat-plate collectors
would no longer be sensible. In that case, use
vacuum tube collectors (horizontal installation
with connection at the bottom), the tubes of
which can be individually oriented.
System with buffer
C.2 Sizing
Fig. C.2.2–4
System with return temperature raising
starts – it is alleged that this reduces heat
losses (by preventing cool-down losses in
standby). In this connection it should be
said that such boiler systems should not be
combined with a solar thermal system but
should, as a priority, be replaced.
With advanced heat sources, such arguments
will not hold. In modulating mode, these
systems generate only that amount of
energy that is necessary for reaching the
flow temperature. Heating the buffer cylinder
means offsetting the system limit. In principle
it increases the surface where conventionally
generated heat is lost – irrespective of the
quality of the cylinder insulation. In addition, it
increases the target temperature of the solar
thermal system, which automatically reduces
its efficiency. For that reason, Viessmann
recommends and prefers the return
temperature raising – subject to there being
no specific requirement for making another
system solution necessary (for example,
integration of a solid fuel boiler).
In systems with return temperature
raising, the heating circuit is
supplied by the boiler. The solar
energy is fed into the heating
circuit when the temperature of the
heating circuit return lies below the
cylinder temperature.
In a system with return temperature raising,
the water heated by solar energy is drawn
when the temperature inside the cylinder
is higher than the heating circuit return
temperature. The boiler starts if the flow
temperature cannot be reached.
Using only one cylinder has the advantage of
needing little space and simple pipework (the
solar thermal system is simply connected to
one cylinder). For this, the maximum draw-off
rates specified in the datasheets of the combi
cylinders must be taken into consideration.
With older boiler systems that are subject
to high losses, it is sometimes said that the
conventionally generated heat should be
brought into the buffer cylinder as quickly as
possible thereby avoiding frequent burner
Fig. C.2.2–5
System with additional mono-mode cylinder
The system with return temperature
raising can also be designed as a
2-cylinder system. This solution is
available in case of high draw-off
DHW cylinder
rates or if an existing cylinder should
Combi cylinder
be integrated.
In the case of high draw-off rates or if the
existing cylinder is to remain part of the
system, a combi cylinder can also be installed
upstream of a mono-mode cylinder that is
heated by the boiler.
but the maximum draw-off capacity of
the freshwater station(s) must be taken
into account.
In systems with separate cylinders, the
solar thermal system heats up several
cylinders separately; the system can be
scaled up as required. In large systems, the
dual-mode cylinder can be replaced by two
mono-mode cylinders.
As an alternative, a heating water buffer
cylinder can be used with a freshwater station
in place of a combi cylinder. Larger systems
can be achieved with this combination,
Fig. C.2.2–6
System with heating water buffer cylinder and freshwater station (EFH)
Larger systems can be realised
with one buffer cylinder and a
freshwater station.
Fig. C.2.2–7
System with separate cylinders (dual-mode)
In systems with separate cylinders,
DHW cylinder
solar heat is stored in the heating
Heating water buffer cylinder
water buffer cylinder as well as in
the DHW cylinder.
C.2 Sizing
Fig. C.2.2–8
System with separate cylinders (mono-mode)
DHW cylinder
Heating water buffer cylinder
In a system with separate cylinders,
the dual-mode DHW cylinder
(Fig. C.2.2–7) can be replaced by
a mono-mode pre-cylinder and a
mono-mode DHW cylinder.
Heating circuit requirements
One frequent misunderstanding is the
assumption that solar central heating backup
is only suitable for underfloor heating
systems. This assumption is incorrect. On
annual average, the yields with radiator
heating systems are only marginally lower.
The reason for this is the slightly higher target
temperature of the solar thermal system
that is always determined by the heating
circuit return.
When comparing the different heating
surfaces it must be considered that the solar
thermal system should essentially deliver
energy to the heating circuit in spring and
autumn. At those times, heating surfaces will
not be operating at their design temperatures
and the return can operate at a lower
temperature level.
However, the correct hydraulic balance of the
radiator circuits is important.
Solar thermal systems and
condensing boilers
Another misunderstanding is that solar
thermal systems cannot be combined with
condensing boilers. That is also not the case.
In actual fact the solar thermal system heats
the cold water (DHW or heating water) in
the system as a priority. If the boiler has to
take up the "rest" load, then the boiler no
longer operates – for example when heating
the DHW temperature from 50 °C (solar
preheated) to 60 °C (target temperature) –
in the condensing range. However, in this
temperature range the condensing boiler
could not do that anyway.
A similar example could be calculated for
solar central heating backup. Generally the
combination with a solar thermal system
has no influence on the efficiency or the
operational reliability of the boiler. In actual
fact the (gross) annual efficiency of the boiler
drops slightly, but that of the system as a
whole rises significantly. What matters are
the absolute energy savings.
Heating several cylinders
If more than one cylinder is heated by solar
energy, there are several possibilities for
designing the solar circuit.
Version with two pumps
With this version, every cylinder is supplied
by its own pump fitted into the solar circuit
return. These pumps are started alternately.
An operation where both pumps run in
parallel is, theoretically, also possible, but in
practical terms only sensible in exceptional
cases. It should be remembered that, as a
consequence, such an operation features
different flow rates in the primary circuit.
Selection criteria
Relative to the operational reliability of the
system and safe planning, both versions are
comparable. The solution with a three-way
valve may be more affordable, the twopump solution has a slightly lower power
consumption (lower pressure drop, no power
consumption for the valve). If more than
two cylinders are supplied, solutions with
pumps generally lead to clearer less complex
systems than several three-way valves in a
linked circuit.
System schemes that
are set up for one of
these two versions are
programmed into many
solar control units. In
Viessmann controllers,
two-pump versions
are pre-programmed.
Settings must be
changed accordingly if
alternative hydraulics
are required.
Version with a three-way valve
With this version, a solar circuit pump runs
to heat both cylinders; the flow is directed,
subject to demand, to the different cylinders
via a three-way valve. The three-way valve is
fitted into the return, as it is more protected
against high temperatures at this point.
Fig. C.2.2–9
Heating several cylinders
Two-pump option
Three-way valve
C.2 Sizing
C.2.3 Utilisation profiles in commercial
The previously calculated examples always refer
to DHW heating with solar backup and central
heating backup in residential applications.
Draw-off profiles and heating times in
commercial applications may vary substantially
from these, which must be taken in to
consideration when designing the solar thermal
system and sizing the individual components.
Detached house with two occupants, 150 l DHW
Doctor's surgery, 150 l DHW consumption (60 °C)
consumption (60 °C) per day
per day
Spread of the DHW consumption:
Spread of the DHW consumption:
Simulation of different cylinder sizes with 4.6 m2
Simulation of different cylinder sizes with 4.6 m2
absorber area:
absorber area:
Utilisation rate
Volume (l)
Additional energy
In the example of the detached house, the
consumption of working days is constant and
a little lower at the weekend. The simulation
(see chapter C.4) with 4.6 m2 absorber area
and different cylinder sizes highlights that
the solar coverage and utilisation rate of the
system no longer increases significantly from
a cylinder capacity of 300 l upwards; the
feasible energy savings have also reached
their maximum. The system is therefore
correctly sized with a 300 l cylinder.
Energy (kWh)
Energy (kWh)
There are no standard
values for systems with
commercial utilisation;
these therefore always
require careful projectspecific engineering.
Utilisation rate
Volume (l)
Additional energy
In the example of a doctor's surgery, the solar
coverage, utilisation rate and energy savings
increase significantly again from a cylinder
capacity of 300 l to 400 l, although the
absorber area and the respective daily
draw-off profile correspond to the example
of the detached house.
The larger cylinder capacity can make the
DHW heated at the weekend by the solar
thermal system available for the start of
the week.
It is therefore important when sizing the system
to consider not only the average amount of
DHW utilised but also its distribution.
C.2.4 Swimming pool water heating
Similar examples can be calculated for solar
central heating backup – a system with
commercial utilisation is different from one
in a residential building, since the heating
circuit temperature in most cases drops at
the weekend.
The ESOP engineering program from
Viessmann (see chapter C.4) offers the option
of creating project-specific draw-off profiles
for the system simulation.
Process heat with low temperature
Heat with low temperature in the case of
process heat is the temperature level that
can still be achieved with flat-plate collectors
or vacuum tube collectors with acceptable
efficiency levels (approx. 90 °C).
Many commercial processes, such as
washing and degreasing, are run with
relatively low temperatures. These processes
are suitable for a supply by solar thermal
systems, particularly when heat is drawn off
continuously. Sometimes very low cylinder
capacities are sufficient – these systems
therefore facilitate a highly favourable price
for heat.
Breweries and other businesses in the food
processing industry are already equipped with
solar thermal systems.
To provide water heating for outdoor pools,
only, unglazed collectors can be used, i.e.
simple absorber mats or hoses. Technically
speaking, these are not collectors. Absorber
mats or hoses are subject to a different test
procedure to EN 12975. The test results for
unglazed absorbers made from polymers can
therefore not be compared to those of glazed
metal absorbers. Furthermore, unglazed
absorbers are utilised in other areas.
These plastic absorbers offer good optical
efficiency, since the losses due to the glazing
are not applicable. However, the absence
of thermal insulation means they are hardly
protected against heat losses, which are
correspondingly high. For that reason,
they are used exclusively where there is a
low temperature differential towards the
ambience, i.e. very low ΔT.
The main area of application for unglazed
collectors is open air swimming pools without
additional connected consumers – here
insolation and heating demand for the pool
water occur simultaneously in summer.
Swimming pool water circulates directly
through the swimming pool absorbers.
These absorbers are generally positioned
horizontally, i.e. on level ground or on flat
roofs and secured to their support base with
straps. They can also be installed on slightly
pitched roofs. These absorbers are fully
drained in winter.
Simple absorber mats are unsuitable for a
combination of solar swimming pool water
heating and DHW heating or solar central
heating backup, which is why they will not be
given further consideration here.
C.2 Sizing
Open air pools without conventional
Outdoor swimming pools are mainly used
between May and September [in central
Europe]. Their energy demand depends on two
loss variables:
t Water loss through leakage and discharge
(that is the volume of water bathers
"carry" with them when leaving the pool) –
this amount of loss would need to be
replenished with cold water.
t Heat losses through the surface, the basin
wall and latent heat.
Poseidon, Hamburg
The following section explains how the heat
demand of swimming pools is included in
the design of combined systems (with glazed
Subject to the type of demand, swimming
pools are split into three categories. The
different rules for their integration into the
overall system can then be derived from the
t Open air pools without conventional
heating (swimming pools of
detached houses)
t Open air pools that are maintained at a
set temperature (public lidos, sometimes
swimming pools of detached houses)
t Indoor pools (pools that are maintained at
a constant set temperature for all year use,
sometimes in detached houses)
The minimum temperature which the basin
water should have at any time is described
as the set temperature. It is safeguarded by
the boiler system. With strong insolation, that
set temperature can certainly be exceeded in
open air pools.
The evaporation loss during periods of nonuse can be substantially reduced by means
of a cover – this will also reduce the energy
consumption. The largest energy input comes
directly from the sun, which shines onto
the pool surface. This way the pool water
obtains a "natural" base temperature – it
can be demonstrated as the average pool
temperature for the entire period of use.
A solar thermal system will not change
anything about this typical temperature
progression, but may raise the base
Fig. C.2.4–1
Water temperature in open air pools
Typical temperature curve for an unheated
outdoor swimming pool
Average pool temperature (°C)
Fig. C.2.4 Lido Schwimmverein
Location: Würzburg; pool surface area: 40 m2,
depth: 1.5 m; position: sheltered and covered at night
The temperature curve in unheated open air pools is the
result of insolation on the pool water surface.
temperature. The magnitude of the
temperature lift depends on the ratio between
the basin surface and the absorber area.
The diagram in Fig. C.2.4–2 highlights the
correlation between the surface/absorber area
ratio and the temperature increase. Due to
the comparatively low collector temperatures
and the utilisation period (summer), the
collector type itself exerts no influence on
these values.
The base value for the "natural" average pool
temperature in high summer is assumed to
be 20 °C. According to experience, a 3 K to
4 K temperature rise is adequate to achieve a
noticeably more pleasant pool temperature.
This is achieved by a collector area that is up
to half as large as the pool surface area.
Open air pools with set temperature
covered by conventional reheating
If the pool water is raised to and held at the
set temperature by a conventional heating
system, the operational characteristics of
the solar thermal system and the effects
on the pool temperature change hardly at
all. The solar thermal system increases the
set temperature compared to pools that are
not reheated.
The system is designed so that the
conventional reheating only operates during
the heat-up phase until the set temperature
has been reached. When the required
temperature has been reached, the solar
thermal system ensures that the required
temperature is being maintained.
For pools that are reheated, the necessary
collector area can be determined by switching
the boiler system off for 48 hours under
sunny conditions and accurately measuring
the drop in water temperature. For the sake
of reliability carry out this test twice. The
collector area is then determined similarly
to the process applied to indoor swimming
pools, as described in the following section.
Fig. C.2.4–2 Temperature increase in open air pools
Fig. C.2.4–3
Typical temperature curve for a heated outdoor swimming pool (conventional plus solar)
Average pool temperature (°C)
Sizing overview
Average temperature rise (K)
Open air pool with set temperature
Ratio – absorber area to pool surface area
Base temperature
Set temperature (conventional)
Solar energy
Season extension (conventional)
Location: Würzburg; pool surface area: 40 m2,
depth: 1.5 m; position: sheltered and covered at night
In open air pools with cover, sizing the absorber area up to
In open air pools with conventionally maintained set temperature the water temperature can be increased
50 percent of the pool water area is adequate.
through solar heat.
C.2 Sizing
Indoor swimming pools
Indoor swimming pools generally have a
higher target temperature than open-air
pools and are used throughout the year. If,
over the course of the year, a constant pool
temperature is required, indoor swimming
pools must be heated in dual-mode. To avoid
sizing errors, the energy demand of the pool
must be measured.
For this, suspend reheating for 48 hours and
determine the temperature at the beginning
and end of the test period. The daily energy
demand can then be calculated from the
temperature differential and the capacity of
the pool. For new projects, the heat demand
of the swimming pool must be calculated.
On average, a collector system used to heat
swimming pool water in central Europe produces
energy of 4.5 kWh/m2 absorber area on a clear
To size this combination, the collector area for
pool water heating is added to the collector
area required for DHW heating. The suitable
cylinder system is determined with reference
to the total collector area. Supplements for
central heating backup are not required.
System with indoor pool
The collector area is calculated similarly to
the process for open air pools (collector area
for pool water heating plus collector area for
DHW heating).
The pool accepts the solar energy provided all
the year round. Consequently, also connecting
the solar thermal system to a heating circuit
is only possible, if the same rules are applied
as are generally applicable to solar central
heating backup (see chapter C.2.2). The
area relative to the summer consumption is
therefore at least doubled. If this factor is
not adhered to, the solar thermal system will
exclusively heat the pool water in autumn,
winter and spring.
summer's day.
Pool surface: 36 m2
Average pool depth: 1.5 m
Pool content: 54 m 3
Temperature loss in 48 hours: 2 K
Energy demand per day:
54 m3 · 1 K · 1.16 (kWh/K·m3) = 62.6 kWh
Collector area:
62.6 kWh : 4.5 kWh/m2 = 13.9 m2
As a rough estimate (cost estimate), in
general an average temperature loss of 1 K
per day can be assumed. With an average
pool depth of 1.5 m, an energy demand of
approx. 1.74 kWh/(d · m2 pool surface area)
is required to maintain the set temperature.
Per 1 m2 pool surface area, approx. 0.4 m2
collector area results.
Designing the system as a whole
The Viessmann
technical guides include
examples of complete
hydraulic schemes with
circuit diagrams for
these system types.
System with open air pool
Pools are only heated in summer. Therefore,
the collector system is available for central
heating backup during the colder months.
Systems that combine swimming pool water
heating, DHW heating and central heating
backup are therefore recommended.
Requirements regarding the swimming
pool water heat exchanger
The heat exchanger that transfers the solar
energy to the pool must be resistant to
swimming pool water and must offer a low
pressure drop, even with a high flow rate.
Conventionally, tubular heat exchangers are
used; under certain application conditions
plate-type heat exchangers can also be used.
On account of the low pool water
temperature, the temperature differential
between the pool feedwater and the collector
return are not as critical as for DHW heating
or solar central heating backup. However, it
should not exceed 10 K to 15 K. Relative to
the installed collector area, the Viessmann
product range offers different tubular heat
exchangers at 10 K temperature differential
(see Fig. C.2.4–6).
Fig. C.2.4–4 System with open air pool and solar central heating backup
A solar thermal system for heating
open air pool water can be used
in autumn, winter and spring for
central heating backup.
Fig. C.2.4–5
System with indoor pool and DHW heating
A solar thermal system for heating
an indoor pool utilises solar heat
even in the autumn, winter and
spring for the indoor pool.
Fig. C.2.4–6
Viessmann swimming pool heat exchanger
Subject to the collector area to
be connected, Viessmann offers
Vitotrans 200
a matching swimming pool
Part no.
Maximum connectable
absorber area Vitosol in m2
3003 453
3003 454
3003 455
3003 456
3003 457
heat exchanger.
C.2 Sizing
C.2.5 Cooling with solar backup
In our latitudes, cooling capacity is required
in summer to condition buildings (residential,
workplace). In other words, the cooling
demand exists during the season with high
insolation. The necessary energy required to
cover constant cooling loads (IT systems, food
storage, etc.) increases in summer.
On the consumption side, the design of a
solar refrigeration system is no different
from a conventional system. Initially, the
cooling capacity and the load profile of the
building must be determined. On that basis,
output and type of refrigeration machine
are determined.
Apart from the widely used electrically driven
compressor refrigeration machine, systems
with thermally driven refrigeration processes
can also be achieved. For liquid refrigerants,
absorption and adsorption machines are used,
for air as coolant, sorption systems with heat
wheels are used.
In most cases, single stage absorber
machines are used in cooling systems
with solar backup. These are available on
the market with comparatively low output.
The refrigerant is water, the sorption
medium is generally lithium bromide. Twostage machines with a significantly higher
COP (Coefficient of Performance) are
unsuitable for operation with commercially
available collectors due to their high
drive temperatures.
In thermally driven refrigeration machines, it
is sensible to consider the utilisation of solar
technology for cooling or air conditioning, as
the energy demand is in direct correlation to
the insolation.
In recent years a number of solar thermal
cooling systems have been built. For these,
some extensively documented operating
experiences and scientific research is
available. Solar air conditioning has left the
pilot phase and is now available as reliable
building systems for real applications.
Fig. C.2.5–1 Cooling with solar
backup in the Environmental
Research Centre, Leipzig.
Subject to manufacturer and application, drive
temperatures even for single stage machines
are around 90 °C; the required collector
temperature is still somewhat higher.
For that reason, only vacuum tube collectors
are suitable for this – flat-plate collectors
could only achieve the required temperature
with exceedingly poor efficiency.
to be reheated. If the system is designed
for a low solar coverage, a correspondingly
large amount of conventional heat with
low efficiency is converted into cooling.
Such conditioning should preferably be
used in projects that enable a mono-mode
solar operation.
The high temperatures require that the
engineering of the collector array is carefully
matched to the output and temperature
spread of the refrigeration machine. The
system must be designed for stagnation-free
operation, i.e. the absorber machine must
be able to absorb solar energy continuously.
Storage on the "hot side" is therefore only
possible to a limited extent on account of the
high temperatures.
For an initial cost estimate, approximately
3 m2 collector area per kW cooling capacity
can be assumed for an absorber machine
with a COP of approx. 0.7. The design output
of the vacuum tube collectors is assumed
to be only 500 W/m2 at these operating
temperatures. Subject to the machine
enabling this, the primary circuit should not
include a heat exchanger; in other words the
heat transfer medium is routed directly into
the absorber machine.
Generally a solar coverage > 50 percent
should be achieved. The refrigeration process
is designed for very low flow temperatures
because of the solar thermal system; the
refrigeration machine consequently operates
with a comparatively poor COP. This must be
taken into account if the system is designed
Fig. C.2.5–2
The high drive temperatures of
Temperature level of absorber machines
absorber machines result in only
vacuum tube collectors being
used to generate solar cooling.
28 °C
6 °C
Air conditioning
12 °C
34 °C
90 °C
85 °C
Typical temperatures when operating a solar-powered absorber machine
C.2 Sizing
C.2.6 High temperature applications
In the case of process heat, heat with a high
temperature level describes a temperature
level that can no longer be achieved with
flat-plate or vacuum tube collectors.
It is only sensible to generate temperatures
> 100 °C with solar thermal technology, if the
incoming insolation is concentrated, i.e. if the
energy density at the absorber is increased.
Very simple concentrating systems are known
as solar cookers with reflective components.
In these, the insolation is bundled at the
focal point of a parabolic mirror, where the
radiation heats up a mat black container and
its contents. Apart from the preparation of
food, solar cookers are also used to pasteurise
potable water.
Concentrating collectors require direct
insolation; diffused light cannot be reflected
onto the absorber. For that reason, this
technology is only used in regions with a high
proportion of direct radiation.
From an economic aspect, the utilisation
of concentrating systems in large scale
systems for solar thermal power generation,
is interesting. The most common form is
parabolic trough power stations.
With this type of power station, parabolic
mirrors are placed adjacent to each other and
follow the sun's progress along a single axis.
Along the focal line runs a vacuum tube with
a selectively coated absorber tube (receiver),
onto which the sunlight is concentrated more
than 80-fold. A thermal oil that is heated to
approx. 400 °C flows through the absorber
tube. The thermal energy is transferred to a
steam turbine process via a heat exchanger.
The turbine then generates electric power.
Further technologies under test are the
Fresnel collectors and the solar tower
power stations.
Fig. C.2.6–1 The Olympic Flame
is ignited by the sun's rays for
the Olympic Games using a
parabolic mirror.
Fig. C.2.6–2
Solar thermal power generation
Steam boiler
and superheater
Booster heater
In regions with a high proportion
of direct insolation, solar thermal
power stations are being
increasingly deployed.
Heating circuit
Steam circuit
C.3 Combinations with renewables
Fig. C.3-1 Leisure pool
Cambomare, Kempten
Combinations with renewables
Due to the rising costs of these fuels, the demand for heat source systems that make do
without oil or gas is constantly increasing. Biomass boilers and heat pumps are a good
combination option for solar thermal systems.
To safeguard a reliable supply, solar thermal
systems are generally combined with
additional heat sources. The principle function
of the solar thermal system does not change
with the different combinations – however
substantial potential exists for optimising the
system as a whole.
A reheating output of adequate magnitude
and good efficiency levels is always on
offer with many advanced gas or oil boilers.
Efficient reheating can also be ensured with
biomass boilers and heat pumps.
C.3.1 Solar thermal systems in
combination with biomass boilers
Boilers for wood or other solid biofuels
feature, by their nature, a large mass. They
are mostly made from metal and contain lots
of water. For the heating operation this is
no disadvantage – however, when reheating
DHW in summer, the utilisation rate is
markedly worse compared, for example, to a
gas condensing boiler. The boiler must heat
up a lot of steel and water to heat up relatively
little DHW.
Therefore, biomass boilers are frequently
combined with systems for solar central
heating backup. This has the benefit of the
system being designed right from the start
for operation in summer almost completely
without reheating. In spring and autumn,
the boiler operates similarly to DHW heating
when there is only a low heat demand; the
necessary heating is essentially provided by
the solar thermal system.
In automatically charged boilers (pellet
boilers) in detached houses, a combination
with a combi cylinder is appropriate. The
design follows the procedure described in
chapter C.2.2.
Manually charged systems require a
complete burnout and are equipped with a
heating water buffer cylinder, the volume of
which is designed for troublefree operation
of the wood boiler. For this, the volume
determination must always be made relative
to the temperature differential between the
expected return temperature (the cylinder
water cannot get colder) and the maximum
cylinder temperature (the cylinder water must
not get hotter). In other words, the cylinder is
sized so that, in case of a complete burnout,
the entire energy can be accommodated
in the heating water buffer cylinder. The
procedure is specified in EN 303-5. In
addition, observe the current order on the
implementation of the Federal Immissions Act
(1st BImSchV) [in Germany].
If this cylinder is preheated by solar energy,
its capacity will be reduced, since the outlet
temperature rises as a result of the preheating
(at the same maximum temperature).
Therefore, the temperature differential
reduces as does the cylinder capacity –
a complete burnout of the wood boiler is no
longer possible.
Consequently, if the boiler system is
to be combined with a solar thermal
system, the cylinder volume must be
increased accordingly.
Fig. C.3.1–1 Vitolig 300 pellet boiler
C.3 Combinations with other renewables
C.3.2 Solar thermal systems in
combination with heat pumps
For detailed information
regarding the
combination with solar
thermal systems,
see the technical
documentation for
Viessmann heat pumps.
Heat pumps combined with solar thermal
systems for DHW heating
Heat pumps combined with systems for
solar central heating backup
The smaller the differential between the
target and heat source temperature, the
more energy efficient the heat pump will be.
For DHW heating, the flow temperature is
therefore kept as low as possible via large
heat exchanger areas. For the dual-mode
operation of a solar thermal system with a
heat pump, Viessmann offers a special heat
pump cylinder.
In respect of their power tariffs, heat pumps
are frequently subject to certain power-OFF
times and must, in those cases, be combined
with a heating water buffer cylinder – this is
also suitable to be heated by solar energy. As
the power-OFF periods generally apply during
the daytime, a "conflict" with the prior heating
time cannot be avoided. This situation can
often be diffused by technical means, but it
cannot be avoided altogether.
The very large internal indirect coil is made
available exclusively for the heat pump; the
solar thermal system heats the cylinder via an
external heat exchanger.
Fig. C.3.2–1
DHW cylinder Vitocell 100-V with
solar heat exchanger set
Viessmann heat pump cylinder
The buffer volume is determined by the
minimum heat capacity that is required to
bridge the power-OFF periods. A matching
collector area can be connected to this
heating water buffer cylinder. It can be
increased in size if a higher coverage is
required. In this case that part of the cylinder
that is required to bridge the power-OFF
periods must be made available to the
heat pump separately from a hydraulic and
control aspect.
Where a heating water buffer cylinder is
not absolutely required for the heat pump
operation, then the buffer cylinder for the
solar thermal system is sized in the same way
as when combining it with boiler systems.
C.4 System simulation with ESOP
System simulation with ESOP
A simulation is a calculation with the aid of a computer model – the result delivers
insights into a real system.
Simulations are carried out if conventional
manual calculations would be too extensive
or might deliver inadequate results. This is
frequently the case with dynamic system
characteristics, i.e. if the system is subject to
constant changes within a defined period.
Simulation programs for solar thermal
systems offer the possibility to recreate
and analyse such systems on the computer.
For this, the parameters in the models
programmed into the simulation program are
matched to the intended system.
Dynamic simulation models are required to
achieve the most accurate results because of
the diverse, time-dependent interactions in
solar thermal systems, that occur daily as well
as with changing seasons.
Fundamental program structure
On the one hand, a simulation model requires
input variables, such as weather details or
load profiles. On the other hand, the individual
components of the system, such as the solar
collector, DHW cylinder or heat exchanger,
must be defined using defaulted parameters.
The simulation program delivers certain
values as output variables, such as the solar
coverage or the annual solar yield.
Input variables
Essential input variables for a dynamic
simulation program are the meteorological
details for the intended location of
the system.
The 'test reference years' are widely used.
These are offered by the German Weather
Service (DWD) [for Germany]. The DWD has
split the Federal Republic into 15 climate
zones and for each of these zones has
collated typical meteorological data, such as
strength of insolation, air temperature, relative
humidity or wind speed.
In addition the simulation program offers the
possibility to load data sets, such as DHW
draw-off rates or heating load details and to
process these as part of the simulation.
The Viessmann ESOP simulation program
includes weather details. ESOP offers a
dynamic simulation model with which the
time-dependent thermal and energetic
characteristics of the individual components
and of the overall system as well as the
energy flow can be entered into a statement
using its numeric calculation process.
Fig. C.4–1
Information flow chart for simulation
Input variables
Simulation model
Output variables
C.4 System simulation with ESOP
Fig. C.4–2
ESOP: Solar thermal system for DHW heating
Setting the model parameters
The necessary work in creating a system
simulation includes setting the model
parameters for the required system concept,
i.e. the input of the component parameters
(e.g. efficiencies or loss coefficients) as
well as the collation of the components into
a single system. ESOP already includes
common system schemes for DHW heating
and solar central heating backup including
both as combinations with swimming pool
water heating.
Fig. C.4–3
ESOP: Input dialogue, collectors
Fig. C.4–4 ESOP: Definition of the heat load
The parameters of the Viessmann system
components such as collectors, cylinders
or boilers can be entered quite easily and
conveniently into the ESOP program. Clicking
on the respective component opens a
selection menu.
For entering the heat load too, there are
predefined profiles enabling parameters to be
set quite easily. These profiles enable daily
and weekly load profiles as well as seasonal
fluctuations or holiday periods to be taken
into consideration.
Fig. C.4-5
ESOP: Result printout
Output variables
ESOP issues all essential parameters
required to assess the system configuration,
such as solar coverage, collector yield and
energy savings.
ESOP was developed to help with design and
engineering and to optimise solar thermal
systems. Furthermore, the system is also
suitable as support for the selling process,
either as an appendix to an offer or for
experienced users "live" on site.
Simulation limits
Some experience is required to carry out
simulation calculations. Errors in entering
parameters can possibly result in a gross
falsification of the simulation – a plausibility
check is therefore always recommended.
In principle, the specific collector yield (see
chapter A.2.4) is a good parameter for the
plausibility check.
For a system for DHW heating with flat-plate
collectors, this value should lie between
300 kWh/(m2 · p.a.) and 500 kWh/(m2 · p.a.). In
addition, the experiences of existing systems
enable some parameters to be defined and
checked with the simulation calculation.
It should also always be observed, that a
simulation always represents fictitious system
characteristics based on synthetic weather
details for a whole year.
The actual weather conditions and the actual
utilisation pattern may, in a genuine system,
indicate significant seasonal fluctuations.
Individual months, weeks or days can strongly
deviate from the simulation, yet without
leading to significant deviations in annual yield
between the simulated and the actual system.
A simulation only
permits an energetic
assessment of the
system. The simulation
result and the graphic
printout neither replace
the building plan nor a
sound planning of the
D Solar controllers
The solar controllers takes over the energy management and ensures
the effective use of the sun's heat.
With the Vitosolic range of control units
Viessmann offers the right device for all
requirements. The Vitosolic ensures that heat
"harvested" from the solar collectors is utilised
as effectively as possible for either heating
DHW or swimming pool water or for central
heating backup.
The controller communicates with the boiler
circuit control unit and switches the boiler off
as soon as sufficient solar heat is available.
146 D.1
Solar controller functions
147 D.1.1 Standard functions
149 D.1.2 Auxiliary functions
154 D.2
Checking function and yield
155 D.2.1 Checking function
156 D.2.2 Checking the yield
D.1 Solar controller functions
Fig. D.1–1
Viessmann solar
controller Vitosolic
Solar controller functions
Solar thermal systems are regulated by the solar controller.
The requirements that a controller must fulfil may vary considerably
– they depend on the type of system and the required functions.
The following describes the standard
and possible additional functions of solar
controllers. The Vitosolic solar controllers
cover all conventional applications.
For concrete system-specific
controller settings, see the respective
technical documentation.
D.1.1 Standard functions
Temperature differential control
With the temperature differential control,
two temperatures are measured, and the
differential between both is determined.
A solar control unit in most systems compares
the collector and cylinder temperatures – for
this the controller uses the actual values
captured by the temperature sensors fitted
to the collector and the DHW cylinder. The
solar circuit pump starts as soon as the
temperature differential between the collector
and the cylinder has exceeded the preselected
value (start temperature differential). The heat
transfer medium transports the heat from the
collector to the DHW cylinder. If a second
smaller temperature differential is no longer
achieved, then the solar circuit pump stops
(stop temperature differential). The difference
between the start and stop temperature
differential is referred to as hysteresis.
The start point for the solar circuit pump must
be selected so that the heat transport from
the collector to the cylinder is worthwhile,
in other words that, at the heat exchanger,
an adequately high temperature differential
between heat transfer medium and cylinder
water is present. Furthermore, when the
heat transport from the collector commences
the system must not immediately switch
off again, as soon as the cold heat transfer
medium from the pipework reaches the
collector sensor.
In conventional solar thermal systems
with internal indirect coils inside the solar
cylinder, a start value of 8 K and a stop value
of 4 K collector temperature above cylinder
temperature has proven useful, subject to
the heat transfer medium temperature being
captured accurately (see Fig. D.1.1–2). A
certain tolerance for inaccurate measurements
is unavoidable for these values. In case of very
long pipework (approx. > 30 m) raise both
values by 1 K per 10 m.
For systems with external heat exchangers,
calculate the start and stop values for the
primary and secondary circuit on the basis of
the line lengths and the selected temperature
differential at the heat exchanger. The
secondary circuit starts and stops at slightly
lower temperature differentials.
The solar controller ensures the
efficient heat transport. Heat will only
be moved from the collector into the
cylinder if it is worthwhile.
Fig. D.1.1–1
Principle of the solar controller
1. Heating the collector
2. Heating the cylinder
3. Storing the heat
D.1 Solar controller functions
Fig. D.1.1–2
Collector sensor position
The location of the temperature
sensors inside sensor wells ensures
optimum measurements for the
solar controller.
Maximum temperature limits
Sensor position
In addition, every solar heating process
can be limited to an adjustable maximum
temperature. This does not replace a high limit
safety cut-out that may be required to prevent
the generation of steam inside the cylinder,
for example.
A precise measurement results if the
temperature is captured immediately in
the heat transfer medium, in other words if
sensors are positioned inside sensor wells.
Sensor wells are standard for all Viessmann
cylinders and collectors.
Temperature sensor
Measures for protecting
the sensor and the
solar controller against
overvoltages are
described in chapter
The temperature sensor at the collector must
be highly resistant to high temperatures, since
solar thermal systems generate temperatures
substantially higher than conventional heating
systems. In addition, the sensor element
must be fitted to a lead that is highly resistant
to high temperatures and weather influences.
All other sensor requirements are the
same as the performance characteristics
of commercially available, high grade
heating controllers.
With Vitosol flat-plate collectors with meander
absorbers, sensor wells must be located on
that side of the collector where the absorber
pipe is soldered onto the manifold pipe
(this is the collector side where the type
plate is located on Viessmann collectors).
This enables the collector sensor to quickly
recognise a temperature rise in the absorber.
D.1.2 Auxiliary functions
Multiple temperature differential
measurements and cylinder priority
In solar thermal systems with several
cylinders or consumers it is necessary to
combine various temperature differential
measurements. Subject to requirements,
different control strategies can be selected
for this.
Priority control
With priority control, one cylinder is given
preference for solar heating. In other words,
when two consumers are being heated – for
example, a DHW cylinder and a swimming
pool without conventional reheating – the
system concept will be controlled so that the
DHW is heated with solar energy as priority.
Only when the cylinder has reached its target
temperature should the solar thermal system
heat up the swimming pool (see Fig. D.1.2–1).
Fig. D.1.2–1
Priority control
T1 Collector sensor
T2 Cylinder sensor
T3 Swimming pool sensor
P1 P2 Solar circuit pumps
The control settings therefore determine
that the solar thermal system heats up the
DHW cylinder as a priority. This represents
an acceptance that the solar thermal system
operates with slightly poorer efficiency since
it will not, as a priority, heat up the colder
pool water.
DHW heating priority: P1 runs, if T1 is greater than T2. P2 only runs, if T2 has reached its target temperature
and T1 is greater than T3. (Observe the respectively necessary temperature differential.)
Control for efficiency
If the solar thermal system is to operate
as efficiently as possible, it should always
operate within the best possible efficiency
range. For a system with two cylinders that
are reheated all the year round, the control
unit must ensure that it is always the cylinder
with the lowest temperature that is heated
(see Fig. D.1.2–2).
Fig. D.1.2–2
Control for efficiency
T1 Collector sensor
T2 T3 Cylinder sensor
P1 P2 Solar circuit pumps
This control concept is used, for example,
when two consumers (living units) are
supplied by a single solar thermal system.
Control for efficiency: P1 runs, if T1 is greater than T2 and T2 is smaller than T3. P2 runs, if T1 is greater than
T3 and T3 is smaller than T2. (Observe the respectively necessary temperature differential.)
D.1 Solar controller functions
Fig. D.1.2–3
Control with bypass pump
Control with bypass pump
A bypass system can improve the starting
characteristics of a solar thermal system, for
example when there are long supply lines to
the cylinder or when vacuum tube collectors
are installed horizontally on a flat roof.
T1 Collector sensor
T2 Bypass sensor
T3 Cylinder sensor
P1 Bypass pump
P2 Solar circuit pump
Control with bypass pump: P1 runs, if T1 is greater than T3. P2 only runs, if T2 is greater than T3. (Observe
the respectively necessary temperature differential.)
Fig. D.1.2–4
The solar controller captures the collector
temperature with collector sensors. The
bypass pump is switched ON if the set
temperature differential between the
collector temperature sensor and the cylinder
temperature sensor is exceeded. This ensures
that the heat transfer medium heated by solar
energy initially heats the pipework only. When
the programmed temperature differential
between the bypass sensor and the cylinder
temperature sensor is exceeded, the solar
circuit pump starts and the bypass pump
stops. This prevents the cylinder from cooling
down too much (when operating with internal
indirect coils) when heating commences.
Control with radiation sensors
From the hydraulic principle this concept
is similar to the control with bypass pump,
however the bypass operation is not started
by means of a differential temperature but by
capturing the level of insolation.
Control with radiation sensor
T1 Bypass sensor
The solar controller captures the level of
insolation via a solar cell. The bypass pump
starts when the set insolation threshold is
exceeded. A value of 200 W/m2 has proven
useful in conventional applications.
T2 Cylinder sensor
SF Radiation sensor
P1 Bypass pump
P2 Solar circuit pump
Control with radiation sensors: P1 runs when the insolation has exceeded the minimum value. P2 only runs, if
T1 is greater than T2. (For this, observe the necessary temperature differential.)
This type of bypass control is particularly
suitable when a consistently precise
temperature capture is not possible at the
collector, for example because of partial
shading (chimneys and similar).
Suppression of reheating
Heating for DHW hygiene
To raise the efficiency of the solar thermal
system, the conventional reheating of the
dual-mode DHW cylinder can be delayed until
no solar energy is being delivered anymore
(solar circuit pump off). This function can
be utilised in conjunction with the Vitotronic
boiler controllers. The current Viessmann
product range features the required software;
older controllers can be retrofitted.
To take care of the DHW hygiene, the entire
DHW volume is once daily heated to 60 °C.
This concerns the lower part of the dual-mode
DHW cylinder or any pre-cylinders used.
As is customary, a reheat temperature for the
DHW is selected at the heating controller. In
addition, a minimum temperature is selected.
If reheating suppression is activated, and
the cylinder is heated by solar energy, the
boiler control unit permits that the DHW
temperature can drop down to the selected
minimum temperature. The DHW cylinder will
then only be heated by the boiler (solar circuit
pump runs), if this minimum temperature is
not achieved.
For this pasteurisation, the necessary amount
of heat must be able to be routed via the
reheating indirect coil into the entire cylinder
volume. The sensor location must ensure that
the entire DHW volume has actually reached
the required temperature.
Optimising pasteurisation
The controller function for optimising
pasteurisation prevents it happening when the
DHW in the pre-cylinder or in the lower part
of the dual-mode DHW cylinder has already
been heated to 60 °C by the solar thermal
system within the past 24 hours.
This function also presupposes that the boiler
control unit is suitable for communication with
the Vitosolic controller.
Fig. D.1.2–5
Control of pasteurisation
1 Vitosolic
2 Boiler control
For pasteurisation, a good interaction between the solar
controller and the boiler control unit is beneficial. Reheating
will be prevented if, during the past 24 hours, 60 °C has
been exceeded at the cylinder sensor.
3 DHW cylinder
4 Pre-cylinder
D.1 Solar controller functions
The cooling functions
of the controller
supplement the
measures in case of
stagnation, but they do
not replace them. For
full details regarding
stagnation, see
chapter B.3.5.
Functions for preventing stagnation
Interval function
Further functions can be activated to prevent
stagnation or to reduce stagnation loads.
However, they are only appropriate for
systems subject to very high pressure and
those used for solar central heating backup,
where frequent stagnation must be expected.
The interval function is used in systems
where the absorber temperature cannot be
precisely determined immediately. This may
be the case, for instance, with horizontally
installed vacuum tube collectors where
there is no adequate thermal buoyancy
ensured in the tubes that the collector sensor
immediately registers the temperature
increase. The solar circuit pump is started for
30 seconds in adjustable intervals to move
the heat transfer medium from the collector
to where the sensor is located. From 22:00 to
6:00 h, the interval function will be disabled.
Cooling function
The solar circuit pump will be switched
OFF in standard mode, if the set maximum
cylinder temperature is reached. The pump
will be started long enough to enable this
temperature to fall by 5 K, if the collector
temperature rises to the selected maximum
collector temperature and the cooling function
has been enabled. With this setting, the
cylinder temperature can then rise further,
but only up to 95 ºC. The magnitude of this
thermal cylinder reserve is selected via the
maximum cylinder temperature.
Return cooling function
This function is only appropriate if the
cooling function has been enabled. If the set
maximum cylinder temperature is exceeded,
the solar circuit pump will be started to
prevent the collector from overheating. In
the evening, the pump will run for as long
as required to cool the DHW cylinder via the
collector and the pipework down to the set
maximum cylinder temperature. With flatplate collectors, this function has a much
more pronounced effect than with vacuum
tube collectors.
The maximum cylinder temperature is adjusted to
70 °C. The solar circuit pump initially stops when
this temperature is reached. The collector heats up
to the selected maximum collector temperature of
130 °C. With the cooling function, the solar circuit
pump starts again and continues to run until the
collector temperature falls below 125 °C or the
cylinder temperature has reached 95 °C.
In the evening, the return cooling functions makes
the solar circuit pump run for as long as required to
cool down the cylinder via the collector to 70 °C or
until the cylinder temperature reaches 95 °C
(safety shutdown).
Thermostat functions
In addition, the Vitosolic 200 controller
offers various thermostat functions. For
this, additional sensors capture respective
temperatures; when set temperatures are
not achieved or are exceeded, an actuator
will be switched. For example, at a specific
cylinder temperature the primary pump for a
swimming pool can be started.
For information regarding the speed control of
the solar circuit pump, see chapter B.3.1.3.
D.2 Checking functions and yield
Checking functions and yield
The solar controller ensures the effective utilisation of solar energy,
and it can also undertake important checking functions.
As with any technical equipment, faults
with solar thermal systems cannot be totally
excluded. In other supply systems, failures
are generally quickly noticed. However, with
a solar thermal system, the conventional heat
source takes over the heating automatically –
technical faults therefore are not always
apparent. Therefore, the planning of a solar
thermal system must also take account of
system monitoring.
A solar thermal system can be checked in
two ways, either through a function check or
through checking the yield.
By means of a function check, the function or
incorrect function of the entire system or of
individual components can be detected. This
can be done manually or automatically.
For the yield check, actual amounts of heat
per unit of time are compared with fixed or
calculated set values. Checking the yield can
also be done manually or automatically.
D.2.1 Checking function
Advanced solar controllers not only ensure
the correct system operation, they also offer
the facility to check the most important
system functions.
Self-test of the controller
A solar controller is comprised of different
assemblies; their functionality and interaction
are monitored by the controller itself.
Should one of these assemblies fail, then
a fault message will be generated or an
alarm triggered.
Fig. D.2.1–1
Unprotected sensor lead
The following is, regrettably,
frequently found in practical
applications: clear traces of bites
and pecking on unprotected
sensor leads.
Checking the sensor leads
A functional controller immediately registers
any faults in the sensor lead. For example,
damage of an unprotected sensor lead at the
collector by a rodent or by birds may result in
a short circuit or a complete break.
For the controller, this means an electrical
resistance either against 0 or infinity, or – in
the "logic" of measuring temperature – a
temperature of "infinitely" hot or cold.
Temperature limits are programmed into
the controller that extend to the generally
expected temperature range of a solar thermal
system. The controller reports a fault once
that range has been breached.
Monitoring temperatures
Maximum temperatures for cylinder
and collector can be defined. If these
are exceeded, the controller generates
a fault message. Before defining these
temperatures, system-specific checks need
to establish how high these values must be to
prevent confusing fault messages.
Monitoring the temperature differentials,
generally between collector and cylinder
provides a further option for checking
functions. This type of monitoring is based on
the assumption that the collector should, in
standard operation, i.e. as long as the cylinder
has not reached its maximum temperature,
not be more than 30 K hotter than the cylinder
(adjustable value), for example. The automatic
function control indicates typical faults that
lead to no energy being transferred from the
collector to the cylinder any more, although
the latter could still accept more energy:
t Faulty collector circuit pump
t Interrupted power supply to the pump
t Hydraulic problems in the collector circuit
(e.g. air, leaks, deposits)
t Valves incorrectly set
t Faulty or severely contaminated
heat exchangers
D.2 Checking functions and yield
In addition, it is possible, in spite of an idle
solar circuit pump, to register temperature
increases at the collector or a positive
temperature differential between the colder
cylinder and the hotter collector (e.g. at night).
This may point to an incorrect function of
a system component that results in gravity
circulation, i.e. the cylinder heats the collector
via gravity.
However, it should be noted that, for example,
a stronger nightly demand at the height of
summer may result in an actual temperature
differential that briefly prevails between
the cold cylinder and the hotter collector
(ambient temperature). Strong fluctuations
of the outside temperature can also possibly
result in confusing fault messages. It is
therefore recommended to inform the system
user about possible fault messages when
this function check is enabled, to prevent
unnecessary service calls.
All fault messages can be checked
immediately at the controller. In addition there
is the option to pass the fault messages to
a building management system or to other
parties via the internet.
Automatic function checks can monitor
current operating conditions with great
reliability and pick up on many incorrect
functions. However, automatic monitoring has
its limits, for example in areas where there is
a high risk of incorrect messages and system
conditions that cannot be illustrated by a
typical fault image for function errors.
If the temperature at the collector does not rise on
account of a severely contaminated or broken
collector pane, the controller cannot "know" whether
there is a fault or whether it is just a case of bad
weather. In such cases, a precise diagnosis
promises better results when it is based on
measuring and assessing yield.
D.2.2 Checking the yield
One easy and effective check is the
comparison between the actual hours run
by the pump and the expected values. For
an average solar thermal system, allow for
1 500 – 1 800 hours per annum. A system
simulation of a full year provides more
accurate values for the expected hours run by
the pumps. This comparison, however, does
not represent an actual yield.
Measuring the yield
Before solar yields can be measured, the
measuring method must first be critically
appraised to avoid incorrect estimates
concerning the system. For this, note that the
capturing of the yield via the solar controller
tends to be more of an estimate than a
measurement. It is, for example, possible
to measure the duration during which the
pump draws current. If the assessment is
supplemented by assumed (not actual) flow
rates and the temperatures of both cylinder
and collector, then this assessment is not a
reliable measure of the yield but is, instead,
only an estimation.
To measure the yield, concrete facts regarding
the flow rate and the measurement of two
temperatures are required. When measuring
the primary circuit, consider that the
viscosity of water and water:glycol mixtures
is different. If, for example, a commercially
available heat meter is installed into the
glycol circuit without applying a correction
factor, then it cannot accurately determine
the amount of heat delivered – it can only
estimate it.
Fig. D.2.2–1
Measuring yield
DHW cylinder
Heating water buffer cylinder
A Measurement in secondary circuit downstream of buffer cylinder
B Measurement in secondary circuit upstream of buffer cylinder
+ Precise measurements can be taken comparatively simply
+ Precise measurements can be taken comparatively simply
+ Takes account of cylinder losses, i.e. measures the amount of
– Does not take account of cylinder losses
usable energy transferred to the system
In systems with external heat exchangers
it is always more appropriate to measure
the secondary circuit. This enables a
determination of the amount of heat that is
transferred from the solar thermal system
to the cylinder with reasonable accuracy.
A capture point downstream of the buffer
cylinder is required if cylinder losses are also
to be taken into account, i.e. only that amount
of heat is to be metered that is transferred as
available heat to the system.
For practical applications, however, it
should be noted that measuring the amount
of heat alone – irrespective of where the
measurement is taken – is unsuitable for
legally binding billing of heat generated
by solar energy to tenants. The relevant
legislation and the description of suitable
billing procedures are currently in flux. If
an investor needs a billing method for solar
heat, we would recommend that current
information is sought from local building
trade associations.
C Measurement in primary circuit
– Very imprecise
D.2 Checking functions and yield
Manual assessment of yield
The captured yield will only then provide an
adequate statement regarding the correct
system function, if it is compared with a
reference variable, in other words a set yield.
This reference variable can either be taken
from a simulation or from actual data that has
been calculated at the system site. A certain
amount of inaccuracy cannot be avoided
with either process. For that reason, a high
tolerance must be applied to measurements
and differences between simulated and actual
weather conditions to make an assessment
meaningful. These tolerances are described
in detail in the VDI guideline 2169, that will be
published during 2009 regarding the subject
of checking the yield.
Always apply assessments of yield on the
basis of simulated weather details to a whole
year. An assessment of shorter periods is only
possible with actual weather details that are
entered into the simulation.
The simulation for a simple system for DHW heating
results in an annual value of 1 500 kWh "Solar
thermal system energy to DHW".
The fixed weather details from the test reference
year (see chapter C.4) that are stored in the
program, can deviate from the actual weather
conditions in the year to be assessed by up to
30 percent.
As only a measure of yield in the glycol circuit can
be given in this type of system, inaccuracies in
measurements result – even when using suitable
heat meters (correction factors) – in additional
deviations of similar magnitude.
Furthermore, when measuring the heat yield in the
glycol circuit the cylinder losses are not taken into
account. These were, however, entered into the
simulation result of 1 500 kWh p.a.
For example, an annual yield of 1 400 kWh is no
actual cause for doubting the correct function of
the system.
Assessing measurements taken over
several years
If yields measured annually are compared with
each other over a longer period, inaccuracies
in measurements and any capture point
that may not be ideal can be ignored, if the
assessment is only made to check the system
function. The correct system function can be
assumed if the actual results fluctuate by no
more than 20 percent.
Manual yield assessments as described here
cannot replace automatic function checks,
since actual measurements only enable faults
to be recognised in retrospect, in other words
following the expiry of longer periods without
yields. If the actual yields are "only" reduced,
faulty functions can only be recognised
through accurate analysis and with lots
of experience.
Handling user data
Frequently, designers/engineers and
installers are confronted with details that
must be supplied by the system user. The
reason for this is frequently the wish for an
"analysis" of such details. However, these
data are frequently of little use if something is
checked and recorded at some random time.
Furthermore, in most cases these absolute
values are rarely relevant.
Nevertheless, statements can be derived
about the operating state of a system with
details noted down by customers, such as
hours run, readings from heat meters or of
power consumption of the solar thermal
system, as long as this data is put into a
relevant context. It improves customer
satisfaction if these data records that have
frequently been collated meticulously, are
not dismissed as "useless", but are instead
interpreted – applying the required limitations
regarding accuracy.
Comparison of simulated with actual values
using the pump runtime as an example
Solar circuit pump hours run
Simulation (monthly)
N Dec
Simulation (cumulative)
The graphic shows the simulated runtimes of a solar
circuit pump, in the bottom line as absolute values
over one month, in the upper line as
cumulative values.
In the upper curve, the actual hours recorded at any
time run can be added:
Solar circuit pump hours run
Simulation (monthly)
N Dec
Simulation (cumulative)
Customer test values
system. The set value of a system then is
not the result of a simulation utilising the test
reference year, but of current actual data. This
enables much shorter assessment periods.
Viessmann participates in the development
and optimisation of the so-called Input/Output
Controller. With this process, the potential
yield of a system is constantly compared to
the actual yield. This is based on the specific
parameters of the system components and
the actual consumption and weather data.
A fault message is generated in the case of
implausible deviations of the actual value from
the set value.
The actual values essentially mirror the simulated
values; the conclusion can therefore be drawn that
the system operates correctly.
In a similar way, the collected data from
the estimation or measurement of the
heat amount can also be used. For this,
it is important to explain to the customer
that it not absolute values that matter, but
the progression.
Automatic yield assessment
If the system operating states and weather
data are automatically captured in situ, then
yield forecasts for the actual day can be
generated and be compared with the amount
of heat actually delivered by the solar thermal
Costs for monitoring functions and
assessing yields
Experience has shown that, the more
accurately the system yields are to be
measured and assessed, the higher
the related costs. The same applies to
monitoring system functions that cannot be
captured with the simple control functions
of the controller. When deciding which cost
framework is reasonable for monitoring and
assessment, orientation on a guide value may
be helpful: costs should be within 5 percent
of the system costs – an orientation along
the lines of this "rule of thumb" generally
leads to a balanced ratio between monitoring
costs and the value represented by the yields
"safeguarded" through that expenditure.
E System operation
The long-term reliable and efficient operation of solar thermal systems depends on well
developed components and clear concepts, as much as the particular care with which
commissioning is carried out.
The utilisation period of a solar thermal
system also depends on the care extended
to it beyond the commissioning. Apart from
instructing the customer, this also concerns
the inspection and maintenance work.
and indicates the points to check during
inspection and maintenance. In addition, the
phenomenon of condensation in flat-plate
collectors that can occasionally occur, will
be explained.
This chapter describes the preparation
and progress of commissioning, refers
to important details for practical work
162 E.1
Commissioning and maintenance
Pressure inside the solar thermal system
Preparing for commissioning
Commissioning steps
Maintenance of heat transfer media containing glycol
172 E.2
Condensation in flat-plate collectors
E.1 Commissioning and maintenance
Commissioning and maintenance
The collector generates heat as soon as sufficient light hits the absorber,
independent of whether the entire system is operational or not.
If the system is being filled and the collector
is not covered, heat will begin to be generated
in the entire primary circuit as soon as there
is insolation. To prevent unnecessary loads,
the solar thermal system will only be filled
when the heat transfer has been ensured. A
test commissioning of a solar thermal system
is impossible.
The prevailing pressures in the solar circuit
have a crucial influence on the operating
characteristics of the system. Whether filling
pressure, system operating pressure or precharge pressure in the diaphragm expansion
vessel – only the correct interaction enables
an optimum system operation to be realised.
E.1.1 Pressure inside the solar thermal system
As part of the investigations into the
stagnation characteristics of solar thermal
systems it has been shown that the pressure
in the solar circuit exerts an important
influence on the system efficiency and
its longevity.
For the design of the pressure maintaining
facility and commissioning of the solar
thermal system, some particular features
need to be considered, which will be
explained in the following.
In its idle state (cold), the system must
indicate a pressure of 1 bar at its highest point
to prevent negative pressure at this point in
operation. The solar circuit pump pushes the
heat transfer medium up to this high point,
then it "drops" via the solar circuit flow back
towards the pump. With this process, gravity
acts on the heat transfer medium so that the
pressure reduces at the highest point. Since
this point is, in most cases, also the hottest
within the system, steam might form here
due to the low pressure.
To protect the pump against excess
temperatures in operation or when the system
stagnates, locating it in flow direction into the
return upstream of the diaphragm expansion
Fig. E.1.1–1
System pressure
vessel has proven to be advantageous. When
pump and diaphragm expansion vessel are
located there, this is referred to as pressure
or end pressure holding. As a result, the
operating pressure of the pump lies below
the idle pressure of the system. To prevent
cavitation through partial boiling of the heat
transfer medium, the actual pressure must
not fall below a minimum supply pressure at
the inlet connector.
This essential supply pressure depends on the
differential pressure of the pump, the boiling
point and the operating temperature of the
transported medium. In conventional solar
thermal systems with a static pressure of at
least 0.5 bar and a filling pressure at the high
point of 1 bar, this problem can be ignored, if
you are using Viessmann solar circuit pumps.
Where designs vary and feature an idle
pressure < 1.5 bar at the inlet connector of
the pump, a calculation under consideration
of the essential minimum supply pressure
is recommended.
When calculating the static pressure, the
differences in density between commercially
offered heat transfer media and pure water
can be ignored; in other words, 0.1 bar per
metre can be assumed.
The minimum pressure at the high point of
the system and the static pressure enable the
Fig. E.1.1-2
Maintaining the pressure
Subject to the static height of the
The pump is fitted in the flow direction upstream of the non-
sloping line (flow), the pressure falls
return valve and the diaphragm expansion vessel, to protect
at the collector outlet.
it against excess temperature during stagnation.
E.1 Commissioning and maintenance
Fig. E.1.1–3
Pressure in the solar circuit
Documentation of pressure
System overpressure at
the highest point
1 bar
Supplement per metre
static ceiling
+ 0.1 bar / m
System operating pressure
(pressure gauge)
_____ bar
System operating pressure
_____ bar
Pressure reserve for venting
+ 0.1 bar
Filling pressure
_____ bar
System operating pressure
_____ bar
Deduction for hydraulic seal
– 0.3 bar
Supplement per metre height + 0.1 bar / m
difference pressure gauge – DEV
Pre-charge pressure DEV
_____ bar
3 Pressure gauge
5 Diaphragm expansion vessel (DEV)
Every solar thermal system requires
such a "Pressure fact file" to
prevent errors in sizing and during
calculation of the system operating pressure
by adding them together. This pressure is
checked at the pressure gauge – for this take
into account that components at a lower
level are subject to a higher pressure. This
is particularly relevant for the determination
of the pre-charge pressure of the diaphragm
expansion vessel. If, for example, the pressure
gauge is at "eye level" and the diaphragm
expansion vessel is at floor level, a pressure
differential of approx. 0.15 bar results.
The pre-charge pressure of the diaphragm
expansion vessel results from the system
operating pressure at the point where the
diaphragm expansion vessel is connected,
less 0.3 bar for the hydraulic seal. The
hydraulic seal is important to balance the
volume loss through cooling down against
the filling temperature. A value of 0.3 bar
in conventional systems ensures that the
required amount of water (4 percent of the
system volume, but at least 3 l) is pushed into
the diaphragm expansion vessel when the
system is being filled.
To compensate for the deaerating of the
medium during the first weeks in operation
(pressure drop through venting), an additional
pressure reserve of 0.1 bar is recommended.
The filling pressure during commissioning
is therefore 0.1 bar higher than the system
operating pressure.
E.1.2 Preparing for commissioning
Minimum requirements of a
commissioning report
Prevention of unintentional heating of
collectors during commissioning
Every commissioning must be recorded. The
commissioning report is a firm component of
the system documentation and a prerequisite
for the correct handover to the user. For
this, it must be noted that some providers of
subsidies may require special reports.
As with the commissioning of any other
piece of technical equipment, the duration
of the process for solar thermal systems
cannot be accurately forecast in terms of
time. Frequently it has proven to be a mistake
to start commissioning before sunrise to be
able to complete the necessary steps in time
before the first insolation hits the collector. If
the process cannot be fully completed before
the collector heats up due to insolation,
breaking off the commissioning temporarily
may cause problems with the system only
partially filled. Therefore, the safest method is
to cover the collectors.
Independent of the selection of the default
commissioning report or of the individual
preference, every report must document the
following values (explanation of the individual
steps in the following sections):
t Pre-charge pressure of the diaphragm
expansion vessel and system operating
pressure (at approx. 20 °C)
t Manufacturer and type of heat transfer
medium, specific gravity test results (frost
protection) and pH values for the heat
transfer medium after filling and venting
t Controller settings
Viessmann flat-plate collectors are supplied
with a foil on the glass cover – it is therefore
recommended not to remove the foil until
after the commissioning has been completed.
Foil covers for vacuum tube collectors
are available.
Whether for installation contractor, user or
system designer/engineer, a commissioning
report is without practical value without
complete details regarding the above point,
and should therefore not be accepted.
Fig. E.1.2–1
To prevent the collectors from heating up prior to
and during the commissioning, Viessmann flat-plate
collectors are supplied with a protective foil.
Collector cover
E.1 Commissioning and maintenance
Checking and adjusting the precharge pressure of the diaphragm
expansion vessel
In chapters B.3.5.2 and E.1.1, the calculation
of the diaphragm expansion vessel volume
and the system operating pressure have
already been described in detail. However,
the most careful calculation is worthless if the
calculated values are not the same as those
of the completed system. In many cases,
the delivered condition of the diaphragm
expansion vessel "determines" the operating
pressure of the system. The first step during
commissioning is therefore to check the precharge pressure of the diaphragm expansion
vessel. Experience shows that this point is
frequently forgotten and can only be done
retrospectively with a great deal of effort,
once the system has been filled.
Experienced contractors have found it useful
to make the person commissioning the
system responsible for the operating pressure
of the system and therefore also for the precharge pressure of the diaphragm expansion
vessel, and not the person installing the
system. During the commissioning, a
complete plausibility check for all data
concerning the system operating pressure
should be carried out (see chapter E.1.1).
Fig. E.1.2–2
The commissioning cannot be
correctly carried out without checking
the pre-charge pressure of the
diaphragm with a pressure gauge.
Manual pressure gauge
Then the pre-charge pressure of the
diaphragm expansion vessel is checked and,
if required, adjusted. Use nitrogen if the
vessel needs to be topped-up with gas. This
prevents the diffusion of oxygen into the heat
transfer medium, since the diaphragm inside
the expansion vessel is never totally gas tight.
In addition, nitrogen takes longer than oxygen
to diffuse through the membrane, in other
words, the pre-charge pressure is maintained
for longer.
Record the adjusted pre-charge pressure
in the commissioning report and, as double
security, on the diaphragm expansion vessel
itself as well. It has proven to be appropriate
in practical use to mark the comment with
"Pre-charge pressure diaphragm expansion
vessel". If the vessel only bears information in
bar, the question might arise during inspection
and maintenance which pressure this remark
refers to – even if readers have written the
note themselves.
E.1.3 Commissioning steps
Pressure test
Prior to flushing and venting, test the system
for leaks. This can, of course, only be carried
out when there is no insolation hitting the
collector. Thirty minutes is adequate for this
provided that the heat transfer medium is not
subject to any temperature changes.
The question regarding the test pressure
is frequently discussed. The essential
components are tested with 1.5-times the
maximum operating pressure. If this test
was to be applied to the entire system, the
safety valve would have to be removed for
the duration of the test and its connection
would need to be shut off. A dangerous
pressure rise could result if the time of day
and the question of a cover for the collector
were to be ignored. Most manufacturers have
therefore agreed that a test pressure of up to
90 percent of the final system pressure
(= 80 percent response pressure of the
safety valve) is adequate – however under
the proviso that the system must be a twocircuit system and the secondary circuit can
be pressure-tested separately (see BDH
information sheet no. 34, 2008).
words, there is a risk that the flushing liquid
remains inside the pipework or the collector.
Flushing the system with water only can lead
to the heat transfer medium being thinned
down, making it lose its required properties.
In critical months there may also be a risk
of frost damage. Experienced contractors
therefore maintain a canister with "Flushing
heat transfer medium" that can be used
several times for this purpose. Here too,
observe the mixability of heat transfer media
(see chapter B.3.4).
Filling and draining the system
Carefully vent the system as part of the
commissioning. At this point, it should
be pointed out once more that venting
facilities on the roof are only intended
as filling aids and are not designed for
deaerating in normal operation (see chapter
B.3.3 and C.1.2). Observe this particularly
during commissioning.
It would be careless to operate the system
during commissioning with an open air vent
valve on the roof. Particularly during the initial
operating phase there is a comparatively high
risk of unintended stagnation – causes could
be, for example, adjusting errors, lack of heat
consumption or a power failure through other
Flushing the system
A solar thermal system must be thoroughly
flushed, just like any other heating installation.
For this ensure that no contamination is
flushed into the collector. Collectors are
delivered in a clean state. Particularly with
welded steel lines, it has proven to be
beneficial to flush these prior to connecting
the collectors. In this case, repeat the
pressure test after the collectors have
been connected.
Fig. E.1.3–1
Flushing container with pump
Filling and venting with an open
vented flushing container and a
powerful pump is state of the art.
This process can be carried out in
With soldered copper lines, flush until all scale
has been removed. Scale, on account of its
oxygen content, leads to a rapid ageing of the
heat transfer medium.
Viessmann recommends that the system
is flushed with heat transfer medium via a
flushing container (see Fig. E.1.3–1). Few
systems ensure that the liquid fully drains
after flushing and pressure test – in other
one step.
E.1 Commissioning and maintenance
When the system starts to operate under
regulated conditions it must already be fully
vented. State of the art for this is the filling
and venting with an open vented flushing
container and a powerful pump.
Filling and venting can then be carried out in
one step. If manual air vent valves have been
installed at the collector array(s), these are
opened for venting and closed again, as soon
as heat transfer medium escapes. For single
array systems, all other steps can be carried
out from the heating room.
Venting via a flushing container takes at
least 30 minutes. With adequate experience,
conclusions can be drawn from the
consistency of the returning heat transfer
medium (foaming, air bubbles) regarding
the state of venting of the entire system.
If in doubt, it is better to vent ten minutes
longer than not vent long enough. For this,
observe the correct operation of the valve
at the container supply side. The valve
prevents negative pressure inside the
collector and the pipework downstream, i.e.
the pressure gauge must always indicate the
static pressure.
If the system comprises sections that can be
shut off (via the return lines), these sections
can be opened individually for venting. It is
therefore very important that this pressure
is held at the container supply side, as
otherwise the heat transfer medium would
deaerate again in the collector arrays that are
shut off on their return side through negative
pressure; this would channel air back into
the collector.
Fig. E.1.3–2
After venting has been completed, the
valve in the flow will be closed and the
system brought to operating pressure. It is
recommended to raise the pressure during
commissioning slightly above normal (approx.
0.1 bar higher), as in operation – in other
words when the temperature rises – the
system will deaerate further and thereby
reduce the pressure (see chapter E.1.1).
Maintaining the pressure during flushing
Pressure gauge
Filling station
Fill & drain valve
Fill & drain valve
To remove any possible residual amounts
of air in very complex collector arrays or
pipework, the system can be operated
during the first few days in manual mode
(providing the extra effort for this can be
justified). This is particularly recommended
when commissioning the system during
bad weather. If the heat transfer medium
is not moved for a longer period following
commissioning, there is a risk of so much air
collecting at the high points in the system,
that the system cannot start at all.
To prevent negative pressure at the collector outlet and in the pipework downstream, the flow rate at the
container supply side (4) is reduced during flushing and filling.
After filling the solar circuit, check and record
the essential parameters of the heat transfer
medium (frost protection and pH value)
(see chapter E.1.4).
Commissioning the controller
Fig. E.1.3–3
Commissioning the controller
When commissioning the controller,
set the start and stop points of the
The controller can be commissioned after
filling and venting the system. Initially, select
and adjust the appropriate system scheme on
the controller. Then all connected components
are checked in manual mode for function, and
the sensor values are checked for plausibility.
Afterwards, the controller parameters are set,
in other words, the start and stop points of
the respective control functions are selected.
Record these settings during commissioning.
respective control functions.
Instructing the user
The user is instructed in much the same
way as for other heating equipment; the
instructions are recorded accordingly.
Although there are no specific regulations
regarding solar thermal systems, the user
should nevertheless be informed of the
possibility of checking the function of the
system. If the system operates as a dualmode system without an automatic function
check, then the user can possibly only notice
failures by manual inspection.
System handover
Initial inspection
An initial inspection after a few weeks in
operation should be part of the contractor's
service and the cost should be included in
the quotation. If the system initially operates
without problems, one can conclude that
the system operates correctly and is likely
to continue to do so over a long service life.
Where operating problems become apparent
during the first inspection, corrections or
adjustments can be made to safeguard the
reliable and efficient function of the system in
the long term.
Good experiences with the initial inspection
following commissioning of a solar thermal
system have resulted in a sector-wide
recommendation that has been formulated
in the information sheet no. 34 issued by the
BDH to its members (see Fig. E.1.3–4). This
makes it easy to establish the initial inspection
in the market that is so important for the
operational reliability of the system. It is a part
of the "Solar thermal system service" which
should not be ignored.
Commissioning can only be completed if the
heat consumption is ensured. Consequently
partial commissioning may be required,
especially with building projects that
take some time to complete. The release
of moneys connected with a handover
must, however, not result in the system
being put at risk through a prematurely
completed commissioning.
Steps such as the pressure test, filling and
setting controller parameters can be carried
out with the collectors covered up. On that
premise, a partial commissioning of the
system can be carried out. It is recommended
to agree this when concluding the contract.
E.1 Commissioning and maintenance
Fig. E.1.3–4
BDH recommendation, extent of inspection
The BDH information sheet no. 34
"Operational reliability of solar thermal
systems" includes a sector-wide
recommendation regarding the extent of
the initial inspection as well as the annual
In addition to the inspection, a visual check
of all essential components (collectors,
pipework, valves and fittings, etc.) is
recommended every three to five years.
Informationsblatt Nr. 34
thermischer Solaranlagen
Für grundlegende und ergänzende Informationen beachten Sie bitte die BDHInfoblätter Nr. 17 „Thermische Solaranlagen“ Teil 1, 2 und 3, sowie die BDHInfoblätter Nr. 27 „Solare Heizungsunterstützung“ Teil 1 und 2.
Dieses BDH-Infoblatt legt den Schwerpunkt auf den Einfamilienhausbereich.
1. Einleitung
Thermische Solaranlagen sind Bestandteil moderner Heiztechnik und reduzieren den Verbrauch von fossiler Energie. Das schützt die Umwelt und senkt
die Energiekosten. Der Trend geht dabei zu größeren Kollektorflächen; fast
die Hälfte der neu gebauten Anlagen dient auch der Heizungsunterstützung.
Moderne Kollektoren sind zudem sehr leistungsfähig: Handelsübliche Flachkollektoren erreichen auf dem Prüfstand Stillstandstemperaturen von deutlich
über 200 °C, bei Vakuum-Röhrenkollektoren liegen sie über 260 °C.
Eine Besonderheit der Solartechnik ist die Energiequelle, denn die Energiezufuhr der Sonne – der „Brenner“ – lässt sich nicht abschalten. Ein Betriebszustand, bei dem die Kollektoren und Teile des Solarkreises bis zur Stillstandstemperatur erwärmt werden, ist daher normal.
Thermische Solaranlagen müssen grundsätzlich eigensicher ausgeführt sein,
d. h., es müssen alle Betriebszustände eigenständig und ohne eingreifende
Maßnahmen von außen durchlaufen werden können. Nur bei eigensicheren
Solaranlagen ist der zuverlässige, störungsfreie Betrieb langfristig gewährleistet.
In der Praxis der vergangenen Jahre stellte sich heraus, welche Anlagenkonzepte besonders betriebssicher sind, wie sich Belastungen reduzieren und
Probleme vermeiden lassen. Dieses Infoblatt fasst die Erfahrungen zusammen
und zeigt auf, wie thermische Solaranlagen über 20 Jahre sicher betrieben
werden können.
7.1 Extent of inspection
The inspection, to be carried out annually, should extend to at least the following
areas (also applies to the initial inspection):
(at initial inspection: initial value)
(e.g. Tmax collector, Tmax cylinder, total yield etc.)
– Flow and return temperatures at thermometers
– Controller display values
The BDH information sheet
no. 34 "Operational reliability of
solar thermal systems" can be
downloaded free of charge at:
The diaphragm expansion vessel and safety valve do not need checking if the system
operating pressure is correct and the safety valve shows no signs of responding
(deposits, drips, level rise in drip container).
E.1.4 Maintenance of heat transfer medium
containing glycol
To ensure that the heat transfer medium
can permanently fulfil its frost and corrosion
protection functions, particularly its loading
through oxygen at high temperatures must be
minimised. For further details in this respect,
see chapter B.3.4. As part of the inspection,
test the heat transfer medium with regards
to its pH value and glycol contents – the
pH value enables conclusions to be drawn
regarding the chemical state of the heat
transfer medium, and the glycol content is
relevant for frost protection.
Checking the pH value
Viessmann heat transfer media are slightly
alkaline and neutralise those acids that can
form through temperature and oxygen loads.
These alkaline buffers will, over the years,
deteriorate. This can turn the heat transfer
medium acidic and this in turn can put system
components at risk. The heat transfer medium
used by Viessmann is or has already been
delivered with the following pH values:
t 5ZGPDPS-4(-4o
t 5ZGPDPS-)5-o
Operation is unproblematic and safe up to
a pH value of > 7; if the value drops below
that value, replace the heat transfer medium.
For checking, a conventional litmus test
is adequate.
Fig. E.1.1–1
Litmus strips
The litmus test indicates the pH value of the tested liquid
through discolouration.
Checking the glycol contents
A glycol tester [antifreeze tester] is a
simple test instrument familiar from the
automotive sector.
Pressure gauge,
refractometer and litmus
strips, etc. are part of
the standard delivery of
the solar test case from
Viessmann provided
for commissioning,
maintenance and
function test of solar
thermal systems.
Subject to the test being carried out at room
temperature, the level of frost protection can
be directly read off the scale in °C. Although
this method is relatively inexpensive, it is also
inaccurate. Compared to the test methods
described in the following it also "consumes"
lots of heat transfer medium.
A refractometer provides a more accurate
test. This instrument determines the glycol
content via a refractive index and displays the
frost protection level relative to temperature
(in °C). A few drops of medium will provide a
comparatively precise measurement.
Heat transfer media are available in different
versions; the refractive indices vary slightly.
For a reliable determination of the level
of frost protection with a refractometer,
draw on the corresponding information
in the respective datasheet of the heat
transfer medium.
The level of frost protection of the medium
is recorded in the commissioning report. A
typical statement would be: "Frost protection
down to – xx °C".
Fig. E.1.4–2 Refractometer
The level of frost protection can be accurately determined with the refractometer using the refractive index.
E.2 Condensation in flat-plate collectors
Condensation in flat-plate collectors
Although the phenomenon of a misted-up pane inside the collector can be noticed
occasionally, it is mostly misinterpreted. The following illustrates and explains
the context.
Most commercially available flat-plate
collectors are equipped with venting apertures
to prevent a permanent precipitation of the
moisture contained in air inside the appliance.
Under normal operating conditions, a 50-fold
air change per day is provided for this.
Especially during the first few days of
operation, increased levels of condensation
can precipitate on the inside of the glass
cover, until the microclimate inside the
collector has stabilised.
The collector "breathing"
Insolation heats up the air inside the collector
and it expands. At the same time, the air
change starts via the ventilation apertures.
Fig. E.2–1
Collector "breathing"
Full insolation
e.g. at midday
As insolation wanes (in the evening or when
the sky clouds over), the air changes stop and
the air inside the collector contracts again.
This sucks colder and more moist ambient
air into the collector. This humidity in the air
settles mostly in the thermal insulation.
As soon as insolation starts again, the
moisture evaporates inside the collector
and initially precipitates as condensate on
the inside of the glass cover. This process
is perfectly normal and is in no way harmful
to the appliance. After approx. 30 minutes
(subject to weather conditions, in other
words on the amount of water inside the
collector) the collector should be dry, i.e.
the pane should be clear again. Therefore,
the insolation can be turned entirely into
heating again.
No insolation
e.g. in the evening
Initial insolation
e.g. in the morning
Humidity can enter into the collector with
the air because of the frequent air changes.
As soon as insolation hits the collector,
it will be dehumidified again.
E.2 Condensation in flat-plate collectors
Reduced or inadequate air change
Every air change means a small heat loss
from the collector. The size of the ventilation
apertures is therefore a compromise
between the speed of drying and the
appliance performance.
Vacuum tube collectors
are hermetically sealed
and cannot, therefore,
condensate. If water
droplets form on the
inside of the tubes, then
the tubes are faulty and
must be replaced.
Under certain conditions, the air change
may become more difficult – with the
consequence that the collector remains
misted-up in the morning for long periods.
t A shallow installation angle will make
convection more difficult inside the
collector and therefore also the removal of
moisture through the apertures.
t Operation with very cold temperatures,
for example when heating a swimming
pool, also reduces the convection inside
the collector.
t A very humid ambience, e.g. near open
waters or in foggy areas, can increase
the amount of moisture brought into
the collector.
t Contamination above the collector (leaves)
restrict or prevent the circulation via
these apertures.
t Incorrect storage prior to the installation
can lead to the collector containing so
much moisture prior to installation, that it
cannot achieve a normal operation.
These circumstances can – but not
necessarily – result in an increased level of
condensate being formed. Where this occurs
it is recommended to take the collector out of
use for a few days and to observe it. After a
well targeted drying operation, the problem is
frequently resolved.
Correct ventilation of the collector can only
be assured if it has been secured with
Viessmann fixing elements. The ventilation
apertures are located out of the reach of rain
in the collector frame. For that reason, the
frame must always have a clearance of at
least 8 mm towards the installation surface.
In addition to the technical information relevant for planning, the appendix includes
information regarding other subjects that are also relevant in conjunction with solar
thermal systems.
When appraising the investment in a solar
thermal system it becomes clear that the
payback period lies well within the range that
is to be expected for residential buildings.
Solar thermal systems have now established
themselves in the field of energy provision
because of their official integration into the
Energy Savings Order [Germany].
The aspects for invitations to tender indicated
here are based on long-term experience with
the engineering, installation and operation of
large solar thermal systems. Consequently,
Use the keyword index of essential terms to
make this manual a useful professional guide
for everyday use.
they offer great benefit to users, particularly
those new to such systems.
178 Viability considerations
182 Information regarding large system tenders
184 Information regarding the Energy Savings Order (EnEV)
186 Keyword index
Appendix – Viability considerations
Viability considerations
When planning a solar thermal system, it is frequently just as important to look at its
viability as its technical aspects.
Small systems (detached house)
In almost 80 percent of all cases, solar
thermal systems are installed in detached
houses as part of the modernisation of a
heating system. For private investors the
question of the viability of a solar thermal
system therefore arises in connection with
the total modernisation costs.
As part of consultations, experts will
undoubtedly raise the subject of solar
thermal backup for the heating system. After
all, over 90 percent of the population are
positively disposed towards solar energy.
The initial consultation cannot avoid the
question of the costs associated with a
solar thermal system. After installing a few
systems, a brief look at the roof will be
enough to provide a rough estimate of the
costs of a solar thermal system. Where the
response to that initial estimate is generally
positive, it is recommended to plan for such
a system and to include it in the quotation
for modernisation.
To make planning and calculating easier,
Viessmann has assembled ready-made
packages for all common system types
and collectors.
In connection with complete system
modernisation, it is useful when putting
together an offer to cleanly separate the
costs for a solar thermal system from the
"miscellaneous costs", in other words to
highlight the actual additional costs separately.
This separate listing of the modernisation
costs makes the decision in favour of using
solar technology easier.
The "miscellaneous costs" are labour and
components that are required anyway,
i.e. even without a solar thermal system.
Nevertheless, they are frequently included in
the part of the quotation dealing with the solar
thermal system. These generally concern
three areas:
t Cold water and DHW connections at the
DHW cylinder
t Connections and control of the (re-)heating
at the DHW cylinder
t Costs for a conventional mono-mode
DHW cylinder
In this case, the specific solar costs refer only
to the additional costs for the dual-mode solar
cylinder; and that should be made very clear.
For calculating solar thermal systems in new
build, one can assume almost the same
costs as for modernisation. In new build,
the installation of the solar thermal system,
as such, is less involved, but in most cases
requires a higher coordination effort and more
frequent journeys to site.
Larger systems (apartment buildings,
commercial premises)
For larger systems, the advance planning and
the concept phase require realistic estimates
to be applied in order to make the decision
as to whether a detailed engineering of the
system is to be undertaken and whether the
invitation to tender text should be formulated.
For this, details regarding the overall volume
of the project that enable the determination of
the costs for solar heating will be required at a
very early stage.
Assessments of subsidy programmes (market
stimulation programme, Solarthermie2000)
are available for different system sizes. These
can be used to estimate costs. Accordingly,
the specific costs as well as the overall costs
decrease with increasing system size.
The factors that flow into the calculation are
comparable with the cost considerations for
alternative heat generators and are defined
as follows:
System costs
Cost estimation subject to system size for systems
with flat-plate collectors, including costs for cylinder
connection and reheating, and VAT.
System costs (€/m2)
System size (m2)
Based on the above assessments, the
proportion of the individual components
and assemblies of the overall costs can also
be demonstrated. However, it should be
noted with this, that the cost proportion for
"collector array and substructure installation"
and for "pipework" are statistical averages that
may, in individual cases, vary substantially.
Cost distribution according to components
Cylinder and heat exchanger
Capital outlay
These include all costs associated with the
solar thermal system and for all ancillary
building costs associated with creating the
system. These include, for example, costs
for a crane, but exclude costs for improving
the roof if such improvement is to be carried
out anyway and simply occurs at the same
time as when the solar thermal system is
being installed.
All subsidies are deducted from the capital
outlay as are all cost savings for components
("miscellaneous costs"). For example, if a
solar thermal system is installed as part of a
the modernisation of a heating system and
this entails a dual-mode cylinder, the cost for
the superfluous mono-mode cylinder can be
deducted (cylinder credit).
Operating costs
These maintenance costs include annual
costs for inspection, maintenance and all
essential repairs. In case of larger systems
(> 30 m2) a value of 1.5 percent of the actual
system costs has proven to be useful for
viability considerations.
Installation, collector array
and substructure
* In unfavourable circumstances, these two
positions together can amount to up to 50 percent
of the overall costs.
Determining the costs for producing
solar heating
A basis for considering the economic viability
of a system is the price for each kilowatt
hour of heat generated by solar energy.
This cost for producing solar heat can also
be described as the price of solar heat and
can be calculated with relative ease. The
calculation is based on the investment costs,
the annual running costs, the interest lost
for the capital employed and the expected
available heat yield.
At this point, the cost determination deviates
from the VDI 6002. There, 1 to 2 percent
of the capital outlay, from which subsidies
have already been deducted from the actual
system costs, are allowed for maintenance
expenditure. Since these subsidies can be
substantially different, this could falsify the
image of the actual maintenance cost of a
solar thermal system. For example, the cost
of replacing a pump depends on whether this
is subsidised as part of the installation or not.
Consumption costs
For this, only the electricity bills for controller
and pumps are included. When utilising
the correct pumps, it can be assumed that
a performance factor of at least 50 will be
achieved, in other words with 1 kWh drive
energy, 50 kWh solar energy can be yielded.
The following calculation, therefore, includes
consumption costs of 1/50th of the cost of
electricity per kWh.
Appendix – Viability considerations
Annuity factor
The annuity factor converts the capital outlay
for the entire system, giving due consideration
to the service life and the assumed interest
rate on capital, into costs per annum. This puts
the investment outlay in relation to the annual
yield. To determine the annuity factor, one can
assume a system service life of 20 years.
Sample calculation 1
System size: 170 m2 collector area
With systems costs of € 100 000 less € 20 000
subsidies, an investment of € 80 000 results. The
heat yield amounts to 81 600 kWh/p.a.
(480 kWh m2 · p.a.). Maintenance and repair are
allowed for at 1.5 percent of system costs; the
electricity price is € 0.2/kWh. An interest on capital
(1 + p)T · p
fa =
(1 +
of 5 percent is applied.
p)T – 1
fa Annuity factor
p Interest on capital as decimal value
T System service life in years
K inv
€ 80 000
€ 1 500
k verbr
€ 0.004/kWh
€ 0.08
Q sol
Source: VDI 6002 part 1
ksol =
81 600 kWh
€ 80 000 · 0.08 + € 1 500
81 600 kWh
Annuity factor
+ € 0.004/kWh
A kilowatt hour generated by solar energy costs
Annuity factor subject to interest rate and a service
€ 0.101.
life of 20 years
Interest rate
Annuity factor
Sample calculation 2
System size: 50 m2 collector area
With systems costs of € 35 000 less € 7 000
subsidies, an investment of € 28 000 results. The
heat yield – amounts to 20 000 kWh/p.a.
(400 kWh m2 · p.a.). Maintenance and repair are
allowed for at 1.5 percent of system costs; the
electricity price is € 0.2/kWh. An interest on capital
of 5 percent is applied.
Solar heat price
Apart from the four variables, the expected
solar yield per annum for the system also
affects the determination of the solar
heat price.
ksol =
Kinv · fa + kbetr
Q sol
+ k verbr
ksol Solar heat price in €/kWh
K inv Capital outlay in €
kbetr Operating costs in €/p.a.
k verbr Consumption costs in €/kWh
Annuity factor
Qsol Solar heat yield in kWh/p.a.
Source: VDI 6002 part 1
The solar heat price ksol is the price for 1
kilowatt hour in euros and applies to the entire
system service life. This calculation process
is described in detail in VDI 6002 part 1 and
can be applied including or excluding VAT. It
is however important that the rule is applied
uniformly to all positions.
K inv
= € 28 000
= € 525
k verbr
= € 0.004/kWh
= 0.08
Q sol
= 20 000 kWh
ksol =
€ 28 000 · 0.08 + € 525
20 600 kWh
+ € 0.004/kWh
A kilowatt hour generated by solar energy costs
€ 0.142.
Sample calculation 3
System size: 5
collector area
At a system price of € 4 000 less € 500 subsidy, the
investment outlay amounts to € 3 500. The heat yield
is 1 750 kWh/p.a. (350 kWh/m2 · p.a.). Maintenance
and repair are allowed for at 1.5 percent of system
costs; the electricity price is € 0.2/kWh. An interest
on capital of 5 percent is applied.
K inv
€ 3 500
€ 60
k verbr
€ 0.004/kWh
€ 0.08
Q sol
ksol =
1 750 kWh
€ 3 500 · 0.08 + € 60
1 750 kWh
combination with a solar thermal system
generally has a positive effect on the
operating characteristics of a boiler system
(reduction of burner starts), the financial
implications for maintenance and repair can
hardly be reduced by these influences.
If the assumed energy price rise falls within
reasonable boundaries, it has a comparatively
small influence on the amortisation time.
The price increase has a greater influence on
financial savings after that time. However, with
a system service life in excess of 20 years it
is very difficult to put accurate figures to that
saving, i.e. by the year 2030.
+ € 0.004/kWh
A kilowatt hour generated by solar energy costs
€ 0.198.
Solar heat price € 0.101/kWh, primary energy price
in the first year € 0.08/kWh, utilisation rate of
The price per kilowatt hour strongly depends
on the assumed interest on capital. In
example 1 it can vary between € 0.071
(excluding interest on capital, i.e. fa = 0.050)
and € 0.137 (10 percent interest on capital),
without changing any other framework
condition. For all capital goods with a long
service life, the required or expected interest
on capital therefore has a fundamental
influence on amortisation.
If the heat price is known, the consideration
of amortisation for a solar thermal system
depends essentially on the development of
costs for the fuel that is saved. The insolation
used gives rise to no costs; price rises for
electricity used as drive energy and the
maintenance costs only exert a minor effect
on this consideration. The solar heat price
enters the amortisation calculation almost
as a fixed value; in every other respect the
amortisation is calculated as for any other
investment appraisal.
When determining the costs for
conventionally supplied energy, it is important
to apply realistic utilisation rates – for example
for DHW heating in summer.
The operating costs for conventional heat
generation should not be taken into account
when calculating savings. Although the
Payback period (years)
conventional heat generation 70 percent
10% 12
18 20%
Annual price increase, primary energy supply
As no one can give an accurate forecast
regarding the energy price increase over the
coming years, it has proven successful to
work with the details supplied by the investor
when determining the amortisation time. This
makes the consideration more plausible, as
the system is not financially "improved" by the
supplier. The investor can then fully account
for each figure, whether 7, 10 or 15 percent are
applied. Using "their" figures the amortisation
time will then always be within a range for
normal structural measures.
Appendix – Information regarding large system tenders
Information regarding large system tenders
The same rules apply to the invitation to tender for larger systems as for any other
construction measures around building services. However, there are some solarspecific aspects which are covered in the following.
Construction time line
Duct/riser planning
The construction time line is part of
the contract and should therefore be
fundamentally sketched into the invitation
to tender – this makes calculating easier for
potential suppliers.
Where the solar circuit line is to be installed
inside the building, allow for this when
planning a duct or riser. For this, not only
consider longitudinal expansion, but also that
for the solar circuit lines, the same thickness
of insulation and clearances to cold water
lines must be maintained as for heating
circuit lines.
Particularly in new build, the installation of
the buffer cylinder is generally a task that
must be carried out early on – under certain
circumstances, it must be carried out before
walls are drawn up or before doors are fitted.
Frequently, the buffer cylinder is the largest
object inside the heating room and must
therefore be in place in good time.
One of the last measures to be implemented
is the collector installation and the system
commissioning that should follow as soon as
possible afterwards.
Provision of a crane
Most collector installations in larger projects
require the use of a crane. As early as
during the tender preparation it should be
established who is to provide the crane or
whether a site crane may still be available at
the time the collectors are installed. In any
case, the construction contract should contain
appropriate provisions.
It is always more appropriate to order a
crane for the collector installation separately,
than to install collectors prematurely and
leave them subject to high thermal loads for
several weeks.
Interface DHW/heating/electrical
The interface must be clearly defined in the
specification if the DHW and the solar thermal
system installations are implemented and
ordered separately. If the water installation
work should create the cold water and DHW
connections at the cylinder or cylinders, the
specification must make this clear and should
also regulate the warranty question. If, for
instance, the female connection at the DHW
connection of the solar cylinder drips, the
specification should already determine who
would be responsible in such a case.
The same applies where the installation of
reheating facilities for cylinders is not to be
implemented by the same firm that installs
the solar thermal system. In such cases
the questions regarding control and control
equipment must also be clarified.
Specification of the thermal insulation
If insulation work is offered for separate
tender, ensure that the thermal insulation of
the solar circuit is adequate for the specific
requirements (temperature, UV radiation,
damage created by small animals).
Central controller and building
management systems (BMS)
The control of the solar thermal system by
a central control unit is, in our experience,
one of the most critical points for the
essential coordination of different equipment.
When assigning tasks in the respective
specifications always work on the basis
that the system installer is unfamiliar with
BMS and that those who will program the
control unit will not be familiar with solar
thermal systems.
Although most commercially available
programmable controllers will include
modules for solar thermal systems, these
will probably need an individual adaptation
anyway. It must therefore be clarified
t who will determine and describe
the principle functions of the solar
thermal system;
t who will document these functions and
maintain this documentation;
t who will create the list of parameters (start
and stop points, pump speed, etc.) and,
most importantly, who will maintain the
system after the initial commissioning;
t that a control engineer is present
during commissioning;
t that the installer of the solar thermal
system determines the kind and extent of
fault messages and that this determination
is documented;
t that the installer of the solar thermal
system is immediately notified in case of
fault messages or that these messages
are directly routed to the installer.
In addition it must be ensured that, when
the central control unit is commissioned and
optimised the solar thermal system is not
switched off by "mistake". It is not uncommon,
particularly in summer, that for work of this
kind on the BMS the entire system is shut
down. Here it is rarely considered that steam
will be generated in the collector system in
just a few minutes. In this case, it must be
ensured that the system installer is on site to
take whatever measures may be required. It
should also be determined how such visits
are to be remunerated. A responsible design
engineer ensures that the parties involved
discuss and clarify these points as soon
as possible.
It may be appropriate to use a separate
solar controller for governing the essential
functions of the solar thermal system, even
if a higher control unit is installed. However,
this arrangement must provide an option for
passing fault messages to the higher control
technology. Additional temperature sensors
or heat meters may also be installed if their
values are to be visualised and documented.
With such solutions, the BMS cannot affect
the functions of the solar thermal system
which is, under normal circumstances,
not required in any case. In these cases,
the solar control unit operates just like a
combustion controller.
Safety on site
The supplier of the solar thermal system must
be able to recognise from the invitation to
tender what safety facilities will already be
on site when the collectors are to be installed
(barriers, scaffolding) or which facilities
they must provide themselves – these
must meet all relevant specifications from
appropriate bodies.
If tying points are provided on flat roofs to
protect against falls (anchorage points), the
construction timeline must ensure that these
will be effective. Missing or inadequate
security facilities will delay work on the roof.
Here too, the order should also clarify who
would be responsible for these costs.
If the site falls under the Construction Site
Order [check local regulations] ensure that
the Health & Safety coordinator is provided
with adequate information about the typical
sequence of a collector installation. In the
past, minuting the conversation between
the coordinator and the system installer has
proven useful.
Collector covers
Just in case the unforeseen happens, the
invitation to tender should always include a
provision for covering the collectors.
Appendix – Information regarding the Energy Savings Order (EnEV) [Germany]
Information regarding the Energy Savings Order
(EnEV) [Germany]
With the Energy Savings Order (EnEV), the legislature has recognised for the first time
that solar thermal systems verifiably make a calculable contribution to energy saving
in buildings.
Assessment of solar thermal systems in
the Energy Saving Order
The first Energy Savings Order (EnEV 2002)
came into force on the 1 February 2002.
It combined the provisions of the Thermal
Insulation and Heating System Orders
that had existed in parallel until then.
Fundamentally new, was the primary energy
approach, i.e. the entire energy chain from
the fuel extraction to the supply of available
heat was taken into account in the energy
provision of a building. System technology
therefore received a much greater relevance
in observing the energy savings requirements
in buildings.
Whilst the respectively applicable EnEV
determines the framework conditions for
the primary energy demand, the actual
calculation regulations for the heating demand
and the efficiency of the system technology
are specified in the associated standards.
DIN V 18599 (previously DIN V 4701 part 10)
provides the calculation basis for system
technology; DIN V 4108 part 6 specifies the
corresponding rule for the building physics.
The primary energetic system expenditure
of energy value ep comprises – in simplified
form – the heat source expenditure of energy
value (conversion of final energy into heat)
and the primary energy factor fp for the
type of energy used. (Conversion of primary
energy into final energy.) In addition, the
losses sustained in the heat transfer chain
(storage losses, line losses, transfer losses) as
well as the required auxiliary energy (power
to operate pumps, burners, control units)
also flow into the system expenditure of
energy value.
Annual primary energy demand for
residential buildings
Q p = (Q h + Qtw) · ep
Annual primary energy demand
Annual heating demand to
DIN V 4108 part 6
Annual heating demand for
DHW heating to DIN V 18599
System expenditure of energy value
relative to primary energy
Solar thermal systems are taken into
consideration in the system expenditure
of energy value ep via the primary energy
factor fp. Thanks to the primary energy factor
fp of solar energy of 0, collectors improve
the system expenditure of energy value ep
subject to the building and solar coverage by
up to 25 percent.
Correlation between final energy, primary
energy and primary energy factor (giving
due consideration to auxiliary energy)
Q p = fp · Qe
Annual primary energy demand
Annual final energy demand of the
individual fuel types
Primary energy factor of the individual
fuel types
Calculation procedures for taking solar
thermal systems into consideration
For the actual heat demand for DHW heating,
DIN V 18599 part 10 assumes as guideline for
detached houses Qtw = 12 kWh/(m2 · p.a.); in
apartment buildings, the guideline relative to
the heated available area assumes
Qtw = 16 kWh/(m2 · p.a.). As an alternative,
these heat demand values can be calculated
as detailed in DIN V 18599 part 8 (calculation
of hot water systems).
Solar thermal systems for DHW heating are
energetically considered by DIN V 18599
part 8 in accordance with the proportion of the
heat demand they contribute. The calculation
process therefore differentiates between
"small" and "large" solar thermal systems.
With small solar thermal systems the
assumption is made that they will be used to
store solar heat in a dual-mode DHW cylinder.
When determining the heat losses of the
cylinder, only the losses from the standby
volume should be considered.
Large solar thermal systems for DHW
heating utilise at least one DHW cylinder
and one separate solar buffer cylinder to
utilise solar heat. Here, only the heat losses
from the DHW cylinder must be taken into
account, since the buffer cylinder is designed
exclusively to store solar heat and its losses
are already taken into consideration in the
solar yield.
For solar thermal systems used to backup
central heating, DIN V 18599 part 5
(calculating heating systems) also offers a
calculation procedure – this enables a systemspecific determination of the energy yield
of the solar combi system. This represents
a significant improvement compared to the
previously applicable DIN 4701 part 10, which
only took the contribution made by the solar
thermal system to central heating backup as a
lump sum.
Appendix – Keyword index
Keyword index
102 ff
38 ff
163 f
140 ff
154 ff
96 f
106 ff
107 ff
182 ff
52 f
Balancing valves, balancing
System handover, partial
Billing of solar heat
Absorber area
Absorber mats
Shutting off the air vent valve
Shutting off the collector array
Shutting off the primary circuit
Air mass
System operating pressure
System documentation
System simulation
System monitoring
Annuity, annuity factor
Aperture area
Safety on site
Rooftop installation
Drip container
Up-current pipe
Expansion volume
Utilisation level
Design output
Design consumption
Orientation of the receiver surface
Collector orientation
Invitation to tender
External lightning protection
167 f
113 ff
163 f
52 ff
Filling the system
Charge circuit
Operating pressure
Operating costs
Biomass boiler
Dual-mode cylinders
Blue Angel
Lightning protection
DHW mixer
Gross collector area
Bypass pump, sensor, circuit
CE designation
Coefficient of performance
89 f
89 f
93 ff
99 ff
Insulating pipework
Steam output
Steam range
Steam hammer
Steam volume
Seals/gaskets in the solar circuit
Sizing system components
Sizing the solar circuit line
Sizing systems for solar central
heating backup
Drainback system
163 f
74 ff
Speed control
Pressure-maintaining device
Pressure test
Pressure inside the solar
thermal system
Pressure drop, external heat exchanger
Pressure drop calculation
Flow meter
Pressure drop
101 f
20 ff
184 f
58 f
184 f
115 f
167 f
84 f
158 f
156 f
140 ff
94 f
Inherent safety
Angle of incidence, insolation
Single array systems
Single circuit systems
Instructing the user
Energy Savings Order
Energy content of cylinders
EnEV [Germany]
Discharge circuit
Venting the system
Venting, air vent valve
Initial inspection
Yield assessment
Checking yield
Measuring the yield
Yield, optimum
Expansion volume
External heat exchanger
47 f
127 ff
31 f
162 ff
155 f
Installation on a wall
Incorrect circulation at the cylinder
Incorrect calculation in the
solar circuit
Solid fuel boiler
Finned absorber
Flat roof installation
Flat roof installation, horizontal
Flat-plate collectors
Open-air swimming pool
Frost protection
Checking the antifreeze level
Filling pressure
Function check
126 f
Commercial use
Glass absorber
Global radiation
Glycol as antifreeze
85 f
Glycol in the heat transfer medium
Checking the glycol
Quality seal
130 f
119 ff
115 f
Indoor swimming pool
Harp-shaped absorber
Heat pipe
Central heating backup
Heating water buffer cylinder
Heating water buffer cylinder, sizing
High flow operation
High efficiency pumps
High temperature applications
162 ff
46 f
Roof integration
Internal lightning protection
Installed output
Internal heat exchanger
Interval function
Annual efficiency (gross)
132 f
79 f
132 f
44 ff
36 ff
100 ff
100 ff
73 ff
25 f
172 ff
Capital outlay
Pump curve
Collector curve
Air conditioning
Collector cover
Collector fixing
Collector yield
Collector array
Hydraulics, collector array
Collector circuit
Collector output
Collector inspection
Clearance between collector rows
Collector efficiency
Combi cylinder
Combi cylinder, sizing
Convection losses
Corrosion protection
Corrosion protection inside the cylinder
Corrosion protection in the heat
transfer medium
Appendix – Keyword index
179 ff
90 ff
90 ff
Corrosion-resistant fixings
Cooling function
Heat sink
Cooling line
Cumulative expenditure of energy
Plastic absorber
84 f
Longitudinal expansion
Low flow operation
Air separator
Air pot
39 f
90 ff
102 ff
90 ff
Meander absorber
Matched-flow operation
Maximum output
Maximum temperature limiter
Multi-array systems
Diaphragm expansion vessel
Mono-mode cylinder
Reheating, control
Reheating, suppression
Inclination of the receiver surface
Optical efficiency
114 f
72 ff
85 f
121 f
110 ff
78 f
Parabolic trough power stations
Parabolic mirror
Collectors connected in parallel
Plate-type heat exchanger
Plate-type heat exchanger, sizing
Primary circuit
Propylene glycol
Process heat with high temperature
Process heat with low temperature
Test pressure
Test seal
Buffer heating
Buffer cylinder
Buffer cylinder sizing
Pump sizing
Pump rate
145 ff
Control unit
Water tightness (rain)
83 f
121 f
Row distance
Collectors connected in series
Residual cooling capacity
Tubular heat exchanger
Pipework fixings
Thermal insulation, pipework
Pipework sizing
Pipe pressure drop
Pipe connections
Return cooling function
Return temperature raising
Check valve
65 ff
45 f
127 ff
131 f
38 f
140 ff
79 f
78 f
146 ff
79 f
62 ff
62 ff
Snow load
Installation on pitched roofs
Black-chrome absorber
Gravity brake
Gravity principle
Swimming pool
Swimming pool absorber
Swimming pool heat exchanger
Selective coating
High limit safety cut-out
Safety valve
Solar Keymark
Solar coverage
Solar constant
Solar circuit pump
Solar controller
Solar station
Cylinder for external heating
Cylinder with internal heat
Cylinder heating
Cylinder material
Storage medium
Storage medium, heating water
Storage medium, potable water
Cylinder priority
Spectral distribution
Flushing the system
Stagnation, prevention
Static pressure
High limit safety cut-out
Idle temperature
Available radiation
65 ff
78 ff
44 ff
163 f
20 ff
Radiation sensor
Radiation level of the sun
128 ff
Flow velocity
Set temperature
Swimming pool
102 ff
25 f
68 f
112 f
107 ff
Sensor well
Partial handover
Partial arrays
Temperature differential
Temperature differential control
Temperature sensor
Temperature stratification
Temperature limiter
Pasteurisation, hydraulic
Pasteurisation, controlled
Thermal oil
Thermosiphon systems
Thermostat function
Thermostatic mixer
DHW cylinder, sizing
DHW heating
Installation on top of a roof
Surge protector
Unglazed collectors
37 f
179 f
73 f
Vacuum tube collectors
Consumption costs
Flow rate in the collector circuit
108 f
Pre-charge pressure, calculating
Pre-charge pressure, testing
Pre-cooling vessel
180 f
70 ff
85 f
25 f
162 ff
178 ff
Heat costs
Heat price
Heat pump
Heat pump cylinder
Heat exchanger
Heat transfer medium
Thermal loss coefficients
Heat losses of the cylinder
Wavelength of radiation
Wind load
DHW circulation, connection
Two-circuit system
Two-way valve
The company
The Viessmann Group
For three generations, the Viessmann family business has been committed to
generating heat conveniently, economically, with environmental responsibility
and in accordance with demand.
With a number of outstanding product
developments and problem-solving solutions,
Viessmann has created milestones which
have frequently made them the trailblazer
and trendsetter for their entire industry.
Viessmann's orientation is decidedly
international – it maintains 13 factories in
Germany, Austria, France, Canada, Poland,
Hungary and China, sales organisations in
Germany and 35 other countries, plus
120 sales offices around the world.
Skilful workforce
Initial and ongoing training is becoming ever
more important. As far back as the 1960s,
we set ourselves the task of offering a tailormade programme of further training to our
competent contractors.
Today Viessmann maintains a modern
information centre at its company head office
in Allendorf (Eder), that is second to none.
Every year at the Viessmann academy, more
than 70 000 contractors bring their knowledge
right up-to-date.
The energy centre of the future
Viessmann has built an energy centre in
line with a homogenous climate protection
concept. This centre is equipped exclusively
with environmentally responsible technology.
This includes the generation of energy, its
use and environmentally friendly production
in the Allendorf factory (Eder). As a result, the
amount of fossil fuels consumed has been cut
by 40 percent compared to previous levels
and CO2 emissions have been reduced by
a third.
Viessmann is committed to meeting its
environmental and social responsibilities.
Viessmann employees form a team acting
on a global footing. This team is defined by
the loyalty, reliability and the responsible
actions of each individual. We ensure all our
processes are environmentally compatible
and encourage the use of renewable forms
of energy.
Furthermore we take an interest in
economics, art and culture and have for many
years engaged in successful international
sport sponsorship.
The comprehensive product range from Viessmann
Oil boilers
Gas boilers
13 to 20 000 kW
4 to 20 000 kW
The comprehensive Viessmann product range
The comprehensive range from Viessmann offers you futureproof systems
for oil, gas, solar, wood and natural heat.
When it comes to saving energy and making
a decision that is secure for the future,
Viessmann will show you the way.
Compared with many specialist suppliers,
your Viessmann engineer can give unbiased
advice about all heat sources and make a
clear recommendation.
Our comprehensive product range sets
new standards
The comprehensive product range from
Viessmann includes advanced heating
systems for all fuel types and for every
output range from 1.5 to 20 000 kW.
The range is divided into the categories 100,
200 and 300, both technically and in terms of
price and so is able to offer a suitable solution
for any requirement, with everything supplied
from a single source, with perfectly matching
system components.
Solar heating
and photovoltaic
Wood boilers/
energy from wood
Heat pumps
1.5 to 1500 kW
Air conditioning System
4 to 13 000 kW
The comprehensive Viessmann
product range in three categories.
The right solution for every
Oil boilers
Viessmann offers a comprehensive range for oil in three categories, including
highly efficient low temperature and condensing boilers from 13 to 20 000 kW
in cast iron and steel, as both freestanding and wall mounted versions.
Gas boilers
The Viessmann gas boiler range in three categories includes freestanding
and wall mounted boilers as low temperature and condensing versions from
4 to 20 000 kW.
Solar heating and photovoltaic
Viessmann is one of the leading European manufacturers of solar thermal systems.
Innovative flat-plate and tube collectors for DHW heating and central heating backup
are available, as are high performance photovoltaic panels for generating power.
Wood burning boilers
Viessmann offers complete solutions for wood – from pellet boilers for supplying
heat to detached houses as well as to complex systems for the generation of
power and heat from biomass, for example for residential complexes, commercial
operations or utility companies (output: 4 to 13 000 kW).
Heat pumps
Utilising naturally occurring heat. The comprehensive heat pump range from
Viessmann extends from compact units for passive houses to cascaded solutions
with several hundred kilowatt output. Brine, water and air can serve as heat sources.
requirement and every budget.
Technical guide
Solar thermal systems
Viessmann Werke, Allendorf (Eder)
Editor & Design
solarcontact, Hanover
Overall production
Grafisches Centrum Cuno, Calbe (Saale)
© 2008 Viessmann Werke
Unless otherwise stated, all graphics and
photographs are by Viessmann.
P 14
P 16
Photocase.de/Andreas Mang
Fotolia.com/Sandra Cunningham
target GmbH/ISFH (edited)
target GmbH (edited)
target GmbH (edited)
target GmbH (edited)
target GmbH (edited)
target GmbH (edited)
DWD (edited)
DWD (edited)
Tyforop Chemie GmbH
Getty Images
Viessmann Werke
35107 Allendorf (Eder)
Telefon +49 6452 70-0
Telefax +49 6452 70-2780
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF