wet central heating systems

wet central heating systems
Heating & hot water
fact sheet 1
A guide to
SevernWye energy advice
Types of heating
There are two main ways that homes may be heated:
1) A central heating system where the heat is produced
centrally and then distributed around each room (usually
using radiators).
2) Heat is produced individually by heaters or fires in
each room.
This leaflet is aimed at those with central heating. If you
have individual heaters or fires in each room, please see
our leaflet on ‘Heaters and fires’.
What is a wet central heating system?
With a wet central heating system, a central heat source
(usually a boiler) produces hot water which is distributed
around the property, heating radiators and becoming
available for use from hot taps and showers.
This is generally considered the most cost effective and
efficient way to provide heat and hot water for a property
and is the most common form of heating in UK homes
today with over 90%*1 having a central heating system.
*For details of the references
used in this document please
scan the QR image, visit
or call us on 0800 500 30 76
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In most wet systems, a boiler heats the water that feeds the system.
The boiler will most commonly use gas as a fuel to do this, but it is also
common to see oil or LPG (Liquid Petroleum Gas) used as a fuel where
mains gas is not available.
Types of boilers
There are two main types of boilers:
• Standard
• Combination (or ‘combi’)
Combination boilers, commonly known as ‘combi’ boilers for short, are the
most popular type in the UK. They provide hot water on demand without
the need for a header tank or cylinder, therefore saving space. Standard
boilers have a cylinder where the hot water is stored ready for use.
In most cases, heating water instantly is more energy-efficient than
‘stored’ hot water systems. It also ensures that you don’t run out of hot
water and have to wait for it to be reheated. However, the flow of hot
water may slow if lots of hot water taps are used at the same time.
Combination Boiler System
Conventional Stored Water System
Wet central heating systems
Boiler efficiency
Most boilers of both types purchased today will be
condensing boilers. These are more energy efficient
because they capture some of the heat
which would normally escape from the
SEDBUK Ratings
flue and re-use it.
Like many appliances today, boilers
are rated for efficiency from A to G
according to their efficiency:
A new A-rated gas, oil or
LPG boiler will be more
than 90% efficient*2. This
means that they will use
at least 90% of the energy
stored in the fuel to heat
the water in the system.
90% - 91.3%
86% - 90%
82% - 86%
78% - 82%
74% - 78%
70% - 74%
Below 70%
Getting your boiler
serviced each year will
help to maintain its
efficiency. This will also
ensure that it is operating
safely, avoiding the
creation of dangerous
gases such as carbon
When you come to get
your boiler serviced,
ensure you use a Gas Safe
engineer if you have a gas
or LPG boiler or an OFTEC
registered engineer for an
oil boiler.
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However, even a well-maintained boiler
will become less efficient over the years
and if your boiler is over 15 years old or
is running at a very low efficiency (ask
your heating engineer when they service
your boiler) you may wish to consider
replacing it with a condensing boiler.
You can find the efficiency of most boilers
at www.homeheatingguide.co.uk/
Upgrading from a G-rated boiler to a
modern A-rated boiler can save as much
as £340 per year on your fuel bills*3.
Wet central heating systems
Controlling your
wet central heating
With the average UK
household spending
more than 50% of
their annual fuel bill
on heating and hot
water*4, large savings
can be made by
making best use of
the heating controls
you have available.
Effective use of
heating controls can
also increase levels of comfort, helping to
ensure that people stay warm and well.
Common heating controls
Most ‘wet’ central heating systems will
have one or more of the following
• A boiler thermostat
• A programmer or timer
• A room thermostat
• Thermostatic Radiator Valves
• A hot water cylinder thermostat
(standard boilers with hot water
storage only)
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Boiler thermostat
Most boilers have a dial, marked with either
numbers or a scale from Min to Max. This controls
the temperature at which the water leaves the
boiler. If it is not set high enough, your system
may struggle to heat rooms to the desired
temperature. However, if there are young children
or vulnerable people in the household, ensure
that radiators do not get so hot that they may
cause a danger if touched.
In most boilers, the boiler thermostat controls the
temperature of water sent to both the hot water
cylinder and the radiators. With ‘combi’ boilers,
the temperature for the heating and hot water
can often be set separately.
Wet central heating systems
Programmer or timer
Programmers or timers let you set when you want the
heating to be on. It is more energy efficient to only have the
heating on when the house is occupied. It is a myth that it
uses less energy to leave the heating on constantly (even if
reduced to a lower temperature when the building is not
However, remember that it will take some time for the house
to heat up and that it will retain the heat for some time after
the heating is switched off. Therefore, factor in these warm
up and cool down times when setting your timer.
Remember, you can also turn your heating off when you go
away on holiday. If it is very cold, leave on a low temperature
just to ensure that the pipes don’t freeze (unless your system
has automatic frost protection).
If you have a standard boiler, it is likely that your programmer
will control the hot water as well. You can sometimes set the
hot water times separately. It is best to set the hot water to
only be on for as long as required to produce the amount of
hot water you need. Experiment with reducing the amount
of time the hot water is on each day to find the minimum
amount of time it can be on whilst still producing enough
hot water for your needs.
Most programmers also have some form of over-ride
allowing you to turn the heating (and hot water if
applicable) on or off manually as required. This can be useful
when your occupancy patterns change from the norm.
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Types of programmer
There are two main
types of programmer:
Mechanical programmers
usually have a set of
sliders (called ‘tappets’)
which you move around
a clock face. You use
these to set the times of
day you’d like the boiler
to switch on and off.
Digital programmers
display information
on a screen. You can
often set different time
patterns for different
days of the week.
Most programmers allow you to set the heating (and
hot water) to turn on and off several times during the
day. They vary in the degree of flexibility they offer.
Some only allow a single setting which applies to every
day of the week. Others allow you to set different times
for weekdays and weekends. The most advanced allow
you to set different times for every day of the week. If you
have irregular occupancy patterns during the week, it
may be worth considering upgrading to a programmer
that offers a greater flexibility of control.
Wet central heating systems
Room thermostat
A room thermostat allows you to set the desired temperature using
either a dial or a digital display. The thermostat senses the temperature
of the air around it and sends a signal to the boiler to switch off once the
desired temperature is reached.
It is worth bearing in mind that you set the temperature of the area
where the thermostat is located. If this is not your main living area,
you may need to experiment with the setting to get the temperature in
your main living area just right.
In terms of saving energy, it is best to have the temperature as low as
possible whilst keeping the temperature in the house comfortable.
Each 1°C reduction results in an approximately 8% saving in heating
consumption*6. Be aware that this should never be done at the expense
of health. Public Health England recommends that main living areas are
heated to at least 18°C. However, the very young, elderly and vulnerable
may require a higher temperature.
It is important to remember that turning the thermostat up higher won’t
make the building heat up any quicker. It will take the same amount of
time to heat up
but will then carry
on getting hotter,
making rooms too
hot and wasting
valuable energy.
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Thermostatic Radiator Valves (TRVs)
TRVs are particularly useful because they
can be used to control the temperature
in each individual room (where a TRV
is present). The higher the number
selected, the hotter the radiator will get.
TRVs measure the air temperature and so
should not be covered up or enclosed.
Being boxed in behind a cupboard or
covered by a curtain will restrict the TRVs
ability to control the radiator.
TRVs can be set fairly high in rooms that are frequently occupied
but can be set much lower in rooms that aren’t used for any
significant period of time (e.g. toilets, utility rooms, spare rooms
etc.). However, be careful that rooms don’t become so cold that
condensation begins to form.
Just as with room thermostats, setting them higher won’t make
a room heat up any quicker. It just means that the room will
eventually become too hot, wasting precious energy.
Hot water cylinder thermostat
If you have a standard boiler, you will also have a
hot water cylinder that stores hot water for hot
taps and showers. A thermostat is usually fitted
between 1/4 and 1/2 the way up the cylinder and is
marked with a temperature scale. This should be
set to 60°C which is hot enough to kill any harmful
bacteria without using excessive energy. If the
water produced is too hot, you could consider
installing a thermostatic mixing valve which will
automatically ensure that hot water comes out of
taps and showers at a safe temperature.
Wet central heating systems
Boiler thermostat
• Controls the temperature at which the
water leaves the boiler
• This can be turned down if radiators are
getting too hot and posing a danger to
children or vulnerable householders
• Allows you to set the times the
heating (and possibly hot water) is on
• Only have the heating on when the
house is occupied bearing in mind
warm up and cool down times
Room thermostat
• Controls the temperature
• Set the temperature as low as possible
whilst keeping the temperature in the
main living areas comfortable
Thermostatic Radiator Valve (TRV)
• Allows you to control the temperature
in different rooms
• Set these fairly high in the main living
areas and lower in bedrooms and
rooms that are not used very often
Hot water cylinder thermostat
• Controls the temperature hot water is
stored at
• Set to 60°C. This is hot enough to kill
off harmful bacteria whilst not using
excessive energy
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What else can I do to
improve my central heating
Insulate your hot water cylinder.
Most modern cylinders are factory insulated which
helps to keep the water at a higher temperature.
However, some older cylinders are not well insulated
and you can buy a hot water tank jacket to improve
the insulation for around £15 from most DIY stores.
This could cut heat loss from the cylinder by more than
75% and could save you around £25-£35 a year, which is
more than the cost of the jacket*7.
Bleed radiators regularly.
Air can become trapped in the heating system,
preventing radiators from working effectively.
This can easily be remedied by bleeding the radiators
to allow the air to escape. For detailed instructions see
Wet central heating systems
Energy saving tips
• Only have the heating on when someone is home.
• Work out your warm up and cool down times to help you set your
timer effectively.
• Have the hot water on for just long enough to produce the amount of
hot water you need each day.
• Turn your heating off when you go on holiday. If it is very cold, leave on
a low temperature just to ensure that the pipes don’t freeze.
• Check that the clock is set correctly and remember to adjust it when
the clocks change (if it doesn’t do this automatically).
• Try reducing the temperature your thermostat is set to. Each 1°C
reduction results in an approximately 8% saving in heating
consumption*8. However, be aware that this should never be done
at the expense of health, particularly where there are young children
or elderly people present.
• If you are cold, try putting on a jumper before resorting to turning the
heating up (bearing in mind the advice above).
• Avoid opening external windows and doors whilst the heating is on as
this lets valuable heat escape (however, some ventilation is required in
order to avoid the build-up of condensation).
• Make sure that heaters are not covered and have plenty of space
around them to allow the heat to circulate effectively.
• Avoid covering heaters or radiators with curtains. Either tuck them
behind or you can purchase ‘radiator shelves’ which the curtains can ‘sit
on’, encouraging the heat to circulate more effectively.
• Fit reflective foil behind your radiators to reflect heat back into the room.
• Set Thermostatic Radiator Valves to a lower setting in rooms which are
not used very often.
• Set your hot water cylinder to 60°C which is hot enough to kill off any
bacteria whilst not wasting valuable energy.
• Insulate your home – It will warm up faster and retain the heat
for longer, saving energy and money. See our separate leaflets on
insulating your home.
SevernWye energy advice
Severn Wye Energy Agency is a not-for-profit company
and charity (charity no. 1083812), established in 1999 under
the European Commission SAVE programme to promote
sustainable energy and affordable warmth
through partnership, awareness-raising,
innovation and strategic action.
While reasonable steps have been made to ensure that the
information in this leaflet is accurate and complete at the time of
writing, Severn Wye Energy Agency cannot be held liable for any
direct, indirect or consequential loss or damage that may result.
Nothing in this leaflet is intended to be, or should be interpreted as
an endorsement of, or recommendation for, any supplier, service
or product. Severn Wye does not intend to and will not provide
advice or make recommendations on financial matters.
Severn Wye Energy Agency
Unit 15 Highnam Business Centre, Highnam, Gloucester GL2 8DN
01452 835 060 | [email protected] |
@Severn_Wye | www.severnwye.org.uk
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Warm and Well has been
running for over 10 years
and has helped thousands
of people tackle their energy
bills and make their homes
more comfortable. Our friendly
advisors provide free advice on
reducing energy use and on the
grants and incentives available.
0800 500 30 76
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This fact sheet is part of a series that includes
information on heating and hot water, home
power generation, lighting and electrical
appliances and reducing heat loss.
The full series of fact sheets can be found at:
Large print copies are available on request
February 2016
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