The modern appliance for solid fuel like every other item of household
equipment work satisfactorily, when they are properly selected and correctly
connected. Then it is little probable any sort of problems to appear. There are
occasions when the problem of the smoky fire presents itself.
Smoke problems and bad draught are caused by poor design and construction of
the chimney – insufficient dimensions and an unfavorable site conditions.
The purpose of this brochure is to describe the possible causes and to explain
what can be done for their removal. It is possible for the brochure describing
mistakes and improvements to create a wrong impression for the degree of severity
of the problems described in it. In order to be comprehensive in reasonable limits,
however, it is desirable to cover the cases which rarely happen as well as the simple
The chimneys aren’t too complex but most of the people really don’t understand
the way of their function. The knowledge of the flue makes it possible to obtain a
good and safe fire and to avoid the problems connected with the chimneys.
Each kind of chimney has its advantages and restrictions. Chimneys constructed
of bricks are traditionally more beautiful, which many owners wish. Chimneys made
of metal tubes are easier to fix and usually are at a lower price.
To determine the best chimney for you requires a complete picture of the
appliances maintained by the chimney (stoves, fireplaces, boilers etc.) and the
necessary ventilation section, which requires each one of them.
If you intend to build a new chimney, first study what passage section the
appliance, which you use, needs. If you install a new appliance (or used one in
another place) to an existing chimney, do the same. Don’t accept in principle that
your chimney is suitable. Check it out.
The chimney is a vertical tube intended to draw the products of combustion from
the heating appliance out and to emit them in the atmosphere outside the limits of the
There are one or more vertical passages located inside the chimney called flues.
Each appliance connected to the chimney requires its own flue. The chimneys made
of brick can have some flues. The metal chimneys, of course have only one in its
inside pipe.
The upward draught or “pull” in a chimney is a result of the combination between
its height and the difference in temperature between the flue gases and the air
outside. It is very simple – the column of hot flue gases in the chimney has smaller
weight than an equivalent column of cold air outside, so that the pressure in the
lowest part of the warm chimney is less than the air pressure outside. This quite
small difference in pressure creates the draught. The warmer and taller is a chimney
the better the draught and the less risk of condensation.
The hollows and the narrows, the rough surfaces and the sharp bends are
resistances to the flow of the gases and reduce the draught, so that as far as
possible they must be eliminated.
Air leakage leads to cooling the flue gases and consequently reducing the
In certain circumstances it may be desirable to reduce the excessive draught by
introducing “dilution” air, but this should only be done in a controlled way, not by
allowing accidental leakage.
Wind at the top of the chimney may have positive or negative effect on draught,
depending on the surroundings and the position of the chimney top in relation to the
The bad draught may also arise with a too weak combustion process as a result
of the low temperature of the flue gases.
When the smoke escapes into the room, the most probable reasons are:
A. Insufficient draught to carry away all the smoke;
B. There is no upward draught at all;
C. The chimney terminates in high pressure zone;
D. Down-draught.
The smokiness can be a consequence of variety of causes, several of which could
operate together, therefore it is not always simple matter to diagnose the cause of the
problem, nor there is an universal means of its determination, and therefore each
case has to be treated depending on the concrete symptoms.
In order to be able to cure smokiness, first of all it must be known why it occurs,
and since there are a great number of possible reasons, not all of which are evident,
it is expedient a sequence of operations to be done in a consecutive order.
The first step is to read through this brochure, because sometimes the problem
can be recognised immediately from a description of it and removed without any
need for a lengthy investigation. If, however, the cause of the problem isn’t evident,
the next step is to check the symptoms, the most common of which are:
A part of the smoke and the fumes but not all, escape into the room without any
sign of being blown back by the wind. Smokiness is constant irrespective of the
climatic conditions, although the wind may have some effect in either worsening or
improving matters. This usually means insufficient draught.
There is no up-draught at all, irrespective of whether wind blows outside or not.
Intermittent blowing back through the chimney with the wind from a definite
direction. The degree of smokiness varies with wind strength.
The draught decreases or reverses with wind from a definite direction. The
draught returns when the wind ceases or changes its direction.
Finally, when one or some simple tests have been carried out, their results will
usually indicate the cause of the smokiness. It is not unusual for two or maybe three
different tests to give positive results – this means that there are several causes,
operating together. For example, the case with the back draught can be combined
with air starvation, or a bad construction of the chimney – with a partially blocked flue.
To cope more quickly with the possible causes of smokiness the methods of
testing and offered remedies are indicated as A.1, A.2, B.1, C1 etc. in connection
with the symptoms described above.
All appliances for solid fuel need an air flow into the room, as some of them, for
example those for open fire need more air. An additional air flow can pass through
special openings in the appliance over the flame. For a closed appliance 15-30 cu. m
per hour are necessary, whereas with a fireplace with open fire this value can reach
250 cu. m. and even more per hour.
Using solid fuel in a room insulated
very well (fig. 1) leads to two
1. If the oxygen is insufficient,
the result is incomplete
combustion. With a complete
combustion the products are
carbon dioxide and water
shortage of oxygen will lead
to formation of carbon oxide
CO, which is very poisonous
gas without smell.
2. There is not sufficient air
available to substitute the air, which
together with the highly poisonous carbon oxide.
TEST: Open a door or a window, preferably when there is no wind outside. If the
smoking into the room ceases, the cause of it is usually “air starvation”.
REMEDY: It is necessary to allow more air into the room but sometimes this can
lead to an unpleasant draughts. The first step is to see whether it is possible to
reduce the volume of the air flowing to the chimney, so that the “demand” for air is
less. An inspection of the throat over the fire may reveal a large throat without any
sign of natural draught and even larger voids at the sides - see Fig.2.
The void should be filled in and the throat – reduced, so that there is a smooth
streamlined entry into the chimney flue, as it is shown in Fig.3. If the throat opening
can be made adjustable for size, it will be better, with a minimum amount of opening
when the chimney is hot and a maximum opening for relighting when the chimney is
The heating appliance must not enter into competition with other appliances,
which throw air away from the house such as aspirations etc. Their action can lead to
a shortage of air and smokiness.
The flow of gases through the chimney flue is limited by its size and the
constructional details of that flue. When the fireplace opening is too large, this flow
increases and it needs a bigger draught, so that it is drawn up. At the same time the
gases are cooled by the entering fresh air, the draught decreases and the smoke
eddies into the room.
TEST: What can be done is the section of the chimney to be compared with the
size of the door. If the section of the open door is more than 8 times than the section
of the flue, there is a likelihood of smokiness. With a short chimney flue (for example
on the top floor flat) this ratio must be reduced to 6:1.
REMEDY: Increase the size of the chimney, where is possible, although it is
difficult and expensive.
This trouble occurs if the inside barrier between two neighbouring flues is
destroyed (there is a break in it.) and one flue isn’t used at the moment. On
insufficient ventilation in the room with the heating appliance the smoke is sucked
downwards through the unused flue. (Fig. 4).
TEST: Open a door or a window and
watch the smoky fire. Check the entirety of
the flues through an inspection or a smoke
REMEDY: Remove the break between
both flues. Improve the ventilation in the
room, where the appliance is working, so
that the pressure increases.
The movement of the air in the building must not impede the work of the chimney.
For example, when a house has a open window on the floor above (Fig. 5), the warm
air goes out through the open window and the whole house begins to work like a big
chimney. An air flow starts from below upwards
in order to substitute the air gown out through
the window, reducing the pressure of the floor
below, where the heating appliance is. If the
effect is strong enough, it will overcome the
draught of the chimney and will pull the gases
downwards back in the house. The badly
insulated roof and ceiling, the staircases
leading to them can cause the same
REMEDY: The insulation on the floors
above shall be improved and it shall be
ensured an outside access of air to the heating appliance on a level as low as
Usually flues are vertical but sometimes horizontal displacements are necessary.
Sometimes these sections are low, with
sharp curves and a big length – the
result is a poor draught and a smoky
TEST: An inspection of the configuration
of the flues is done, as, if it is necessary
it is thrust with a flexible rod.
REMEDY: The main features of a good
passage are indicated on Fig. 6.
It may be due to a bad workmanship or there is a foreign substance or soot
accumulation. A banal example is shown on Fig. 7, where the mortar fallen during the
construction and has caused a partial blockage.
TEST: The sweep’s brush
often reveals the availability of
sometimes it leads to their
removals. Another method of
checking for obstructions is to
lower a metal ball on the end of
a rope down the flue, which
shall have a clearance not more
than 25 mm next to the walls.
Through ticks on the rope,
when the ball gets stopped, will
determine the position of the
blockage, and sometimes it
may be dislodged.
REMEDY: If the blockage
cannot be removed by other
means, the flue at that point will
have to be opened for clearing.
The conventional brick chimneys, although not ideal, are generally suitable for all
types of domestic heating appliances. The small section leads to increasing
resistance and thence – to smoky fire. On the other hand the too big size leads to
difficult warming and again to reducing the draught.
TEST: Measure the size of the flue
REMEDY: Use flues with a section suitable for the respective heating appliance.
Some flues have plain ends and a butt joint,
others are overlapped. A fault often found is a
protruding ring with cement which should have
been removed during construction – Fig. 8. In
other cases the flues have no jointing between the
liners and brickwork omitted.
TEST: Scraping in the place of the joints
indicates if there is a problem. A smoke test
described at the end of the brochure may indicate
by visible smoke the extent of leakages.
REMEDY: If the trouble is due to a cement ring,
it sometimes can be removed by careful scraping.
With overlapping flues in graver cases it may be
necessary to break off the brickwork and even to
change them.
In some cases, a round-base chimney pot is fitted to a
square flue by placing small pieces of tile across the
corners to protect. The four obstructive ledges, added to
the other eventual problems, may turn out the “last
TEST: By inspection
REMEDY: A square-based pot s with parallel sides in
its lower part shall be used.
When smoke flows through a flue, the particles of soot are attracted to cold
surfaces inside of the chimney pot, mainly
on the top of the chimney, especially if there
is a chimney pot as the shown in Fig.9.
Usually the soot is removed by the sweep’s
brush, but if a fuel with high tar content is
used, only a hammer and a chisel may be
necessary to be used.
TEST: Cleaning with a sweep’s brush
reveals if there is such a trouble.
REMEDY: If the trouble is recurrent,
change the fuel for a fuel with lower tar
content. The variant shown in Fig.10, which
is intended for protecting from rain and wind,
can also replace the used square or circular
termination. The combined area of the four
openings should not exceed twice the area
of the flue.
On fig.11 -14 it is shown how some of the faults, which most often result in poor
draught can be fixed. In all the examples the pipe projects too far into the flue so that
the free flow of gases is hindered.
TEST: By inspection
REMEDY: Sharp changes in direction of the flue gases should be avoided. The
pipes should be projecting into the brick flue via bends of 135°. The brick flue should
be plastered smoothly, and the pipe – caulked with a fireproof rope.
The soot cleaning door must be double to avoid excessive cooling of the gases. It
should also be slightly below the entry of the flue pipe so that the hot gases not
impinge on it and so a space to be ensured to avoid the blocking of the pipe by the
fallen soot.
The cold air which is leaking into the flue naturally cools the gases and decreases
the draught, because of which it is very important that the chimney should be airtight.
The most usual points of air leakage are around the register plate where the pipe
projects into the brickwork; in the joints around flue pipes and the soot cleaning
doors. It is possible to leak air if the plaster of the brick flue are badly filled or have
cracked. In the described cases it is possible that potentially dangerous outward
leakage of the fumes rise.
TEST: Inspection usually is done with a lighted match or a candle. If the flame is
drawn, it is a certain evidence for the leakage. The determination of the leakage
through the brick work is done by a “smoke test”, described in the end of the
REMEDY: All faulty flue pipe joints shall be discovered and repaired. The cement
setting compound shall not be liable to crack. Soft gaskets such as heat-resisting
ropes or tapes shall be used.
If the flue is not kept properly cleaned, soot deposits in time will completely stop
the draught; the same thing can happen when pieces of the chimney pot, slate or
bricks etc. fall down the flue.
TEST: Put a sweep’s brush through and it will reveal whether there is a blockage.
REMEDY: Try first with a hard rod or a wire. It this doesn’t help, lower a metal ball
tied with a rope through the top of the chimney. Even if this turns out unsuccessful, it
will allow you to find the exact position of the stoppage for opening the flue. Regular
cleaning remains the most certain prevention.
An uninsulated chimney cools the gases more rapidly and respectively the
draught can’t be as good as when it is well insulated. Besides, when the fire has
been out, or the flue damper of the heating appliance has been closed for several
hours, the draught may be reduced sufficiently to cause smokiness.
Uninsulated metal one-layer pipes should
never be used be used as a chimney outside a
house. The severe chilling of the flue gases,
particularly when the appliance is burning
slowly, not only results in unsightly
condensation but also seriously reduces the
flue draught so much that dangerous fumes
may be emitted from the appliance, because
there is no draught to carry them away.
TEST: Insert a lighted piece of newspaper
in the flue. After a brief delay, the smoke
should be carried up the flue.
REMEDY: Insulation of the flues
Fig. 16 shows a chimney lower than a nearby object, in this case the roof and on
the windward side. This is a region of high wind-pressure. On the other side (in the
lee) underpressure turns out. That means that the air tends to be drawn out of the
house on the leeward side and to be blown in on the overpressure side. This can
affect the chimney, as a counteraction of the draught, and if the underpressure is
high enough, causing a down-draught.
As this condition occurs only with
the wind in a certain direction, the
situation of the chimney in relation
to the roof indicates the cause of
trouble. Furthermore, smokiness
occurs when there are open doors
and windows on the lee side but
not on the high-pressure side.
TEST: Open a door or a window
on the windward side. This should
equalize the pressures and restore
the draught.
REMEDY: If possible, the chimney should be extended beyond the region of high
pressure. In most cases with a height 0,5 – 1 m higher than the ridge of the roof or
other object, causing the overpressure, no trouble occurs.
Metal uninsulated pipes should not be used as permanent extensions, because they
chill the gases and may cause further troubles.
If the possible extension is insufficient, probably the only alternative is to prevent the
air being drawn out of the house towards the lee side. This can only be done by
making the doors and the windows airtight on the lee side and if possible closed.
This trouble occurs mainly with short chimneys. Suction in the room, although
normally associated with the condition described in C.1. can cause smokiness
regardless of chimney position. Fig. 17 shows a situation, which is difficult to cure.
The flow of air around the
house creates regions of
high and low-pressures,
which may cause downdraught if the disposition of
doors and windows is such
created in the room is low
TEST: Light aromatic
sticks and follow with your
eyes the movement of the
smoke; usually it drifts
towards doors and windows,
to the other side where the
pressure is reduced, thereby
indicating from where air is
being drawn out of the room.
REMEDY: Some possibilities can be tried to
restore the normal up-draught.
- fitting chimney draught-inducing cowls to
increase up-draught, as the indicated ones
on Fig. 18;
- restricting the throat over the fire (Fig.3) to
increase the temperature in the flue;
- draught-proofing doors and windows, which
are adjacent to the low-pressure area.
Very often the chimney top is exposed on air turbulence, caused by drawing the
wind upwards or to the sides by some adjacent object (Fig. 19) – a building, a tree, or
the chimney on the lee slope of a hill.
The air occurs in the vicinity of the chimney top in direction from above downwards
and blows the smoke back. This usually is related to a wind from a definite direction
and a definite speed.
TEST: Observe the position of the chimney in relation to adjacent higher objects
such as buildings and trees and in respect of surrounding land contours.
REMEDY: Try to raise the chimney
terminal out of the turbulent zone. It
this is impossible, protect it in one of
the ways indicated in Fig. 10 or Fig. 18.
If a cluster of chimney pots close
together on the same stack, it is
possible as a result of the downdraught smokiness to be caused by
one working chimney through the
adjacent unused flue in a room where
no heating appliance works. In such
case it is better the unused flues to be
temporarily capped in a way that the
flue can be opened if needed later.
Water vapour is one of the products of combustion produced when any kind of
fuel is burnt. Fuels with relatively high hydrogen content produce more water vapour
than others. As long as the water remains in its vapour state until it leaves the flue,
no problems arise, but when the flue gases are cooled below a certain level, moisture
will condense on the surfaces of the flue. Unfortunately, condensation often
combines with sulphur compounds in the flue gases and with sulphates in brick work
to form weak acids, which over a period of time, attack brickwork and mortar joints. In
some cases condensation may cause staining the walls in the rooms, particularly on
upper floors. Other causes contributory condensation are flues too large for the
heating appliance and using wet fuels. The remedy is to use a flue with an
appropriate diameter which is insulated.
 The heating appliance should not be alight.
 Arrange for access into all the spaces of the building through which the
chimney flues run, and to the roof.
 When a chimney stack is shared by two or more houses or flats, make sure
that there is no other heating appliance connected to the stack will be in use
during the test.
 Warm the flue by burning some paper at the bottom of it.
 Have available suitable means to seal off the top and the bottom of the flue
after it has been filled with smoke. For example a plastic bag and sealing tape
is useful for “capping” the chimney pot.
 Light a smoke cartridge in the bottom of the flue.
 As soon smoke issues from the chimney pot immediately seal it off, leaving the
smoke cartridge to burn out and fill the flue with smoke.
 Observe all parts of the chimney for smoke leakage.
 Observe whether smoke issues from any other flue in the same stack.
 Any smoke test should continue for at least 10 minutes.
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