THE CHIMNEY SWEEP
Don Crole Chimney Services
THE IMPORTANCE OF CLEAN AND SAFE CHIMNEY'S:
A fireplace or wood burning stove "can on occasion send smoke back into the room" there are many reasons for this phenominon; we will mention a few of the more common reasons below. Continual downdrafting or smoking back can mean a plugged or blocked chimney and requires an inspection by a qualified chimney sweep to ensure good draft is restored and the "dangers of cabonmonoxide contamination" is avoided; see our information on unwelcome visitors in your chimney.
Sometimes we forget why our forfathers changed to central heating systems. Wood burning appliances require regular maintenance to be safe and reliable.
COMMON REASONS FOR BACK DRAFTING OR
SMOKE IN THE ROOM:
Any obstacle that affects airflow over and around the top of the chimney can negatively
effect it's performance. This can include trees,
buildings, or adjacent roof-lines which are common obstacles to airflow.
If it appears that nearby obstacles may be having an affect on chimney draft, an
extra metre or more of chimney height may be one of the answers to help to prevent
Chimney cap design or blockage can affect the amount of draft produced by
air flowing over the chimney. Some specialized chimney caps can sometimes help
to prevent downdraft due to airflow turbulence or air flowing
down towards the top of the chimney. Always consult the manufacturer of the chimney
or a qualified chimney sweep; do not try and second guest what the problem might be.
THE STACK EFFECT IN HOUSES;
Just as draft is created in chimneys by the buoyancy of the hot
gases, a form of draft is created in houses because of the
difference in temperature between the air inside the house and the
outside air. The warm air inside the house tends to rise because
it is less dense and lighter than the outside air.
The tendency of the warm house air to rise results in pressure
differences at various levels in the house. The pressure in the
basement and lower levels of the house will be less than the
atmospheric pressure. This effect is similar to the low pressure at
the base of a chimney. In upper levels of the house the pressure
will be higher than the atmospheric pressure as the rising air pushes
against the ceiling. At some point between the high and low
pressure zones, the pressure will he neutral. This point is called
the neutral pressure plane. These variations in pressure is called the stack effect.
The taller the house is, the greater the stack effect.The stack effect in your house can
make a considerable difference in the venting performance of a wood burning appliance.
Basement wood burning appliance installations are the most susceptible to
problems caused by the stack effect since they are located at the
lowest point in the building. Wood burning appliances installed in basements
commonly operate in this negative pressure environment and are the most susceptible to backdrafting problems.
In general, appliances and chimney openings located below the
neutral pressure plane of the house must work against the negative
pressure induced by the stack effect. Conversely, appliances and
chimney openings located above the neutral pressure plane are
assisted by the positive pressure induced by the stack effect.
FACTORS AFFECTING THE VENTING FUNCTION
The neutral pressure plane is often, but not always, located at
the vertical mid-point of the house. A house with similar leakage
rates at all levels will have a neutral pressure plane at
approximately its mid-point.
A house with a well-sealed basement
but a leakier upstairs will tend to have a neutral pressure plane
higher than the mid-point, and a house with a leaky basement
and sealed upstairs will have a lower neutral pressure plane.
The neutral pressure plane tends to move towards the leakiest level
of the house. This movement of the neutral pressure plane
explains why opening a basement window can improve chimney
flow in a basement installation. The open basement window
represents a large enough leak to bring the neutral pressure
plane down to basement level, reducing or eliminating the
negative pressure the chimney has to work against.
If you experiance occasional downdrafting opening a door or window
for a few minutes to change the neutral pressure plane will help
prevent that unpleasant smoke filled room.
The first two suggestions are simple ways
to test if you have negative air pressure before lighting your fire and the third is the solution to eliminating the problem.
So now you should understand and be able to eliminate the stack effect in your house.
Remember the warm air in a house tends to rise, producing a stack effect similar to the
draft in chimneys. The resulting negative pressure in a basement can cause
venting problems in appliances located there so always do one of the two tests before lighting your fire and you will be able to eliminate any unpleasent smoke effects in the room!!
- 1) hold your hand inside the fire box and up near the damper (make
sure the damper is open) and see if you feel cold air coming down the chimney.
- 2) Light a good size sheet of paper and hold it in that same position up near the
damper; if the fire goes out or the flames are being blown back the stack effect is
- 3) Open a window or door to balance the air pressure in the room; light another
piece of paper and hold it in the fire box the flames should draw up the chimney if the pressure has been equalized (leave the window or door open until you determine that a good
draw has been establised in the chimney)
The black dots illustrate the sources of negative air pressure, the small cloud of smoke in front of the wood stove shows the stack effect and smoke entering the room.
OTHER FACTORS THAT CAN AFFECT THE VENTING FUNCTION
A chimney installed on the outside wall of a house and serving
a basement installation is most susceptible to
backdrafting caused by the stack effect. At standby (no fire in the
appliance), the air in the flue can cool to outside temperature.
With no temperature difference, no draft is produced in the
chimney, and the minor negative pressure acting on the air inlets
of the appliance can readily cause reverse flow in the chimney.
Backdrafting at standby is clear evidence of the stack effect in
buildings. Such backdrafting may also be caused by other
exhaust ventilators creating negative pressure, but this possible
source is easily checked by carrying out a ventilation inventory. It is sometimes suggested that reverse flow in a chimney can be cured
by holding a burning piece of paper or a hair drier at the base of
the chimney to heat the flue.
But heating the flue only treats the
symptoms of the problem, not the problem itself. Remember, a
chimney that backdrafts at standby can also backdraft during a
receding fire, especially in cold weather when an exterior
chimney is being severely chilled.
Backdrafting during a receding
fire spills poisonous carbon monoxide into the house.
COMPETITION FOR AIR Energy conservation measures are making houses more and
more airtight. Sealed vapour barriers, caulking, and carefully
installed and weather-stripped doors and windows all reduce the
leakage of air into and out of houses (infiltration and exfiltration,
respectively). The shell, or envelope, of such houses resists
airflows created by exhaust ventilators.
Fuel-burning appliances are exhaust ventilators, as are kitchen
and bathroom fans, clothes dryers, and central vacuum systems.
All draw air from the house and exhaust it outside.When an
exhaust ventilator such as a kitchen fan or clothes dryer operates in a well-sealed house, it may not get
all the air it needs to function properly.
of a clothes dryer or high-volume kitchen exhaust can cause
backdrafting of a fireplace or other woodburning system. In
theory, all exhaust ventilators should be supplied with ducted
sources of outdoor air. However, the many complications
involved make the complete isolation of all ventilators very rare. Exhaust ventilators operating on natural draft cannot usually compete with mechanically-powered ventilators. The chimney serving a wood stove may backdraft
when the negative pressure produced by mechanical ventilators overcomes
OUTDOOR SUPPLY OF COMBUSTION AIR
A directly-ducted supply of outdoor combustion air can assist the woodburning appliance and
chimney with added air to help prevent the negative pressure that can be caused by the natural stack effect or the effect of mechanical ventilators.
THE USE OF DIRECT AIR SUPPLY
To assist in reliable operation some woodburning appliances installed in well-sealed
houses can be provided with a directly-ducted source of outside combustion air.
The connection of an air duct to a conventional woodburning
appliance can be difficult, depending on the design of the combustion air
Appliances certified for use in mobile homes must be
provided with duct connections to their combustion air inlets. They also tend
to have a relatively low heat output, which can be suitable for well sealed houses.
Fireplaces present a particular problem because they consume
so much room air, especially open fireplaces or when the doors are open. Even
with doors closed, most fireplaces need large amounts of room
For this reason it is best to provide a fireplace with close-fitting glass
doors and some fireplaces can have a directly ducted source of outdoor air for
Some appliances, notably cook stoves, have multiple air inlets to
which ducts cannot be connected. If you cannot attach a ducted
source of outside air to an appliance, you must compensate for
the room air it consumes.
One solution is to equalize the pressure
in the house is by bringing outside air in and distributing it through a
forced air distribution system.
It is possible, however, to bring
in only enough outdoor air to compensate for air removed by
continuously-operating ventilators such as woodstoves.
The simple natural solution may be easier than you think, visit our Pure Air Plus Division and see how a passive residential ventilation system can provide makeup air that brings in only the required outside air warms it and distributes it through your forced air system.
As an bonus it provides the added benefit of UV purification to the air intake filling your house with pure fresh air every day
out our page on this product use this link Pure Air Plus
To see why you should also
be concerned about clean air in your home use this
link Indoor air quality
When diagnosing a draft-related problem in a basement
installation, always keep in mind that negative pressure induced
by stack effect is be a contributing factor.
The following symptoms can be caused by the stack effect:
- chimney flow reversal when the system is not in use,
- lighting a fire is difficult or impossible
- weak, unreliable draft
when fire is burning
- excessive smoking when loading door is opened
- chimney flow reversal or backdrafting, especially during a receding fire
The stack effect in houses is a natural phenominon but you can reduce or
eliminate the effects that it has on the venting of a woodburning
The surest way to overcome the stack effect is to provide
a directly ducted source of outside air to the combustion air inlet
of the appliance. This isolates the combustion and venting
systems from the rest of the house. When an appliance installed
in a basement has a fitting to accept an outdoor air duct,"always" connect the duct to prevent possible backdrafting.
Indirect sources of outside air are possible but not usually
desirable. A hole in the basement wall may serve as an entry
combustion air, but the air flowing through the hole would mainly
compensate for air leaking out the upper parts of the house.
Qnly a small proportion of the incoming air would be used for
combustion in the appliance The effect of creating the opening
is similar to that of opening a basement window - it lowers the
neutral pressure plane and therefore reduces the negative
pressure in the basement.
FACTORS OR SOLUTIONS TO THE VENTING FUNCTION
Another way to overcome stack effect is to open a window in the room using the outside air
help equalize the
pressure problem and provide makeup air for the fire to use.
The effect would be to improve the quantity of available air the basement, or at
least reduce the amount of negative pressure there. This solution
is available only when a window or source of air is in the same room.
All draw air from the house and exhaust it outside. When an
exhaust ventilator operates in a well-sealed house, it may not get
all the air it needs to function properly. When two or more
exhaust ventilators operate at once, they may compete with each
other for available air. In a competition for air, mechanical
systems usually win out over natural draft systems causing the natural draft appliance to become starved for air.
"FEEL GOOD CHOOSE WOOD"
"Understanding Wood Heat"