Chimneys and Draft
The performance of your woodburning system depends
more on the chimney than on any other single component.
The chimney ‘drives’ the system by producing the draft
that draws in combustion air and exhausts smoke and
gases to outdoors. Give as much attention to the chimney
as you do to the appliance that it serves.
How Chimneys Work
It is well known that hot air rises. This principle is at work
inside chimneys and is the key to understanding how
chimneys function.
The hot exhaust gases from the appliance are lighter than
the outside air. This buoyancy causes the gases to rise in
the chimney. As they rise, a slight negative pressure is
created inside the appliance. Air rushes into the appliance
through any available openings to balance this negative
pressure.
The force caused by the rising gases is called draft. Draft
is created by the difference in temperature between the
gases in the chimney and the outside air. Greater
temperature differences produce stronger draft.
Factors That Effect Draft
There are several factors that interfere with draft and most
woodburning systems have one or more of these features.
It is usually a combination of conditions that make a
chimney fail to function properly.
Here are the main factors that influence draft:
Cold Chimney Liner
An uninsulated chimney that runs up the outside of a
house and is exposed on three sides is chilled by outside
cold. This means that the flue gases give up their heat
rapidly to the liner. As they cool, they lose their buoyancy
and draft is reduced. Insulation between the liner and the
chimney shell can help to reduce the heat loss, but a
chimney that is enclosed within the house is preferable.
Large Liner
Chimney liners that are much larger than the flue collar of
the appliance allow flue gases to move too slowly. This
slow movement gives the gases more time to cool and
lose their buoyancy. Oversized liners are the reason that
many fireplace inserts vented through the fireplace
chimneys tend to perform poorly. Ideally the liner should
have the same internal areas as the flue collar of the
appliance.
Chimney Height
Taller chimneys tend to produce stronger draft. We
recommend that the top of the chimney should be at least
36” (900mm) higher than the highest point at which it
contacts the roof and 24” (600mm) higher than any roof-
line or obstacle within a horizontal distance of 10 feet
(three metres). These figures produce the minimum
allowable chimney height. Chimneys higher than this are
often needed for performance reasons. A chimney serving
a cookstove located on the main floor of a single-storey
house or cottage may not be tall enough to perform well,
even though the minimum heights in the building code
have been followed. A good rule of thumb to use states
that the top of the chimney should be at least 16 feet (4.9
metres) higher than the floor on which the cookstove sits.
Negative Pressure in the House
The draft produced by chimneys is a weak force that can
be influenced by pressures inside the house. A
woodburning cookstove acts as an exhaust ventilator by
removing air for combustion from the house. A typical
house may have several other exhausts, clothes dryer,
gas or oil furnace, fireplace or central vacuum system.
When one or more of these other exhaust ventilators is
running, it may compete for the same air that the
woodburning appliance needs for combustion. This
competition for air supply can make a fire slow to kindle or
cause a stove to smoke when its door is opened.
Chimneys are often blamed for this type of performance.
Stack Effect in houses
In winter, the air intake is much warmer and, therefore
more buoyant than the outside air. The warm air in the
house tends to rise, creating slightly negative pressure in
the basement and slightly positive pressure at higher
levels. This negative pressure in the basement can
compete with chimney draft to a stove or furnace located
there.
Checking an Existing Chimney
Before an existing chimney is used to vent your new
cookstove, a thorough inspection should be done to
determine its suitability. The inspection should be
performed by an experienced professional because of the
many factors that must be considered. A reputable
chimney sweep or retailer can give you good advice on
the suitability of an existing chimney.
Trouble Shooting
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