23
TROUBLESHOOTING
Chimneys and Drafts
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
the 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 chim-
neys 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 Affect 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 chim-
ney fail to function properly. Here are the main factors that
influence draft:
1) 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 en
-
closed within the house is preferable.
2) 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 fireplace chimneys tend to
perform poorly. Ideally, the liner should have the same in
-
ternal area as the flue collar of the appliance.
3) Chimney Height
Taller chimneys tend to produce stronger draft. We recom-
mend that the top of the chimney should be at least 36"
(900 mm) higher than the highest point at which it contacts
the roof and 24" (600 mm) higher than any roofline or
obstacle within a horizontal distance of ten feet (three me
-
ters). These figures produce the minimum allowable chim
-
ney height. Chimneys higher than this are often needed for
performance reasons. A chimney serving a cookstove lo
-
cated 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 chim
-
ney should be at least 16 feet (4.9 meters) higher than the
floor on which the cookstove sits.
4) Negative Pressure in the House
The draft produced by chimneys is a weak force that can
be influenced by pressures inside the house. A woodburn
-
ing 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 perfor-
mance.
5) Stack Effect in houses
In winter, the air in houses 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 lev-
els. 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 deter
-
mine 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.
Masonry chimneys should be checked for deterioration
including damaged bricks, crumbling and missing mortar,
cracks in the drip cap at the top of the chimney, and loose
flashings at the roof line. The liner should be checked for
cracks and misalignment, and its size should be 6"
(15.24 cm).
