Heartland The Artisan - Manual (Page 19)

9
Trouble Shooting
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 fireplace chimneys tend to
perform poorly. Ideally, the liner should have the same internal
area 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" (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 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 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 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