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Your stove is only one part of a system that includes the
chimney, the operator, the fuel, and the home. The other
parts of the system will affect how well the stove works.
When there is a good match between all the parts, the sys-
tem works well.
Wood stove operation depends on natural (unforced) draft.
Natural draft occurs when exhaust gas is hotter (and there-
fore lighter) than the outdoor air at the top of the chimney.
The greater the temperature difference, the stronger the
draft. As the hot exhaust gas rises out of the chimney, it
generates suction that draws air into the stove for combus-
tion. A slow, lazy fire when the stove’s air inlets are fully
open indicates a weak draft. A brisk fire, supported only by
air entering the stove through the air inlets, indicates a good
draft. The inlets are passive; they regulate how much air can
enter the stove, but they don’t force air into it.
The efficiency of a modern woodburning appliance, (in which
the amount of air available for combustion is regulated),
depends on the chimney to keep exhaust gases warm all the
way outdoors. The characteristics of your chimney - whether
it is steel or masonry, interior or exterior, matched or mis-
matched to the stove outlet, - determine how quickly it will
warm up and how well it will sustain the optimum tempera-
tures necessary to maintain strong draft and efficient com
-
bustion. Here follows a description of various flue system
characteristics and the related effects on stove performance.
Masonry Chimney
Although masonry is the traditional material used for
chimney construction, it can have distinct performance
disadvantages when used to vent a controlled-combustion
woodstove. Masonry forms an effective ‘heat sink’ - that
is, it absorbs and holds heat for long periods of time. The
large mass, however, may take a long time to become hot
enough to sustain a strong draft. The larger the chimney (in
total mass), the longer it will take to warm up. Cold masonry
will actually cool exhaust gases enough to diminish draft
strength. This problem is worse if the chimney is located
outside the home or if the chimney flue has a cross-sectional
volume much larger than the stove outlet.
Steel Chimney
Most factory-made ‘Class A’ steel chimneys have a layer of
insulation around the inner flue. This insulation keeps the
smoke warm and protects the surrounding structure from the
high flue temperatures. Because the insulation is less dense
than masonry, the inner steel liner warms up more quickly
than a masonry chimney; this makes the steel chimney
support a good draft more quickly than masonry does. Steel
chimneys are not as attractive as masonry, but they are very
durable and generally outperform masonry.
Indoor/ Outdoor Location
Because the chimney’s function is to keep the smoke warm,
it is best to locate it inside the house. This location uses the
house as insulation for the flue and allows some radiant
heat release from the flue into the home. Since an interior
chimney doesn’t continuously lose its heat to the outdoors,
less heat from the stove is required to get it warm and keep
it warm.
Flue Sizing
The flue size for a controlled-combustion appliance should
be based on the cross-sectional volume of the stove flue
outlet. In this case, more is definitely not better. Hot gases
lose heat through expansion; if a stove with a six-inch flue
collar (28 square inch area) is vented into a 10” x 10” flue,
the gases will expand to over three times their original vol-
ume. As gases cool with expansion, draft strength decreas
-
es. If an oversized flue is also outside the house, the heat it
absorbs will be conducted to the outdoor air and the flue will
remain relatively cool.
It is common for a masonry flue to be oversized for the
stove. Such a chimney can take quite a while to warm up
and the stove performance will likely be disappointing. The
best solution to an oversize flue problem is the installation
of an insulated steel chimney liner of the same diameter as
the appliance flue outlet. The liner keeps the exhaust gas
warm and the result is a stronger draft. An uninsulated liner
is a second choice - although the liner will keep the exhaust
restricted to its original volume, the air around the liner will
require time and heat energy to warm up.
Check your local codes. You may be required to install a flue
liner in any oversize or masonry flue.
Pipe & Chimney Layout
Every bend in the flue will act as a brake on the exhaust as
it flows from the firebox to the chimney cap. The ideal pipe
and chimney layout is straight up from the stove through a
completely straight chimney. Use this layout if at all possible
as it will promote optimum stove performance and simplify
maintenance.
If the stovepipe must elbow to enter a chimney, locate the
elbow about midway between the stove top and the chimney
thimble. This configuration lets the smoke speed up before
it must turn, keeps some pipe in the room for heat transfer,
and allows long-term flexibility for installing a different appli
-
ance without relocating the thimble.
There should be no more than eight feet of single-wall stove
pipe between the stove and a chimney. Longer runs can
cool the smoke enough to cause draft and creosote prob-
lems. Use double-wall stove pipe for longer runs.
Draft Management