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Vermont Castings Madison
Draft Management
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 stove works well.
Wood stove operation depends on natural
(unforced) draft. Natural draft occurs when exhaust gas
is hotter (and therefore 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 combustion. A slow, lazy
fire with the stove’s air inlets fully open indicates a
weak draft. A brisk fire, supported only by air entering
the stove through the normal inlets, indicates a good
draft. The inlets are passive; they regulate how much
air can enter the stove, but they do not move air into it.
The efficiency of a modern woodburning appli-
ance, (in which the amount of air available for combus-
tion is regulated), depends on the chimney to keep
exhaust gases warm all the way outdoors. The charac-
teristics of your chimney - whether it is steel or ma-
sonry, interior or exterior, matched or mismatched to
the stove collar - determine how quickly it will warm up
and how well it will sustain the optimum temperatures
necessary to maintain strong draft and efficient com-
bustion. Here follows a description of various flue
system characteristics and 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-combus-
tion 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 compounded if the chimney is located outside the
home or if the chimney flue has a cross-sectional size
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.
Although steel chimneys are not as attractive as their
masonry counterparts, they are very durable and
generally outperform masonry.
Inside/Outside 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 does not continuously
lose its heat to the outdoors, it takes less heat from the
stove 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 volume. As gases cool
with expansion, draft strength decreases. If an over-
sized 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 appliance without
relocating the thimble.
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