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Intrepid II Woodburning Stove
2000966
A stove is part of a system, which 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 system
works well.
Wood stove operation depends on natural (unforced)
draft. Natural draft occurs when the smoke is hotter
(and therefore lighter) than the outdoor air at the top of
the chimney. The bigger the temperature difference, the
stronger the draft. As the smoke rises from the chimney
it provides suction or ‘draw’ that pulls air into the stove
for combustion. A slow, lazy fire with the stove’s air inlet
fully open indicates a weak draft. A brisk fire, supported
only by air entering the stove through the normal inlet,
indicates a good draft. The stove’s air inlet is passive;
it regulates how much air can enter the stove, but it
doesn’t move air into it.
Depending on the features of your installation - steel or
masonry chimney, inside or outside the house, matched
to the stove’s outlet or oversized - your system may
warm up quickly, or it may take a while to warm up and
operate well. With an ‘airtight’ stove, one which restricts
the amount of air getting into the firebox, the chimney
must keep the smoke warm all the way to the outdoors.
Some chimneys do this better than others. Here’s a list
of features and their effects.
Masonry Chimney
Masonry is a traditional material for chimneys, but it
can perform poorly when it serves an ‘airtight’ stove.
Masonry is a very effective ‘heat sink’ - it absorbs a lot
of heat. It can cool the smoke enough to diminish draft.
The bigger the chimney, the longer it takes to warm up.
It’s often very difficult to warm up an outdoor masonry
chimney, especially an oversized one, and keep it warm
enough to maintain an adequate draft.
Steel Chimney
Most factory-made steel chimneys have a layer of in-
sulation around the inner flue. This insulation keeps the
smoke warm. The insulation is less dense than mason-
ry, so the inner steel liner warms up more quickly than a
masonry chimney. Steel doesn’t have the good looks of
masonry, but it performs much better.
Indoor/Outdoor location
Because the chimney must keep the smoke warm, it’s
best to locate it inside the house. This uses the house
as insulation for the flue and allows a little heat release
into the home. An indoor chimney won’t lose its heat to
the outdoors, so it takes less heat from the stove to get
it warm and keep it warm.
Flue sizing
The inside size of a chimney for an ‘airtight’ stove
should match the size of the stove’s flue outlet. When
a chimney serves an airtight, more is not better; in fact,
it can be a disadvantage. Hot gases cool off through
expansion; if we vent a stove with a six-inch flue collar
(28 square inch area) into a 10 x 10” flue, the gases
expand to over three times their original size. This cools
the gases, which weakens draft strength. If an over-
sized flue is also outside the house, the heat it absorbs
gets transferred to the outdoor air and the flue usually
stays cool.
It’s common for a masonry flue, especially one built for
a fireplace, to be oversized for an airtight stove. It can
take quite a while to warm up such a flue, and the re-
sults can be disappointing. The best solution to an over-
sized flue is an insulated steel chimney liner, the same
diameter as the stove or insert’s flue outlet; the liner
keeps the smoke at its original volume, and the result is
a stronger draft. An uninsulated liner is a second choice
- the liner keeps the smoke restricted to its original size,
but the smoke still must warm up the air around the
liner. This makes the warm-up process take longer.
Pipe & Chimney Layout
Every turn the smoke must take in its travel from the
stove to the chimney top will slow it down. The ideal
pipe and chimney layout is straight up from the stove,
to a completely straight chimney. If you’re starting
from scratch, use this layout if possible. If the stove-
pipe must elbow to enter a chimney, locate the thimble
about midway between the stove top and the ceiling.
This achieves several goals: it lets the smoke speed up
before it must turn, it leaves some pipe in the room for
heat transfer, and it gives you long-term flexibility for
installing a taller stove 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 problems. Use double-wall stove pipe for long
runs.
Single Venting
Each ‘airtight’ stove requires its own flue. If an airtight
stove is vented to a flue that also serves an open fire-
place, or a leakier stove, it’s easier for the chimney draft
to pull air in through those channels than it is to pull air
through the airtight, and performance suffers. Imagine
a vacuum cleaner with a hole in the hose to see the ef-
fect here. In some cases the other appliance can even
cause a negative draft through the airtight, and result in
a dangerous draft reversal.
Draft Management