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Vermont Castings Resolute Acclaim
2000893
Draft Management
A stove is part of a system, which includes the chim-
ney, 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
insulation around the inner flue. This insulation keeps
the smoke warm. The insulation is less dense than
masonry, 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
results can be disappointing. The best solution to an
oversized 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 stovepipe 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
fireplace, 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 effect here. In some cases the other appliance
can even cause a negative draft through the airtight,
and result in a dangerous draft reversal.