Filter
Top Products
24
Vermont Castings Defiant
DRAFT MANAGEMENT
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 or insert 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 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
stove’s air inlets are passive; they regulate how much
air
can
enter the stove, but they don’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 in order for the stove to work
well. 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 a steel chimney 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 some 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
heat it up and keep it warm.
Chimney Height
The common wisdom tells us that a taller flue draws
better than a short one. This isn't necessarily so. If a
chimney is tall enough to meet the safety requirements
of the 2/3/10 foot rule, then adding more height isn't
the right answer to a draft problem. In fact it could
make the problem
worse
, by adding more mass to the
chimney system, which the smoke must warm up, at
the far end from the heat source (the stove). Don't
make a chimney taller unless you must to meet the
safety rules, or unless there's some nearby feature
causing a downdraft. Even then, there are downdraft-
preventing chimney caps available, which are probably
the smarter choice.
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 lose heat 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
oversized 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
serving a fireplace, to be oversized for the 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 warm, 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 as it travels 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 different stove without relocating the
thimble.