Vermont Casting 1945 - Manual (Page 25)

25
Defiant Woodburning Stove
30001693
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 ma-
sonry, 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 un-
less 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 stove, 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 diam-
eter 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 chim-
ney 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.
Draft Management