Draft Management - Vermont Castings Defiant FlexBurn Homeowner's Installation And Operating Manual

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 (un-
forced) draft. Natural draft occurs when the exhaust 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 hot gases rise through the
chimney they provide 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, sup-
ported 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
stove's exhaust 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 chimney gases enough to diminish draft. The
bigger the chimney, the longer it takes to warm up. It's of-
ten 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 chimney
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.
30005220
Defiant FlexBurn 1975 Non-Catalytic / Catalytic Woodburning Stove
® ™

Draft Management

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 must be
warmed up, a distance from the heat source (the stove).
Don't make a chimney taller unless you must in order to
meet the safety rules, or unless there's some nearby feature
causing a downdraft. Even then, there are downdraft-pre-
venting chimney caps available, which are probably the
smarter choice.
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 faster as they
travel slower through a chimney; if we vent a stove with
a six-inch flue collar (28 square inch area) into a 10 x 10"
flue, the gases slow to one third their original speed. This
allows the gases to cool more rapids, 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 is 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 exhaust
warm, and the result is a stronger draft. An non-insulated
liner is a second choice - the liner keeps the exhaust
restricted to its original size, but the hot gases still must
warm up the air around the liner. This makes the warm-up
process take longer.
Every turn the exhaust must take as it travels to the chimney
top will slow it down. The ideal pipe and chimney layout
is to vent vertically into a completely straight and vertical
chimney. If you are 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 allows the
gases to speed up before they 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.
There should be no more than eight feet of single-wall
stove pipe between the stove and a chimney; longer runs
chimney height
Flue Sizing
pipe & chimney layout
27