17
Vermont Castings Aspen
®
II Multi-Fuel
30003892
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 there-
fore 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 suc-
tion 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, especial-
ly an oversized one, and keep it warm enough to maintain
an adequate draft.
Steel chimney
Most factory-made steel chimneys have a layer of insula-
tion 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 out-
doors, 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
draft management
serves an airtight stove, more is not better; in fact, it can
be a disadvantage. Hot gases cool off through expansion;
if we vent a stove with a 152 mm (6”) flue collar [181 sq
cm(28 square inch) area] into a 254 x 254 mm (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 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 com-
pletely 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 relo-
cating 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 nega-
tive draft through the airtight, and result in a dangerous
draft reversal.
creosote
Creosote is a by-product of slow woodburning. It’s an
organic tar that can condense in the flue if it’s dense in the
smoke, and slow-moving, and cools off to less than 143° C
(290° F). Condensed creosote is volatile, and can gener-
ate chimney fires if it gets hot enough. All the features that
