Vermont Castings Intrepid II Installation & Operating Manual page 27

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 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.
27 Vermont Castings • Intrepid II Installation & Operating Manual_R48 • 11/17
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.
Creosote:
Creosote is a by-product of slow wood-burning. 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 290°F. Condensed
creosote is volatile, and can generate chimney fires if it gets
hot enough. All the features that affect chimney draft also
affect creosote condensation - so use whatever combination
of installation features and operational steps will encourage
good draft and minimize creosote production.
Because letting the smoke cool off and slow down is one
of the keys to creosote production, it makes sense to line a
chimney to match the stove's outlet size, for safety reasons
as well as performance. Canadian law requires a matching
liner to serve any stove or insert vented through a fireplace
chimney; in the US, the National Fire Protection Association
recommends one if the flue is more than three times bigger
(in square area) than the outlet on the stove or insert. Some
localities enforce the NFPA guidelines as part of their building
codes.
3-90-2000966