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Vermont Castings Defiant
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. With prefabricated chimney,
bring it down to six to eight feet from the stove. With a
masonry chimney, arrange the pipe layout to elbow into
the chimney within eight feet of the stove.
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 degrees F (130˚C). 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.
Fuel
Even the best stove installation will not perform
well with poor fuel. The best fuel is hardwood that has
air-dried 12-18 months. Softwood burns, but not as
long as hardwood. Fairly ‘green’ wood has a lot of
moisture in it; it will burn, but some of the heat potential
is used to drive the remaining moisture off the wood.
This reduces the amount of heat that reaches your
home and can contribute to a creosote problem. There
are moisture meters available for firewood; you can
also judge your wood by its appearance and weight. If
you get it green, lift a piece and get a sense of its
weight; it can lose a third or more of its weight as it
dries. Also look at the ends of a log; as it dries it
shrinks and often cracks. The more weathered and
cracked a piece is, the drier it is.
Dry wood burns readily with a good chimney draft.
But with modern stoves, especially catalytic ones, the
wood can be too dry. While extra-dry wood has little
creosote in it, the remainder can ‘gas out’ from the
wood quickly and densely enough to overload the
catalytic burner. If you hear a rumbling or roaring
noise, like a propane torch, from the stove, that is a
sign that the catalyst is seriously overfiring. The
catalyst is a platinum film on a ceramic base; the metal
can get to higher temperatures than the ceramic can
take, and overfiring the catalyst can break it. Dry wood
can also burn out faster than you want. If your dry
wood burns out too quickly or overloads the catalyst
you can mix in greener wood to slow the fire down.
Back-puffing
Back-puffing results when the fire produces volatile
gases faster than the chimney draft pulls them out of
the firebox. The gases back up in the firebox till they’re
dense enough and hot enough to ignite. If your stove
back-puffs, you should open up the damper to let the
smoke rise to the flue more quickly, let more air into
the firebox, and avoid big loads of firewood. You
should always see lively, dancing flames in the firebox;
a lazy, smoky fire is inefficient and can contribute to
creosote buildup in the chimney.
Draft Testing
An easy way to test your chimney draft is to close the
stove’s damper, wait a few minutes to let the airflow
stabilize, then see whether you can vary the strength
of the fire by swinging the air control open and closed.
Results are not always instant; you may need to wait a
few minutes for a change in the air control setting to
have an effect on the fire. If there’s no change, then
the draft isn’t strong enough yet to let you close the
damper, and you’ll need to open it for a while longer
and manage the fire with the air inlet until the draft
strengthens. If you keep track of your burning habits
and relate them to their effects on the stove’s opera-
tion, you’ll be rewarded with good performance and a
safe system.