Vermont Castings Defiant FlexBurn Homeowner's Installation And Operating Manual page 28

Defiant ® FlexBurn ™ 1975 Non-Catalytic / Catalytic Woodburning Stove
can cool the exhaust 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 so that it turns 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 and performance of the stove
suffers. Imagine a vacuum cleaner with a hole in the hose
to understand the effect here. In some cases the other ap-
pliance can even cause a negative draft through the stove,
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 is present in
the exhaust, slow-moving, and cools to less than 290°F
(130°C). Condensed creosote is volatile, and can gener-
ate 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 op-
erational steps that will encourage good draft and minimize
creosote production.
Because letting the exhaust cool off and slow down is one
of the keys to creosote accumulation, 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 (NFPA) recommends a chimney liner 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.
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.
'Green' wood has a lot of moisture in it; it will burn, but some
of the heat potential is used to boil the extra moisture from
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, wood can be too dry and too volatile.
Smoke and combustible gases can 'gas out' from the wood
quickly and densely enough to overload the combustion
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Fuel
system. If you hear a rumbling or roaring noise (like a
propane torch) from the stove, that is a sign that the stove
is overfiring.
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 until they are concentrated
enough and hot enough to ignite. If your stove back-puffs,
the stove needs to cool down. You should open the damper
to let the smoke rise to the flue more quickly, allow more
air into the firebox, avoid big loads of firewood and check
that the wood moisture is not too low.
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 awhile longer and manage the fire with the air inlet
until the draft strengthens. If you keep track of your burn-
ing habits and relate them to their effects on the stove's
operation, you'll be rewarded with good performance and
a safe system.
Negative pressure
Good draft also depends on a supply of air to the stove; a
chimney can't pull in more air than is available to it. Sluggish
draft results when a house is tight enough to prevent the
ready flow of air to the stove, or by competition between
the stove and other equipment that sends indoor air outside
- especially power-driven equipment like range hoods,
clothes dryers, etc. If the chimney draws well with all other
equipment turned off (or sealed, in the case of fireplaces
and/or other stoves), then you simply need to be careful
with timing the use of the other air consuming equipment.
If you need to crack a nearby window or door to enable
the chimney to flow well, it may be a good idea to install
an outside-air intake to bring combustion air directly to the
stove. An outside air kit is available to connect the stove
directly to a source of outdoor combustion air.
conclusion
Wood-burning is an art rather than a science. Once the
stove and chimney system are in place, you can only vary
your technique, mostly your timing, to achieve good results.
If you keep track of your burning habits and relate them to
their effects on the stove's operation, you'll be rewarded
with good performance and year of reliable heating.
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