28
Defiant
®
1975 Non-Catalytic / Catalytic Woodburning Stove
30005220
transfer, and it gives you long-term flexibility for install-
ing 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 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 chim-
ney 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 fire-
place, 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 appliance 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 pres-
ent in the exhaust, slow-moving, and cools to less than
290°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 installa-
tion features and operational 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) recom-
mends 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 guide-
lines 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. ‘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 ap-
pearance 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 system. If you hear a rumbling or roar-
ing 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. Re-
sults 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 burning 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 avail-
able 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.
