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Vermont Castings • Defiant® Model 1975-CAT-C Installation Manual_R8 • 02/21
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 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 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.
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 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 present 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 installation 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) 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.
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 contains a lot of moisture; 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 system. If
you hear a rumbling or roaring noise (like a propane torch)
from the stove, that is a sign that the stove is over-firing.
