Vermont Casting 1945 - Manual (Page 26)

26
Defiant Woodburning Stove
30001693
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 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 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 ef-
fect 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 (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 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. 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 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 operation, you’ll be rewarded
with good performance and a safe system.