Vermont Casting 2550 - Manual (Page 26)

26
Encore Woodburning Stove
2000956
Flue Sizing
The inside size of a chimney for an ʻairtightʼ stove
should match the size of the stoveʼs flue outlet. When
a chimney serves an airtight, more is not better; in fact,
it can be a disadvantage. Hot gases lose heat through
expansion; if we vent a stove with a six-inch flue collar
(28 square inch area) into a 10 x 10” flue, the gases
expand to over three times their original size. This cools
the gases, which weakens draft strength. If an over-
sized flue is also outside the house, the heat it absorbs
gets transferred to the outdoor air and the flue usually
stays cool.
Itʼs common for a masonry flue, especially one serving
a fireplace, to be oversized for the stove. It can take
quite a while to warm up such a flue, and the results
can be disappointing. The best solution to an oversized
flue is an insulated steel chimney liner, the same diam-
eter as the stove or insertʼs flue outlet; the liner keeps
the smoke warm, and the result is a stronger draft. An
uninsulated liner is a second choice - the liner keeps
the smoke restricted to its original size, but the smoke
still must warm up the air around the liner. This makes
the warm-up process take longer.
Pipe & Chimney Layout
Every turn the smoke must take as it travels to the
chimney top will slow it down. The ideal pipe and chim-
ney layout is straight up from the stove, to a completely
straight chimney. If youʼre starting from scratch, use this
layout if possible. If the stovepipe must elbow to enter a
chimney, locate the thimble about midway between the
stove top and the ceiling. This achieves several goals:
it lets the smoke speed up before it must turn, it leaves
some pipe in the room for heat transfer, and it gives you
long-term flexibility for installing a different stove without
relocating the thimble.
There should be no more than 8ʼ (2.4m) of single-wall
stove pipe between the stove and a chimney; longer
runs can cool the smoke enough to cause draft and
creosote problems. Use double-wall stove pipe for long
runs.
Single venting
Each ʻairtightʼ 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, itʼs 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. Itʼs an
organic tar that can condense in the flue if itʼs dense
in the smoke, and slow-moving, and cools off to less
than 290°F. 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 stoveʼs 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 ʻgreenʼ wood has a good amount 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 that reaches your home and
can contribute to a creosote problem. There are mois-
ture 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 ʻgas outʼ 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 the catalyst is seri-
ously 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.