Vermont Casting 2477 - Manual (Page 28)

28
Dutchwest Non-Catalytic Convection Heater
30002314
Your stove is only one part of a system that includes the
chimney, the operator, the fuel, and the home. The other
parts of the system will affect how well the stove works.
When there is a good match between all the parts, the
system works well.
Wood stove operation depends on natural (unforced)
draft. Natural draft occurs when exhaust gas is hotter
(and therefore lighter) than the outdoor air at the top of
the chimney. The greater the temperature difference, the
stronger the draft. As the hot exhaust gas rises out of
the chimney, it generates suction that draws air into the
stove for combustion. A slow, lazy fire when the stove’s
air inlets are fully open indicates a weak draft. A brisk fire,
supported only by air entering the stove through the air
inlets, indicates a good draft. The inlets are passive; they
regulate how much air can enter the stove, but they don’t
force air into it.
The efficiency of a modern woodburning appliance,
(in which the amount of air available for combustion is
regulated), depends on the chimney to keep exhaust
gases warm all the way outdoors. The characteristics
of your chimney - whether it is steel or masonry, interior
or exterior, matched or mismatched to the stove outlet,
- determine how quickly it will warm up and how well it will
sustain the optimum temperatures necessary to main-
tain strong draft and efficient combustion. Here follows a
description of various flue system characteristics and the
related effects on stove performance.
Masonry Chimney
Although masonry is the traditional material used for
chimney construction, it can have distinct performance
disadvantages when used to vent a controlled-combus-
tion woodstove. Masonry forms an effective ‘heat sink’ -
that is, it absorbs and holds heat for long periods of time.
The large mass, however, may take a long time to be-
come hot enough to sustain a strong draft. The larger the
chimney (in total mass), the longer it will take to warm up.
Cold masonry will actually cool exhaust gases enough
to diminish draft strength. This problem is worse if the
chimney is located outside the home or if the chimney
flue has a cross-sectional volume much larger than the
stove outlet.
Steel Chimney
Most factory-made ‘Class A’ steel chimneys have a layer
of insulation around the inner flue. This insulation keeps
the smoke warm and protects the surrounding structure
from the high flue temperatures. Because the insulation
is less dense than masonry, the inner steel liner warms
up more quickly than a masonry chimney; this makes
the steel chimney support a good draft more quickly than
masonry does. Steel chimneys are not as attractive as
masonry, but they are very durable and generally outper-
form masonry.
Indoor/ Outdoor Location
Because the chimney’s function is to keep the smoke
warm, it is best to locate it inside the house. This location
uses the house as insulation for the flue and allows some
radiant heat release from the flue into the home. Since
an interior chimney doesn’t continuously lose its heat to
the outdoors, less heat from the stove is required to get it
warm and keep it warm.
Flue Sizing
The flue size for a controlled-combustion appliance
should be based on the cross-sectional volume of the
stove flue outlet. In this case, more is definitely not better.
Hot gases lose heat through expansion; if a stove with a
six-inch flue collar (28 square inch area) is vented into a
10” x 10” flue, the gases will expand to over three times
their original volume. As gases cool with expansion, draft
strength decreases. If an oversized flue is also outside
the house, the heat it absorbs will be conducted to the
outdoor air and the flue will remain relatively cool.
It is common for a masonry flue to be oversized for the
stove. Such a chimney can take quite a while to warm up
and the stove performance will likely be disappointing.
The best solution to an oversize flue problem is the instal-
lation of an insulated steel chimney liner of the same
diameter as the appliance flue outlet. The liner keeps the
exhaust gas warm and the result is a stronger draft. An
uninsulated liner is a second choice - although the liner
will keep the exhaust restricted to its original volume, the
air around the liner will require time and heat energy to
warm up.
Check your local codes. You may be required to install a
flue liner in any oversize or masonry flue.
Pipe & Chimney Layout
Every bend in the flue will act as a brake on the exhaust
as it flows from the firebox to the chimney cap. The ideal
pipe and chimney layout is straight up from the stove
through a completely straight chimney. Use this layout
if at all possible as it will promote optimum stove perfor-
mance and simplify maintenance.
If the stovepipe must elbow to enter a chimney, locate
the elbow about midway between the stove top and the
chimney thimble. This configuration lets the smoke speed
up before it must turn, keeps some pipe in the room for
heat transfer, and allows long-term flexibility for installing
a different appliance 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 smoke enough to cause draft and creosote
problems. Use double-wall stove pipe for longer runs.
Draft Management