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November 22, 2008
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Wood Stoves
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MSU Extension
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What's in this article
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 Introduction
Wall Protection
Floor Protection
Stove Pipe
Additional Precautions
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 Return to the Knowledge Base
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Stove Pipe
The stove pipe or chimney connector runs from the
stove to the chimney. Many fires associated with wood
stoves are caused by unsafe stove pipe installation. A
safe installation requires proper material, construction
clearances and does provide proper draft. A 24 gauge or
thicker metal is recommended; lower gauge numbers
indicate thicker metal. This gauge will provide better
protection in the event of a chimney fire and will also
resist chemical corrosion longer. Most stoves use either
a 6 or 8-inch stove pipe. Using stove pipe that is
smaller in diameter than the fire box outlet will reduce
combustion efficiency and may cause improper draft.
Keep the connector pipe as short as possible. lt
should not be longer than 75% of the vertical chimney
height above the flue inlet (where the connector pipe
enters the chimney). The maximum length is 10 feet. If the
pipe runs horizontally, it should have a rise of at least
 Figure 3 - Causes of creosote formation |
1/4-inch per linear foot from the elbow or stove outlet to
the chimney inlet. Use 45" angles to create an upward
slope in the flue connector pipe. Try to have no more than
one right angle turn between the stove and chimney.
Additional right angle bends can cause soot and creosote
to collect in the smoke pipe or chimney, blocking flue
gas flow and increasing the danger of a fire.
The connector pipe diameter should be as large as the
flue collar (where the connector pipe joins the stove).
When joining sections of the pipe, overlap the joints at
least 2 inches, with the crimped (male) end pointing down
to prevent creosote drip or leak. Many house fires have
resulted from stove pipe joints vibrating apart during a
chimney fire. Secure each joint with at least 3 sheet
metal screws. A fireproof sealant may be used in addition.
(See Figure 3)
Clearances from a connector pipe must be 3 times the
pipe diameter (a 6-inch pipe needs 18 inches clearance)
unless the wall is protected. (See Figure 4) You should not pass
a stove pipe through a combustible wall but if a stove
pipe must pass through an interior combustible wall in
order to hook up with a chimney flue, there are 4 ways to
do this safely. (See Figure 5)
 Figure 4 - Stove pipe clearance with ceiling or wall protection
A - See Table 1 for this distance
B - Minimum distance here is 3 times the diameter of the smoke pipe |
1) Use an U.L. "All Fuel" thimble extending through the
wall, with a wall hole 4 inches larger than the thimble
diameter. This permits the placement of an insulating
material such as fiberglass or rock wool between the
thimble and the wooden framing of the wall.
2) Use a ventilated thimble that is as least 3 times
larger than the stove pipe. For a 6-inch stove pipe, use
a thimble that is 18 inches in diameter. This type of
thimble is not readily available but can be fabricated by
a sheet metal shop. Ventilation through this thimble is
an essential aspect of its design; the ventilating holes
on either side must not be blocked.
3) Use a fire clay thimble surrounded by 8 inches of
brick work or non-combustible material such as rock wool
insulation.
4) Use no thimble but remove all combustible materials
within 18 inches on all sides of the stove pipe. Material
for closing this opening must be non-combustible, with
insulating properties.
When the wall is cut between supporting studs for the
thimble, inspect the opening to make sure there are no
electrical wires or conduit in the space between
adjoining wall studs. Heat from the stove pipe may be
sufficient to melt the insulation on wire in this space,
causing an electrical fire.
Stove pipe should not pass through ceilings, closets,
or outside a building. Holes in the ceiling (including
hot air registers) permit fires through upper floors. A
closet fire could smolder and spread undiscovered.
 Figure 5 - Thimbles for safely passing stove pipe through combustable walls |
Running a stove pipe out a window and up the outside
wall of the house is a dangerous practice, because the
pipe cools faster than a prefabricated metal chimney and
allows a rapid creosote buildup. Wood burners sometimes
recommend long spans of single thickness stove pipe as a
heating device. This idea had some merit when used with
old fashioned inefficient stoves where much of the heat
went up the pipe. Today's airtight stoves are more
efficient and this practice may cause rapid creosote
buildup.
Some stove installations require a damper either
built into the stove or in the pipe near the stove to
control draft and loss of volatile gases. Check the
recommendation of the stove manufacturer.
When connecting the stove pipe to the chimney make
sure the fitting is snug at the flue inlet. Use the
proper thimble. The pipe must not project into the flue
itself, since it would hamper draft.
Long stove pipes and those with restrictions should
be cleaned frequently to prevent creosote buildup and
possible chimney fires. The entire length of the stove
pipe must be easily inspected, firmly fastened at the
joints and kept free of all combustible materials. Tap
your pipe to check its condition several times during the
heating season and before starting the stove each year.
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