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July 20, 2008
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Chimney Problems
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MSU Extension
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What's in this article
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 Introduction
Creosote
Reducing creosote problems
Be Prepared for A Chimney Fire
If you have a chimney fire
Chimney Cleaning
Cleaning the Chimney Yourself
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 Return to the Knowledge Base
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The combustion process when wood is burned is never
complete. The smoke from a wood fire usually contains a
dark brown or black substance which has an unpleasant
odor. This tar-like substance is called creosote and is
found almost anywhere in a wood heating system.
At temperatures below 250 degrees F creosote will
condense on the surfaces of stove pipes or chimney flues.
When the temperature gets below 150 degrees F the
creosote deposit will be thick, sticky and similar to
tar. This tends to trap carbon from smoke which dries and
bakes inside pipes and flues. This flaky substance is
very flammable.
Creosote is more of a problem with wood stoves than
fireplaces since the exhaust gases from stoves are cooler
than those from the fireplaces.
The amount of creosote condensing on the surfaces of
the system varies according to the density of the smoke
and vapor from the fire (less smoke means less creosote),
the temperature of the surface on which it is condensing
(higher temperatures reduce chance of creosote
condensation) and the type and dryness of wood being
burned. (
 Figure 1 - Causes of creosote formation |
Creosote may build up to a considerable
thickness on the interior surface of the chimney and the
draft opening may subsequently be reduced. A serious fire
may be ignited if creosote is allowed to build up. Most
problems with creosote are due to poor chimneys with a
low draft and cold walls.
You can reduce the creosote problem several ways.
Smoke density can be lowered somewhat in an airtight
stove by using small amounts of wood and stoking more
often or by using larger pieces of wood. Creosote
formation can be limited by leaving the air inlet or
stove door slightly open after adding wood to promote
more rapid burning until the wood is mostly reduced to
charcoal. Then close the inlet as desired.
Allowing this extra air causes more complete
combustion lowers the potential creosote-forming gases and
generates additional heat to the surrounding area. Vapor
in the flue gases may be controlled by using the driest
wood possible and using only small pieces of wood during
mild weather when combustion is relatively slow. The stack
temperature can be raised by insulating the stove pipe
connection so that it cools as little as possible before
reaching the chimney. Using an insulated pipe also aids
in increasing the stack temperature.
Draft can be increased by having as few bends as
possible between the appliance and the chimney, having
the proper height and diameter, keeping the chimney in
good repair and by having a separate flue for each
appliance. Also use proper sized stove pipe. In a large
chimney, draft can be increased by decreasing the flue
size. This can be done by installing a new smaller flue
or a stainless steel stove pipe liner.
In many airtight stoves, a sealed overnight fire will
deposit creosote even with dry hardwood. To dry the
creosote always open the draft caps and let the fire burn
hot for at least 5 minutes every morning and again before
bedtime.
Opening the direct draft damper 20 to 30 minutes to
dry the creosote in chimneys is a questionable practice.
This should only be done in a new or clean chimney and
should be done daily or every time you use the wood stove.
Allowing hot flame in the chimney at intermittent times
can result in a small chimney fire. The heat generated
from these hot flames also may cause deterioration of the
metal or crack mortar in the chimneys.
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