State University Extension
Home Maintenance And Repair - 01500598
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
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.
1) 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
Be Prepared for A Chimney Fire
No wood burning system is 100% safe and fire-proof. A safe installation
and extra care help prevent fire, but accept the idea that there could
be a fire, and be prepared to handle it. Chimney fires are most likely
to occur during a very hot fire, as when cardboard or Christmas tree
branches are burned or even when a stove burns normal wood but at a
higher than normal rate.
Make certain everyone in the house is familiar with the warning signs
of a chimney fire --- sucking sounds, a loud roar and shaking pipes.
Instruct everyone on what to do in case of fire. Practice fire drills
and instruct all adults on how and when to use a fire extinguisher. Put
the fire department phone number in an obvious place near the phone.
If you have a chimney fire:
a) Call the fire department immediately.
b) If all the stove pipe joints are tight and no other appliance is
connected to the same flue, close all openings and draft controls if
you have an air-tight stove. Close the stove pipe damper in a
c) You can attempt to cut off the air supply to a fireplace by using a
wet blanket or sheet metal to seal off the fireplace opening. Be
careful since a strong draft can make this difficult and dangerous. Use
only non combustible materials.
d) If you have a leaky stove or fireplace you may have to wait for the
fire to burn out.
e) Get everyone out of the house, and put them to work watching for
sparks or signs of fire on the roof or nearby. One adult should stay in
the house to check the attic and upper floors for signs of fire.
f) Discharge a class ABC dry chemical fire extinguisher or throw baking
soda into the stove or fireplace if the chimney is not sound or there
is a danger of the house or surroundings catching on fire. The chemical
travels up the chimney and often extinguishes the flame.
g) Throwing water in a stove will cause the metal to warp, but if it's
a choice between the house or the stove, use water.
h) Check the chimney after a fire. A chimney fire can range from 2000
to 3000 degrees F which is hot enough to cause deterioration of metal
or cause masonry to weaken. Metal chimneys can deteriorate after 2 or 3
i) If a chimney fire occurs once, chances are that it will occur again.
Find the cause.
A problem with frequent chimney fires is the possibility of the framing
catching on fire. The ignition temperature of new house framing is
about 500 degrees F Over a period of years, as this wood is repeatedly
heated by chimney fires, the wood will ignite at a much lower
Chimneys need to be cleaned to remove creosote and soot deposits. This
will prevent chimney fires and improve the draft as well. How often the
chimney is cleaned depends on how frequently the wood burning appliance
is used, how it is operated and the type of installation. Some
authorities recommend cleaning the chimney after every third cord of
wood is burned and most recommend at least once a year. Any time you
observe excessive soot and creosote, the chimney should be cleaned.
After you once have cleaned the chimney, you may want to check it after
2 weeks, 1 month, 2 months, etc., to determine how often your chimney
needs to be cleaned.
You may have the chimney cleaned for you by professional chimney sweeps
or you can clean it yourself.Costs for chimney sweeps vary with the job
but usually amount to about $40. ln addition to cleaning your chimney,
a good chimney sweep can act as a inspector for your installation.
Chimneys are normally cleaned by mechanical means to scrape off any
loose creosote build-up. Stiff wire chimney cleaning brushes are
available at reasonable cost. (Vis.
2) They are constructed to match the size of the chimney flue and
can be pushed through the chimney with extension rods or pipe or can be
pulled with ropes on either end of the brush. You can attach a weight
to the bottom of some brushes. The weight will drag the brush to the
bottom of the chimney so it can be pulled up with a rope. Other
cleaning methods are to lower a burlap bag containing wire netting
weighted with chains or rocks up and down the chimney or to use tire
chains or wire netting without a bag.
3)Don't swing a length of heavy chain down the chimney. The impact
can damage the flue lining.
Many people start chimney fires deliberately by building hot fires or
by tossing in compounds designed to remove soot and creosote by
controlled burns. Under some circumstances this practice may be
reasonable, but generally it is a risky way to keep a chimney clean.
Any chimney fire could build into a house fire, but in addition a
chimney fire causes wear on a chimney. The high temperatures increase
corrosion rate of metals and cause thermal expansion of masonry
materials which can lead to cracks. Some of the compounds used in
controlled burns have been known to explode in stoves.
Chemical chimney cleaners are commercially available.These are not
intended for use in chimneys already containing heavy deposits of soot
and creosote. Chemicals such as sodium chloride, or table salt, are
sometimes used as a chimney cleaner. These chemicals combine with water
released from a hot fire to form a weak acid that dissolves small
amounts of creosote. Sodium chloride is corrosive to metal and is not
recommended for metal chimneys. Cleaners that contain copper sulfate
will coat any soot in the chimney and act as a catalyst to allow soot
to burn away at lower than normal temperatures.
Chemical cleaners are intended to be used after chimneys are cleaned or
new. Use the chemicals as directed--- approximately 1 ounce per week.
If not used as directed, the chemicals can cause intense chimney fires
that will result in rapid deterioration of the chimney. THE ONLY
EFFICIENT AND EFFECTIVE METHOD OF CLEANING IS TO USE A CHIMNEY BRUSH,
SINCE THE BRUSH SCRUBS THE ENTIRE SURFACE UNIFORMLY.
Cleaning the Chimney Yourself
If you plan to clean the chimney yourself, you will need to obtain some
or all of the following tools and supplies:
Drop cloth or other appropriate covering
Treble light or portable lantern
Hand wire brush
Hand scraper or stiff putty knife
Hammer and screwdriver
Heavy duty vacuum cleaner
Whisk broom and dust pan
Can of furnace cement
Rope and a weight or extension rods
Before starting to clean the chimney, be sure all doors and windows are
shut to prevent any drafts. Remove damper, if possible. Seal fireplace
opening with a drop cloth and masking tape. You will need proper
protective clothing, including a mask to cover your mouth and nose and
glasses or goggles for your eyes. The material that collects in
chimneys is of such a nature that you should avoid contact with it as
much as possible. Wear good shoes with slip resistant soles and
be careful when climbing on high, steep roofs to clean a chimney.
When cleaning the chimney from the roof, the easiest method is to
attach a line to the brush with a weight on the opposite end. This
weight should be of such a size and shape that it cannot swing free
into the tile liners and cause damage. The purpose of the weight is to
pull the brush down into the chimney. A solid 15- to 20-pound weight is
required to move the brush downward. This will depend on how tightly
the brush fits and how dirty the chimney is.
Another method is to attach a rope at each end of the brush with a
person at the top of the chimney and one at the bottom, taking turns
pulling the rope. This method may be somewhat messy.
More effective is the use of rigid extensions such as a pipe or tubing
with a flexible leader. This allows you to control and feel the
scrubbing action of the brush in the chimney. This method is used by
most professional chimney sweeps. Fiberglass rods are available for
this purpose. If metal pipe is used, be careful of power lines above.
Lower the brush into the chimney being careful not to disturb any loose
brick mortar or any device in the chimney. Cleaning can be accomplished
by passing the brush through the chimney a number of times in the same
direction or by raising and lowering the brush in short strokes in a
scrubbing action. If your brush is too large, it will not reverse in
the chimney and may even lock up.
Experience will tell you how many passes to make to get the chimney
clean. Once this process is finished, remove the seal from the
fireplace opening. Use a drop cloth in your working area. Slowly open
the damper if you were unable to remove it, vacuum up debris from the
bottom of the hearth, smoke shelf or catch pit. If you can't open the
damper you may have to drop a hose down the chimney to vacuum out the
While cleaning masonry chimneys, check for cracks in the brick or
masonry. Cracks allow cool air to come in, thus reducing the efficiency
of the fireplace or wood stove and allowing creosote to form.
Stove pipes on the wood burner are critical to safety and require
additional attention. When cleaning an inside flue, remove the
connected sections. Be careful to protect the area from soot. Take the
sections outdoors and brush inside them with a hand wire brush or a
flue or chimney brush that is the same diameter as the pipe. Remove all
the soot and creosote build-up from the breech and the loose
accumulation in the fire box. Stove pipes need to be cleaned regularly.
Check pipes at least once every 2 or 3 months of stove operation.
After using your chimney brush, rinse it in a cleaning solution such as
kerosene and store it away in a dry place. lt is a valuable tool.
This information comes from Michigan State University Extension
The Creosote Problem.
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not mentioned. This information becomes public property upon
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are open to all without regard to race, color, national origin, gender,
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status. Issued in furtherance of MSU Extension work, acts of May 8 and
June 30, 1914, in cooperation with the U.S. Department of Agriculture.
Thomas G. Coon, Director, MSU Extension, East Lansing,MI 48824. This
information is for educational purposes only. Reference to commercial
products or trade names does not imply endorsement by MSU Extension or
bias against those not mentioned.
information.was reviewed as
of June 2008. For more information about the contents please
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