As you snuggle in front of a cozy fire or bask in the warmth
of your wood stove, you are taking part in a ritual of
comfort and enjoyment handed down through the centuries. The
last thing you are likely to be thinking about is the
condition of your chimney. However, if you don’t give some
thought to it before you light those winter fires, your
enjoyment may be very short-lived. Why? Dirty chimneys can
cause chimney fires, which damage structures, destroy homes
and injure or kill people.
Chimney fires can burn explosively – noisy and dramatic
enough to be detected by neighbors or passers-by. Flames or
dense smoke may shoot from the top of the chimney.
Homeowners report being startled by a low rumbling sound
that reminds them of a freight train or a low flying
airplane. However, those are only the chimney fires you know
about. Slow burning chimney fires don’t get enough air or
have enough fuel to be as dramatic or visible. But, the
temperatures they reach are very high and can cause as much
damage to the chimney structure – and nearby combustible
parts of the house - as their more spectacular cousins. With
proper chimney system care, chimney fires are entirely
CREOSOTE & CHIMNEY FIRES:
WHAT YOU MUST KNOW
Fireplaces and wood stoves are designed to safely contain
wood-fueled fires, while providing heat for a home. The
chimneys that serve them have the job of expelling the
by-products of combustion – the substances given off when
As these substances exit the fireplace or wood stove, and
flow up into the relatively cooler chimney, condensation
occurs. The resulting residue that sticks to the inner walls
of the chimney is called creosote. Creosote is black or
brown in appearance. It can be crusty and flaky … tar-like,
drippy and sticky … or shiny and hardened. Often, all forms
will occur in one chimney system.
Whatever form it takes, creosote is highly combustible. If
it builds up in sufficient quantities – and catches fire
inside the chimney flue – the result will be a chimney fire.
Although any amount of creosote can burn, sweeps are
concerned when creosote builds up in sufficient quantities
to sustain a long, hot, destructive chimney fire.
Certain conditions encourage the buildup of creosote,
restricted air supply, unseasoned wood and
cooler-than-normal chimney temperatures are all factors that
can accelerate the buildup of creosote on chimney flue
Air supply: The air supply on fireplaces may be restricted
by closed glass doors or by failure to open the damper wide
enough to move heated smoke up the chimney rapidly (the
longer the smoke’s “residence time” in the flue, the more
likely is it that creosote will form). A wood stove’s air
supply can be limited by closing down the stove damper or
air inlets too soon and too much, and by improperly using
the stovepipe damper to restrict air movement.
Burning unseasoned firewood: Because so much energy is used
initially just to drive off the water trapped in the cells
of the logs – burning green wood keeps the resulting smoke
cooler, as it moves through the system, than if dried,
seasoned wood is used.
Cool flue temperatures: In case of wood stoves, fully-packed
loads of wood (that give large cool fires and eight or 10
hour burn times) contribute to creosote buildup.
Condensation of the unburned by-products of combustion also
occurs more rapidly in an exterior chimney, for example,
than in a chimney that runs through the center of a house
and exposes only the upper reaches of the flue to the
HOW CHIMNEY FIRES DAMAGE CHIMNEYS
When chimney fires occur in masonry chimneys – whether the
flues are an older, unlined type or are tile lined to meet
current safety codes – the high temperatures at which they
burn (around 2000oF) can “melt” mortar, crack tiles, cause
liners to collapse and damage the outer masonry material.
Most often, tiles crack and mortar is displaced, which
provides a pathway for flames to reach the combustible wood
frame of the house. One chimney fire may not harm a home. A
second can burn it down. Enough heat can also conduct
through a perfectly sound chimney to ignite nearby
Pre-fabricated, factory-built, metal chimneys.
To be installed in most jurisdictions in the United States,
factory-built, metal chimneys that are designed to vent wood
burning stoves or pre-fabricated metal fireplaces must pass
special tests determined by Underwriter’s Laboratories (U.L.).
Under chimney fire conditions, damage to these systems still
may occur, usually in the form of buckled or warped seams
and joints on the inner liner. When pre-fabricated,
factory-built metal chimneys are damaged by a chimney fire,
they should no longer be used and must be replaced.
WAYS TO KEEP THE FIRE YOU WANT…
From Starting One You Don’t!
Chimney fires don’t have to happen. Here are some ways to
- Use seasoned woods only (dryness
is more important than hard wood versus soft wood
- Build smaller, hotter fires that burn more completely and
produce less smoke;
- Never burn cardboard boxes, wrapping paper, trash or
Christmas trees; these can spark a chimney fire;
- Install stovepipe thermometers to help monitor flue
temperatures where wood stoves are in use, so you can adjust
burning practices as needed;
- Have the chimney inspected and cleaned on a regular basis.
Clean chimneys don’t catch fire. Make sure a CSIA Certified
Chimney Sweep TM inspects your solid fuel venting system
annually, and cleans and repairs it whenever needed.
Your sweep may have other maintenance recommendations
depending on how you use your fireplace or stove.
CSIA recommends that you call on certified sweeps, since
they are regularly tested on their understanding of the
complexities of chimney and venting systems.
SIGNS THAT YOU’VE HAD A CHIMNEY FIRE
Since chimney fires can occur without anyone being aware of
them … and since damage from such fires can endanger a home
and its occupants, how do you tell if you’ve experienced a
Here are the signs a professional chimney sweep looks for:
If you think a chimney fire has occurred, call a
WETT (Wood Energy Technical
Training) certified chimney sweep for a professional evaluation. If
your suspicions are confirmed, a certified sweep will be
able to make recommendations about how to bring the system
back into compliance with safety standards. Depending on the
situation, you might need a few flue tiles replaced, a
relining system installed or an entire chimney rebuilt. Each
situation is unique and will dictate its own solution.
- “puffy” creosote, with rainbow coloured streaks, that has
expanded beyond creosote’s normal form;
- warped metal of the damper, metal smoke chamber, connector
pipe or factory-built metal chimney;
- cracked or collapsed flue tiles, or tiles with large
- discoloured and distorted rain cap;
- creosote flakes and pieces found on the roof or ground;
- roofing material damaged from hot creosote;
- cracks in exterior masonry;
- evidence of smoke escaping through mortar joints of
masonry or tile liners.
WHAT TO DO IF YOU HAVE A CHIMNEY FIRE
If you realize a chimney fire is occurring, follow these
- Get everyone out of the house, including yourself.
- Call the Fire Department.
If you can do so without risk to yourself, these additional
steps may help save your home. Remember, however, that homes
are replaceable, but lives are not:
Put a flare type chimney fire extinguisher into the
fireplace or wood stove.
Close the glass door on the fireplace.
Close the air inlets on the wood stove.
Use a garden hose to spray down the roof (not the
chimney) so the fire won’t spread to the rest of the
Monitor the exterior chimney temperature throughout the
house for at least 2 or 3 hours after the fire is out.
Once it’s over, call a WETT
(Wood Energy Technical Training) certified chimney sweep to
inspect for damage.
Chimney fire damage and repair normally is covered by
homeowner insurance polices.