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 preventable.
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
wood burns.
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 walls.
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 elements.
Masonry 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 combustibles.
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.
From Starting One You Don’t!
Chimney fires don’t have to happen. Here are some ways to avoid them:
·
Use seasoned woods only (dryness is more important than hard wood
versus soft wood considerations);
·
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.
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 chimney fire?
Here are the signs a professional chimney sweep looks for:
·
“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 chunks missing;
·
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.
If you think a chimney fire has occurred, call a CSIA Certified Chimney
Sweep TM 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.
If you realize a chimney fire is occurring, follow these steps:
1) Get everyone out of the house, including yourself.
2) 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:
1. Put a flare type chimney fire extinguisher into the fireplace or wood stove.
2. Close the glass door on the fireplace.
3. Close the air inlets on the wood stove.
4. Use a garden hose to spray down the roof (not
the chimney) so the fire won’t spread to the rest of the structure.
5. 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 CSIA Certified Chimney Sweep TM to inspect for damage.
Chimney fire damage and repair normally is
covered by homeowner insurance polices.
Copyright ©1994-1999 Chimney Safety Institute of America