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
Chimney fires can burn explosively -- noisy and dramatic enough to be detected by neighbors or
passersby. 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 air
plane. 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.
Creosote and Chimney Fires
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.
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 the 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.
How Chimney Fires Damage Chimneys
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 2000' F) 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 prefabricated
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
Ways to Avoid Chimney Fires
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
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
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.
Have your solid fuel venting system inspected annually, and
have it cleaned and repaired whenever needed.
Your chimney sweep may have other maintenance recommendations depending on how you use
your fireplace or stove.
The Chimney Safety Institute of America recommends that you call on certified chimney
sweeps, since they are regularly tested on their understanding of the complexities of
chimney and venting systems.