CHIMNEY FIRES can burn explosively. Flames or dense
smoke may shoot from the top of the chimney. Described as
sounding like a freight train running at high speeds.
Chimney fires typically can be
caused in three ways:
1. Creosote deposits
ignite and burn at very high temperatures,
from combustibles is not adequate, and
3. Pyrolysis, a
chemical composition of the home framing that is changed by
the heat and is capable of spontaneously igniting.
Sometimes chimney fires burn unnoticed by the occupants of
your home because they as they smolder, they are slow burning
fires and sometimes spread into the home. 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 both to chimney structures
and ignite other structural or flammable sections close to the
house. Other factors such as bird's nest, inadequate clearance
to combustibles contribute to structural chimney related
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
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.
glass doors may restrict the air supply on fireplaces 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
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.
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.
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
Chimney fires don't have to happen. Here are
some ways to avoid them:
woods only (dryness is more important than hard wood versus
hotter fires that bum more completely and produce less smoke,
Never burn cardboard boxes, wrapping paper, trash or
Christmas trees; these can spark a chimney fire,
stovepipe thermometers to help monitor flue temperatures where
wood stoves are in use, so you can adjust burning practices as
Have the chimney inspected and cleaned on a regular