The following article is courtesy of HouseMaster Home Inspection.
Dirty fireplaces and solid-fuel burning stoves are a major cause of chimney fires. Defective stoves, improperly constructed fireplaces, and faulty heating equipment can also lead to chimney fires. So can the improper use of any fuel-burning equipment.
Chimney fires can burn explosively with flames or dense smoke shooting from the top of the chimney. They can be noisy enough to be heard, creating a sound sometimes described as being like a rumbling freight train or a low-flying airplane. More often than not, however, chimney fires don't get enough air or have enough fuel to become visible. But the temperatures reached within the chimney can still be hot enough (around 2000° F, 1093° C) to cause damage to the chimney and ignite adjacent combustible house components – even without direct exposure to flame.
Fireplaces and wood stoves are designed to safely contain the wood-burning fuel, while providing heat for a home. The chimneys they are connected to are also designed to safely expel the by-products of combustion. As these by-products exit the fireplace or wood stove, and flow up into the relatively cooler chimney, condensation occurs. The resulting residue – creosote – sticks to the inner walls of the chimney. Creosote is typically black or brown in appearance. It can be crusty and flaky; tar-like, drippy and sticky; or it can be shiny and hardened. Often, all these forms will occur within one chimney system.
Whatever form it takes, creosote is highly combustible. If it builds up and catches fire inside the chimney flue, the result is a chimney fire. Although any amount of creosote can burn, the greatest concern exists when the creosote has built up in sufficient quantities to sustain a long, hot, destructive fire. Generally a ¼ inch or more of creosote creates a high hazard situation; however, any buildup can be a problem.
Certain conditions encourage the formation of creosote. Restricted air supply, the use of unseasoned wood, and cooler-than-normal chimney temperatures are all factors that contribute to the buildup of creosote on chimney flue walls.
- Air supply – The air supply to a fireplace may be restricted by closed glass doors or by the failure to open the damper wide enough to allow heated smoke to move rapidly up the chimney (the longer the smoke stays in the flue, the more likely creosote will form). A wood stove's air supply can be limited by closing down the stove damper or air inlets too soon or too much.
- Burning unseasoned firewood – Burning green wood results in a cooler fire than if dried, seasoned wood is used. This means cooler smoke and greater potential for the byproducts to condense and buildup on the chimney interior.
- Cool flue temperatures – Condensation of the unburned by-products of combustion also occurs more rapidly in an exterior chimney (one with one or more sides exposed to the exterior from the base to the roof) than in a chimney that runs through the center of a house and exposes only the upper reaches of the flue to the elements. The colder the climate, the quicker the buildup.
Whether a masonry or factory-built metal chimney, if there is damage from a chimney fire, corrective action needs to be taken. One chimney fire may not cause outward damage to a chimney; but a second one can lead to a house fire. Enough heat can even be conduct through the wall of a perfectly sound chimney to ignite nearby combustibles.
Since chimney fires can occur without anyone being aware of them, and since damage from such fires can endanger a home and its occupants, all chimneys used for fireplaces and stoves – particularly those used often, should be inspected and cleaned on a regular basis.
Roof Sheathing Charred by a Chimney Fire
Note: These tips are only general guidelines. Since each situation is different, contact a professional if you have questions about a specific issue. More home safety and maintenance information is available online at www.housemaster.com.
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