What to know about chimney fires

Ontario Municipal Fire Prevention Officers Association
Fireplaces and wood stoves are designed to safely contain woodfuelled 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. Certain conditions encourage the build-up of creosote: restricted air supply, unseasoned wood, and cooler-than-normal chimney temperatures are all factors that can accelerate the build-up 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 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 build-up. 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 centre of a house and exposes only the upper reaches of the flue to the elements.