Is There a Time Bomb in Your Chimney?

Fireplaces and wood-stoves are designed to safely contain wood fuelled fires, while providing heat for a home. The chimneys that serve them have the job of expelling the by-products of combustion (substances given off when the wood burns). These substances include smoke, water vapour, gases, unburned wood particles, hydrocarbon volatiles, tar and assorted minerals. As these substances exit the fireplace, 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 tar or creosote.

The build-up of tar in your fireplace, wood-stove, and chimney is unavoidable. A natural by-product of the wood burning process, tar forms a black/brown crusty, powdery, flaky, tar like, drippy and sticky or hard and shiny glazed coating on the inside of your chimney. It is not uncommon to see all forms of tar in one flue system. Whatever form it presents itself, creosote is highly combustible and a potential fire hazard: it’s the primary fuel in most chimney fires.

During a chimney fire, the outside surface of the chimney can become hot enough to ignite the surrounding walls, floor joists, rafters, insulation, or roofing materials. Suddenly, this can develop into an uncontrolled structure fire.

Even without a chimney fire, creosote and soot can reduce the draft and diminish the efficiency of your heating system.

Certain conditions encourage the build-up of tar. Restricted air supply, unseasoned/wet wood and cooler than normal chimney temperatures are all factors that can accelerate the build-up of tar on flue walls. Air supply on fireplaces may be restricted by  failure to open the damper wide enough to move heated smoke up the chimney rapidly or operating a stove with an unlined chimney [The longer the smoke lingers in the chimney, the more likely it is that tar will build up in the flue].

Burning dry, seasoned, wood allows for higher burning temperatures. If the wood is not seasoned, energy is used to initially drive off the water trapped in the cells of the (unseasoned) logs, which also results in cooler smoke temperatures.

Burning hot fires with dry, seasoned, wood can ultimately help lower the amounts of tar accumulation.