CHIMNEY FIRES

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Your best prevention is regular care by a chimney sweep. Here's what to do when that precaution isn't enough.

One of the true challenges of parenthood is facing questions like, "Mommy, where do the pieces of tree you put in the woodstove go?" What do you say? Do you launch into a speech on oxidation of carbon, vaporization of water, temperature-induced draft and flue gas velocity? Might as well invoke gremlins. A more honest and effective answer would be to explain that much of the wood turns into goo — a substance resembling day-old oatmeal, except it's black — that sticks to the inside of the chimney.

That glop is, of course, creosote, the wood burner's bane. It's a nuisance to clean out and a hazard to leave alone. Touched off by the right combination of heat and oxygen, creosote can show a demonic mind of its own, turning a normally smoky chamber into an inferno. Flame temperatures in a chimney fire can exceed 2,000°F, sufficient to crack a flue liner or send showers of sparks onto the roof. More than 20% of all house fires are caused by woodburning appliances, and around half of those are related to chimney problems.

What Is It?

Creosote consists of particles and condensed vapors left over from the incomplete combustion of wood. It comes in many different forms — chimney sweeps identify types such as tar glaze, slag, flakes and soot — but all are combustible. Besides posing the threat of chimney fires, creosote is messy; it stains pipes, connectors, chimneys and roofs and corrodes metal parts of the chimney system. Some of its constituents may be highly carcinogenic.

The best way to combat creosote is to bum wood as completely as possible in one of the new clean-burning woodstoves. Look for a heater that has been certified by the state of Oregon's Department of Environmental Quality or the U.S. Environmental Protection Agency. However, an efficient stove is still no substitute for a good chimney system. The following suggestions will help any wood burner work more efficiently and safely.

Good Design

A properly designed and installed chimney will accumulate creosote more slowly than will an inadequate one, and it will be better able to withstand the ravages of a chimney fire, should one occur. If you're considering installing a chimney, take comfort in the fact that it doesn't cost a lot more to do it right; and if your present chimney doesn't measure up, consider the potential costs of neglect.

The basics. Safety dictates that a chimney used to vent a woodstove should be made of either tile-lined masonry with an air gap between tile and brick or insulated stainless steel, and it should serve only the stove. All openings other than the stove's connector and the cleanout (which should have a tightfitting cover) should be sealed. Air leaks make it impossible to control a chimney fire if one gets started.