CHIMNEY FIRES

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Safe clearances. Even the most effective chimney requires a clear air space (without insulation) of at least 2" to the nearest combustible material. All wooden framing members and sheathing should be held back from the outer surface of the chimney. This may seem unnecessary when you lay a hand on a brick chimney and realize that it's little more than warm to the touch, but the provision is meant to deal with the sorts of extremes that occur during a chimney fire.

Likewise, you should observe safe clearances for the stove and its connector. If these aren't listed on the appliance, consult a book on wood heating.

Creosote accumulation. The creosote accumulation rate in a chimney is affected by five factors: the density of the smoke, the temperature of the smoke, the amount of time the smoke spends in the chimney, the turbulence (swirling) of the flow and the temperature of the chimney walls. The first two are mainly matters of operating technique, which we won't discuss here, but the last three are directly affected by chimney design.

Chimneys should have about the same cross-sectional area as the stove's outlet, a size which offers a good compromise between smoke residence time and turbulence. Creosote problems often occur when a woodstove is connected to a fireplace chimney, which may be three or four times too large. Such chimneys should be relined to reduce their size.

Of the three chimney design factors, wall temperature may have the largest influence on creosote deposition. What's more, a warm chimney is a win-win situation. You'll get less creosote and more draft. The temperature of the chimney's inner wall is determined by the temperatures of the flue gases inside and the air outside and by the insulation value of the material between. Most important, an interior chimney — one that is wrapped in warm house air — stays clean much longer than one that runs up the side of the house.

Of the various types of chimney materials, insulated metal is most effective at retaining heat. The insulation may be either a special fiberglass and ceramic material or a sealed air space. (Avoid chimneys cooled by circulating air.) Masonry is less effective, though some reliners use an insulating refractory that has a considerably higher R-value than either brick or block.

An indoor connector between a woodstove and chimney isn't usually made with insulated pipe. This short section of single- wall material is a compromise between efficiency (since the pipe radiates heat that would otherwise go up the chimney) and creosote. As a general rule of thumb, keep uninsulated connectors to less than 6' in length. Use the minimum number of elbows with the most-gradual turns possible to allow smoke to flow smoothly up the chimney. And secure all joints with sheet metal screws.

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