Preventing Chimney Fires

Keeping chimneys clean

Dirty Business

As mentioned, dirty chimneys—or, more properly put, dirty flues—are prime breeding grounds for chimney fires. But it's not just the ashes and wood particles that travel through chimneys that make them dirty. Creosote, a natural by-product of burning wood, makes them dirty, too, and is at the heart of most chimney fires.

Wood smoke consists of tar acids, organic vapors, and water. When it goes up into a chimney, it wafts over cooler surfaces. As it does, the substances it contains condense and forms a residue—creosote—that sticks to the inner walls of the chimney.

How much creosote forms and how quickly it forms depends on the following:

• How thick the smoke and fumes are

• How hot the fire burns

• The temperature of the stove pipe or flue

Cooler temperatures and slow-burning fires cause greater buildup.

Creosote is pretty unsightly stuff. It can be black or brown, crusty or tar-like, and drippy, or even shiny and hard. Look up inside your chimney, and you'll probably see more than one form of it. But appearance is secondary. What's most important to know is that all forms of creosote are highly flammable. Even small amounts of it can cause a fire. Let it build up over time, say over a winter or two, and there will be enough to fuel a long, hot fire that can destroy your chimney -- and possibly your home.

Getting rid of creosote buildup in its entirety isn't possible. Moisture and cooler temps produce the gases that form creosote. All wood contains moisture and no fire can burn bright and hot forever. But you can keep creosote in check by using wood-burning appliances correctly, and by taking good care of them.

As mentioned, every fire produces creosote, and it builds up in every chimney. However, certain factors can make it worse. They are…

• Poor air supply. Fires need oxygen to burn well. On fireplaces, not opening the damper wide enough or closing glass doors restricts the amount of air that flows into the fire. When this happens, the heated smoke from the fire can't travel as quickly up the chimney as it should. The longer the smoke stays in the flue, the more creosote it forms. With woodstoves, stopping down the damper or air inlets too soon or too much or not using the stovepipe damper correctly can cause the same problems.

• Green wood. Green wood is higher in moisture than seasoned wood is. Burning it takes a lot of energy, which results in a cooler-than-normal fire that doesn't burn very efficiently.

• Cool chimney temperatures. Creosote-creating condensation forms faster in exterior chimneys that are exposed to the elements than in chimneys that run through the interior of a house. Packing woodstoves tightly for an all-day burn creates large, cool fires instead of small, hot ones. When fires burn cool, chimneys can't heat up like they should.

• Burning things other than wood. Plastic substances in things like boxes, wrapping paper, and trash can emit a corrosive acid that can worsen an existing creosote problem.

Woodstoves, fireplaces, and the chimneys they're attached to cause an estimated 100,000 residential fires every year and some 200 deaths. Makes you want to avoid one at your home, doesn't it? What's more, chimney fires are not only deadly, they're expensive. A good-sized chimney fire can warp a metal chimney and crack or break the tile liner on masonry chimneys.

These are major repairs, and they should be made before using the chimney again. Let even a small crack or hole in a flue go unfixed, and the next chimney fire could be much more serious. Sparks from a regular fire could float into the attic or framework near a chimney and cause a house fire.

As already mentioned, there are two basic ways to keep chimney fires from happening: using fire-burning appliances correctly and keeping chimneys clean.