What is a chimney fire and how does it happen?
Chimney fires happen when products of incomplete combustion (soot) build up in a chimney for a fireplace or wood stove. The build-up is called creosote. Creosote is usually black or dark brown and is readily combustible. When creosote burns in a chimney it can be explosive. Often you will see flames shooting out of the top of the chimney. Creosote can also burn slowly with no exterior visible flame and put out tremendous heat. Either form of chimney fire can spread to structural members of the house through loose mortar or by the warping of metal chimneys or just from the extreme heat radiating from the fire in a metal chimney.
How can I prevent creosote from building up in my chimney?
Maintenance is the key. You can’t completely prevent creosote since it is a natural result of combustion. That is why it is critical that your chimney be professionally cleaned at least once a year with normal use. There are certain things that people unwittingly do that actually accelerate creosote accumulation. Some people try to make the fire last longer by cutting off most of the air supply. This does make the fire slow down but it also increases creosote which is what smoke becomes as it deposits soot on the walls of the chimney. Some people also burn firewood that is "green" (high moisture content). Water, when vaporized, expands 1700 times in size and acts as a cooling agent. Burning green wood cools the fire and the smoke allowing creosote to build up faster. Remember these key points:
- Have your chimney professionally cleaned at least once a year (more often with heavy use)
- Burn only seasoned dry wood
- Keep your fires active and hot to keep the products of combustion moving up the chimney. Remember that the longer smoke is in the chimney the more likely it will build creosote on the walls of the chimney
- Never burn trash, plastic or paper in your stove or fireplace since this can ignite creosote in your chimney
What are the signs that a chimney fire is happening or that is has happened?
- The roar of a fire in the chimney (often heard in the wall of an upper floor from fireplace or stove or on a lower floor)
- Warping of metal anywhere in or on the chimney
- Creosote flakes and debris found on the ground, roof, or gutters
- Discolored or warped rain cap (at top of chimney)
- Scorching or burns in the roofing material from burning debris falling on the roof
- Visible cracks in the masonry of the chimney
- Smoke escaping through cracks in the chimney
- Smoke in the attic or visible soot build-up in the attic
What do I do if I have an active chimney fire?
- Get everyone out of the house and call 911