When wood is burned the by-products of combustion go up the chimney. These include smoke and water vapor, as well as unburned wood particles and other things. As these hit the relatively cooler chimney, condensation occurs, like steam on a mirror. This creates a residue which gets deposited on the inside of the chimney walls.
This is known as creosote and is flammable.
Cleaning the chimney removed the creosote build-up so it can’t start a chimney fire.
There are several factors which contribute more or less to creosote build-up. Burning wet wood will create more creosote than burning dry wood. Hard woods create less generally than soft woods, as they burn hotter and increase the temperature in the flue and reduce condensation.
A wood stove or fireplace insert (wood stove in a fireplace) is normally burned differently than an open fireplace. In a normal fireplace there isn’t usually a way to control the rate of burn, so the wood burns rapidly, heating the chimney.
A wood stove or insert is normally used for heat, and there is usually an effort to conserve fuel. Plus a metal box weighing several hundred pounds can get pretty hot with a fire inside, so people understandably “damp” them down, causing the temperature in the inside of the chimney (“flue”) to be lower than in an open fireplace. This increases condensation and creosote build-up.
People often ask how often a chimney should be cleaned. There is no easy answer, as it depends on how much it was used, what kind of wood, etc.
We recommend that chimneys be inspected once a year for safety and structural reasons. If one is used regularly it could need to be cleaned yearly. We have some customers who burn a stove more or less around the clock in winter and have us clean it twice during the season.
Hope this has been helpful.
Oregon Chimney Repair and Cleaning, Inc.