Creosote - Build-up in Chimneys

Build-up in Chimneys

Burning wood and fossil fuels at low temperature causes incomplete combustion of the oils in the wood, which are off-gassed as volatiles in the smoke. As the smoke rises through the chimney it cools, causing water, carbon, and volatiles to condense on the interior surfaces of the chimney flue. The black oily residue that builds up is referred to as creosote, which is similar in composition to the commercial products by the same name, but with a higher content of carbon black.

Over the course of a season creosote deposits can become several inches thick. This creates a compounding problem, because the creosote deposits reduce the draft (airflow through the chimney) which increases the probability that the wood fire is not getting enough air to burn at high temperature. Since creosote is highly combustible, a thick accumulation creates a fire hazard. If a hot fire is built in the stove or fireplace, and the air control left wide open, this may allow hot oxygen into the chimney where it comes in contact with the creosote which then ignites—causing a chimney fire. Chimney fires often spread to the main building because the chimney gets so hot that it ignites any combustible material in direct contact with it, such as wood. The fire can also spread to the main building from sparks emitting from the chimney and landing on combustible roof surfaces. In order to properly maintain chimneys and heaters that burn wood or carbon-based fuels, the creosote buildup must be removed. Chimney sweeps perform this service for a fee.

73% of heating fires and 25% of all residential fires in the United States are caused by failure to clean out creosote buildup. Since 1990 the number of creosote caused fires has decreased in the United States by 75%. This is partly due to the use of efficient wood-burning stoves that fully burn the volatiles in the smoke.