Corry, Bob. Understanding and Investigating Chimney Fires. Fire and Arson Investigator. March, 1992. pp. 22-24.

Abstract: This article discusses the formation and nature of creosote and how the substance starts chimney fires. It also describes the building codes concerning the installation of factory-built and masonry chimneys.

Creosote is an organic compound comprised of different pyrolytic decomposition compounds that change states as they cool while traveling up a chimney. The amount of creosote that condenses and accumulates on chimney walls is increased with low temperature fires because creosote will not reach its gas state and exit the chimney. Most fires in fireplaces and wood-burning stoves are ventilation-controlled to conserve fuel and control the heat production. This produces low temperature fires, which contributes to creosote build-up.

The article describes the conditions creosote requires to ignite. Most tests conclude that temperatures of 1,000 degrees F lasting for three to four minutes will ignite a fire in the flue. A chimney fire usually begins in the lower half of the chimney and burns upwards. The oxygen supply for the fire is usually the damper or the clean out door. The fire gradually burns towards the top as it gains more oxygen. A chimney fire will usually last 15 minutes, lasting longer if the chimney is larger.

To check a chimney for creosote levels, examine the chimney walls with a mirror and a flashlight. If the walls appear black and dull, the chimney is lined with soot and safe. If the walls are black and shiny, the walls are covered with creosote and should be cleaned professionally. The article describes the building codes concerning the installation of factory-built and masonry chimneys.

For more information, contact:
International Association of Arson Investigators (IAAI)
300 Broadway Suite 100
St. Louis, MO 63102-2808
Phone: 314-621-1966