Creosote is a chemical product that has many forms and uses. We will be specifically discussing wood-tar creosote and it's relationship with chimneys.
Creosote is formed in chimneys when wood (or other fossil fuels) is burned incompletely. The oils in the wood that are not burned properly are evaporated and travel up the chimney as part of the smoke. As the gases cool on their way up the chimney, they condense into an oily residue we know as creosote. This residue builds up on the inner walls of the chimney flue.
Creosote is a flammable substance and when heated to its ignition temperature and exposed to oxygen it will burn. This leads to chimney fires. Chimney fires can spread to the structure of the house in several ways, such as damage to the chimney caused by the fire itself of present at the time of the fire allowing heat or fire to escape the chimney enclosure. Sparks and embers can also be emitted from the top of the chimney and land on a nearby roof and cause a fire.
Creosote is generally formed from cooler burning fires, such as when burning unseasoned wood or when dampering the fire down to achieve longer burn times. With cooler burning fires comes cooler flue temperatures allowing easier condensation of creosote in the chimney. This is why burning properly seasoned wood is so important. Hot fires promote complete burning of the wood as well as higher flue temperatures that are able to induce sufficient draft to expel the products of combustion before they have cooled enough to condense.
Creosote can present itself in several forms. Normally it is a liquid that condenses in the flue. It is an oily, sticky reside that coats the inside of the flue and can build up in thick layers. As it dries out (such as when exposed to heat) it can become more solid. It may become flaky or glazed. As it builds up it reduces the flue opening and slows the draft, creating cooler fires and more creosote generation. It is a cycle that will eventually lead to a blocked flue or chimney fire if not taken care of.
Creosote should be removed regularly by sweeping the chimney. A certified chimney sweep is the best choice as he or she will have the knowledge, skills, and tools to properly sweep and inspect your chimney. It is recommended that every chimney be swept at least once a year, and you should check for buildup several times throughout the heating season.
The typical cycle of creosote and chimney fires is predictable and completely preventable. First, creosote formation is encouraged in one or more of several ways including burning unseasoned wood, burning for long periods of time while dampered down, and using old inefficient or malfunctioning woodburning appliances. As the creosote builds up in the chimney it can start to affect the draft which leads to even more creosote buildup. At some point a fire is built hot enough to cause the creosote in the chimney to reach its ignition temperature. This hotter fire increases draft in the chimney bringing fresh oxygen in contact with the hot creosote and the creosote starts to burn. Thus a chimney fire is born. Remember, clean chimneys don't have fires!
Chimney fires will be discussed on another page, but keep in mind that any time you think you are having a chimney fire you should close all doors and air controls on your appliance, evacuate the house, and call the fire department. After the situation is under control you should have an experienced chimney professional inspect your chimney for any damage that may have been caused by the fire. Do not use the chimney until your chimney professional has inspected and passed the chimney.
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