To prevent soot and Creosote build up your chimney should be swept annually or before your first fire of the season, log burning stoves every three months when in use. What is Creosote?​ Creosote is a flammable and corrosive substance that can build up on the walls of your fireplace and chimney. Creosote is formed when unburned wood particles, fly ash and other volatile gases combine as they exit the chimney. If there’s a poor draft, these unburned particles and gases can condense and build up on the walls of your chimney. Over time, these creosote or soot buildups can become a danger as they can potentially result in a chimney fire. Causes of Chimney Creosote Problems "Smoke" or better termed, flue gas, is released by the initial fire, or primary combustion. Burning wood, no matter how it is done, releases pollutants in the form of gases and particulate matter. Flue gas, comprised of steam and vaporized but unburned carbon based byproducts (vaporized creosote). If smoke exiting the chimney is cooled to below 250 degrees Fahrenheit, the gases liquefy, combine, and solidify to form creosote. A flue too large for the woodburning appliance may increase the likelihood that creosote will build up.