By definition creosote is a combustible deposit that originates from condensed wood smoke. It also includes tar, vapors, and other organic compounds. It’s a natural by-product of burning wood. Once inside the chimney, creosote usually under goes pyrolysis, a chemical alteration of the fuel molecules as a result of the application of heat.
The by-products of pyrolysis are gases and solid compounds. It takes on many different appearances. It can be sooty or ash like, sticky, tacky and runny tar glaze, dry honeycombs or curly flakes. It can also be dense, hard and shiny black tar glaze. Several variables that affect the amount of build-up deposited in the wood heating system are smoke density, flue gas temperature, and residence time.
The smokiest fires and the coolest chimneys produce the greatest amount of build-up. In addition over-sized equipment usually causes more build-up. Contrary to popular belief, burning seasoned hardwood does not eliminate build-up. Wood can actually be too dry.
Proper operation of the wood heating system is the single, most important contributing factor in minimizing build-up. Burning smaller fuel loads, refueling more frequently and avoiding slow, smoldering burns is also important.
Avoid over firing and keep internal flue gas temperatures under 900 degrees F. You want internal flue gas temperatures over 400 degrees F. The build-up must also be removed from chimneys because it can restrict the draft, corrode metals, cause foul odors and lower the systems heat transfer efficiency.