What is creosote, and how does it form? (And how can I reduce it?)

Creosote is the byproduct of the incomplete combustion of wood. It forms in the chimney by two methods. One is called the aerosol method in which the smoke comes in contact with the sides of the chimney, much like spray paint on a wall. The other method is condensation, where temperatures in the chimney fall below the condensation point of the smoke. Creosote initially forms as a liquid then dries to a tar or glaze type consistency. With enough heat it then goes through pyrolysis, which breaks down the creosote, driving off alot of the volatiles, turning it into a brushable flaky consistency. Without adequate heat in the chimney, the creosote will continue to form in the liquid to glaze consistency never completing its cycle of development. Knowing these facts helps to understand the proper installation and use of your appliance. The faster the smoke is up and out of the chimney the less time it has to come in contact with the sides and the less heat loss and less chance of condensation. What this basically means is the chimney should be properly sized to the appliance and that burning smaller, hotter fires will help reduce the amount of creosote build up. It is important to note that heat output from your appliance should be controlled by the amount of fuel as much or more than the combustion air to the fire.

The three things governing the amount of creosote build up are:

1. Time of smoke in chimney -- Oversized chimneys and lower interior temperatures create slower drafts
2. Temperature of interior of chimney -- Oversized chimneys, exterior chimneys and smoldering fires reduce temperature of smoke
3. Density of smoke or tar fog -- Smoldering fires increase the density

Back to FAQ's