Chimney Creosote From Wood Burning
Burning wood in any manner, releases pollutants in the form of gases and particulate matter. Consequently, one of the primary dangers of wood burning in the fireplace is excessive chimney creosote buildup.
Chimney creosote is an acidic, foul smelling, gummy, and very flammable substance. It is formed when volatile gases emitted by the initial fire, “primary combustion,” combine and condense as they leave the chimney with the smoke, or “flue gas.” The flue gas consists of steam and vaporized but unburned carbon based byproducts (vaporized creosote). As the flue gas leaves the fireplace, it drifts up into the relatively cooler flue. If the smoke is cooled to a temperature below 250 degrees F, the carbon particles in the warm vapor will liquefy, combine, and solidify to form creosote.
Creosote can also form when the smoke is vented into a chimney that is too large for the fireplace. This increases the smoke’s “residence time” and lowers air draw. This is especially prevalent in older chimneys. In such conditions, creosote builds up rapidly because the large air space cannot heat up sufficiently. Relining a large chimney will increase heat output and efficiency by 200%, and reduce creosote condensation to minimal levels. With a reduced volume of air inside a smaller flue, the temperature will stay much hotter and cause a stronger draw that will let the fireplace burn hotter.
The smoke’s “residence time” within the flue is a factor also. The longer it is in the flue, the more likely creosote will form. If a fireplace’s air supply is restricted by closed glass doors or by not opening the damper wide enough to let heated smoke move up the chimney, creosote will condense rapidly.
A final source is the burning of wet wood. Its combustion creates less heat and fails to consume most of the wood. Because much energy is consumed initially just to drive off the water trapped in the logs, the resulting fire and the flue are cool. The “smoke” of green wood is consequently filled with unburned creosote that quickly condenses within the flue.
Chimney creosote takes several forms, which are all combustible. It can be the fine black dust called soot, (1st stage); or a hard layer coating the chimney liner that is porous and crunchy, (2nd stage); or it can be a tar-like liquid, drippy and sticky, until it hardens into a shiny glaze, (3rd stage). All forms of creosote can occur in the same chimney. If it builds up in sufficient quantities and ignites within the chimney, a volcanic chimney fire can erupt.
The best measures for preventing chimney fires are:
- Using a high efficiency heater insert that wholly burns the carbon volatiles in the smoke.
- Using dry, well seasoned wood.
- Inspecting and cleaning the chimney yearly.
- Installing proper chimney insulation and lining.
Chimney creosote is a hazardous substance that must be controlled in order to prevent a chimney fire and let homeowners enjoy their fireplaces safely.