Burning wood with a high moisture content compromises both efficiency and safety. Wood is considered wet when its moisture content is above 20%, requiring significant energy to dry out before it can effectively combust. This drastically reduces the fire’s heat output and creates hazards, including local air pollution and a serious risk of chimney fires.
The Energy Cost of Evaporation
The most immediate consequence of burning wet wood is the drain on the fire’s potential heat energy. Before the wood can ignite and release its stored energy, the fire must first heat the liquid water trapped inside the wood fibers to its boiling point and convert it into steam. This process consumes an immense amount of thermal energy known as the latent heat of vaporization.
This energy expenditure means that a substantial portion of the heat generated by the fire is used simply to dry the fuel, rather than warming the surrounding space. For every pound of water that must be vaporized, approximately 1,200 British Thermal Units (BTUs) of heat are consumed and lost up the chimney as steam. The result is a fire that struggles to achieve the high temperatures needed for complete combustion, dramatically lowering the net heat delivered into the home.
Consequences of Incomplete Combustion
Because of the energy wasted on evaporation, the fire’s temperature remains too low to properly break down the wood’s complex chemical structure, leading to incomplete combustion. When wood burns at a lower temperature, it fails to fully ignite the released volatile gases and chemical compounds. This failure is visible as heavy, dense smoke, often appearing white, gray, or black.
The visible smoke contains uncombusted carbon particles (particulate matter) and various harmful volatile organic compounds (VOCs). These substances represent fuel that was not converted into heat, resulting in a smoldering fire. Releasing this dense smoke contributes to local air pollution and can cause respiratory irritation for people nearby. Seasoned wood burns hot enough that these gases are consumed, producing minimal visible smoke once the fire is established.
The Danger of Creosote Buildup
Incomplete combustion caused by wet wood creates a hazard within the chimney system. When the fire is cool, unburned gases and organic compounds escape up the chimney flue. As these hot gases mix with the cooler air inside the flue, they rapidly cool and condense.
This condensation deposits a highly flammable, tar-like residue known as creosote onto the inner walls of the chimney. Burning wet wood accelerates the formation of dangerous creosote, which can be hard, shiny, and glazed. This glazed form ignites easily and is the primary fuel source for a chimney fire. Regular use of wet wood rapidly builds up this flammable layer, increasing the risk of a high-temperature fire that can damage the chimney structure.