Anything that disrupts the burning process burners or results in a shortage of oxygen or burners, can increase carbon monoxide production. Wood, coal, and charcoal fires always produce carbon monoxide, as do gasoline engines. Any appliance or implement which burns a fossil fuel. Incorrectly installed venting systems and chimneys, chimneys plugged by bird nests or tree leaves, deteriorating chimneys, chimneys too short to vent correctly, appliances with no venting system, and house air flow patterns and downdrafting can all cause vent failure. Downdrafting, also called backdrafting, is flow reversal in a vent pipe or chimney.
Outside air enters the house through the vent.
During downdrafting, hot combustion gases spill into the house instead of flowing up the vent. Anything that moves air out of a house and depressurizes the house can cause downdrafting. This includes exhaust fans, any vented heating appliance, fireplaces, even holes in the ceiling. Vents in tight homes are especially susceptible to downdrafting. Warm air is lighter than cold air and normally rises.