Back drafting is the condition where flue gas enters the living space rather then exit through the venting system of a combustion appliance. The majority of heating appliances and water heaters use room air rather then draft fans. Back drafting is one of the conditions that a professional home inspector will look for while performing your home inspection.
All heating appliances have a smoke or vent pipe which is used to discharge the products of combustion. Exhaust gas is hot and generally is lighter then room air so the gas exists through the flue with little problem. Back drafting can occur with a loss of indoor air pressure. This reduction of air pressure can be caused by operating a fireplace or exhaust fans in the home. With the loss of air pressure exhaust gas can enter the living space. Improperly configured smoke pipes and obstructed chimneys or smoke pipes can also cause the back drafting condition.
There are several things that home inspectors will look for to try to determine if back drafting is occurring.
– Combustion gas may leave a dark residue on the top of the boiler or water heater cabinet. The existence of soot is an indication of back drafting.
– A carbon monoxide detector can be used to determine if back drafting is occurring.
– A home inspector can hold a lighter under the draft diverter to determine if there is draw or pull to pull the flame upward.
– An inspector can use a smoke pen or stick to see whether it gets sucked into the duct or if it spills back into the room.
Some inspectors will turn on all the exterior drafting appliances to simulate a worst case scenario in the home.
Several types of drafting:
The majority of back drafting is caused by atmospheric draft. Exhaust gases rise through the smoke pipe only by the force of convection, which might not be strong enough to counter the pull from dips in indoor air pressure.
Induced draft: This type of system has a fan that creates a draft. The potential for back drafting is reduced because the draft fan is usually strong enough to overcome any competing pull from an indoor air-pressure drop.
Sealed draft: The combustion and venting systems are completely sealed off from indoor air. Combustion air is drawn in from the outdoors through a pipe that is designed for that purpose. The potential for back drafting is almost eliminated because the rate of ventilation is not influenced by indoor air pressure, and the vented gas has no pathway into the home.