All fuel-burning appliances require air for combustion and for diluting and exhausting the products of combustion out of the house. If there is not enough air, the chimney or flue could reverse its flow and backdraft or spill dangerous gases back into the house (see Figure 9-5).

Backdrafts may be caused by competition for air. For example, a powerful kitchen fan, a barbeque-style down draft range or even a roaring open fireplace exhausts air from the house. The resulting negative pressure can pull air into the house down chimneys or vents.

Signs of combustion air problems include

The first line of defence is to replace spillage-susceptible space and DWH equipment with direct vent or sealed combustion appliances. For example, electricity, or better yet, consider heat pump technologies.

Conventional open fireplaces can be a wonderful feature of a home, but they are also responsible for leaking heated air to the outside and are prone to backdrafting. When at the end of their burn cycles, fireplaces release large quantities of carbon monoxide and are also more vulnerable to backdrafting at this point. Minimize this problem by installing tight-fitting glass doors and consider opening a window slightly when you operate the fireplace.