How do outdoor combustion air intakes work?

expert EXPERT April 26, 2007

In the 1970's, supplying outdoor combustion air for fireplaces became popular. This was done -

• to make fireplaces more efficient, and
• because modern tighter construction made it harder to supply air to fireplaces.

Houses are becoming better sealed, so that less outdoor air can get into the house. Appliances such as furnaces, water heaters, and exhaust fans compete for house air. This can starve fireplaces, causing poor draft and smoking problems.

Some jurisdictions require a separate combustion air intake for fireplaces. Atypical configuration includes a screened outdoor air intake and a 2- to 4-inch diameter round metal duct terminating at or near the front of the firebox floor. In some areas the duct area must be between 6 and 55 square inches. For a round duct, this is 13/8 inch to 12 inch diameter. This is quite a range! A 3-inch duct is typical. In the firebox, the inlet may be screened and hooded to prevent embers getting into the duct.

The duct is often insulated to prevent condensation as the cold outdoor air enters the duct, cooling the warm, moist house air around it. Without insulation, considerable condensation can develop on the outside of the duct. The insulation also helps to shield combustible materials from the heat that can travel through the duct.

In many cases, the combustible clearance required from the duct is 2 inches for at least the first 3 feet of duct measured from the firebox.

The outdoor intakes not only have to be screened, they should be high enough so they won't be blocked by snow. They should be oriented to minimize the effect of wind pressures on the duct. Many intakes face straight down or are hooded.

The duct should include a damper that can be closed from inside the room near the fireplace. In many cases, the hood can be pushed down flush with the floor to close the air intake.

In the early to mid 1990's, concern has arisen with respect to the wisdom and safety of outdoor combustion air intakes. In some situations, particularly as the fire dies down, exhaust gases have been found to be leaving the house through the combustion air intake, rather than the chimney. As the chimney cools, the combustion air intake sometimes becomes a better chimney than the chimney itself! This can be a fire hazard. Some areas are considering removing the requirement for outside combustion air for fireplaces and perhaps even prohibiting it.