Venting Standards for Gas Appliances

Furnaces and Water Heaters

• Gas furnaces and water heaters operate unattended for long periods, often while people are sleeping, so safe venting is critical. Older furnaces use naturally drafting vents that rely on the buoyancy of exhaust gases for an adequate flow of exhaust through a vertical chimney. Newer furnaces use fan-assisted venting and may vent horizontally, but exhaust ducts should be metal and not plastic. With both furnaces and water heaters, the appliance's heat exchanger should be checked regularly for proper operation; a malfunctioning heat exchanger can result in carbon monoxide being released into the indoor air.

Clothes Dryers

• Because the exhaust produced by a gas dryer contains a high level of moisture and particulate matter in addition to harmful carbon monoxide, these appliances should always be vented to the outside. The dryer vent duct should be at least four inches in diameter and not exceed 25 feet in length. Ducts should be constructed of rigid metal pipe or flexible metal tubing with a smooth interior, and joints between segments of ductwork should be wrapped with metallic foil tape.

Ranges and Ovens

• Gas ranges and ovens do not necessarily need to be vented to the outside; ranges with re-circulating vents draw in air from the range top and filter it before returning it to the room. The vent filters remove odors and particulate matter, but they do not remove significant amounts of moisture or carbon dioxide. Cooking flames are typically not used for long enough periods to cause a dangerous buildup of carbon monoxide, but gas ranges or ovens should never be used for heating if a power failure occurs.

Fireplaces

• Conventional gas fireplaces must vent through a chimney in the same way that wood-burning fireplaces are vented; gas fireplaces vent through a four-inch diameter pipe, called a B-vent pipe, which must vent to the outside above roof level. Direct vent gas fireplaces draw air from the outside through the outer layer, a double-layer vent pipe, and then vent exhaust through the inner layer of the same pipe; direct vent fireplaces may vent through a wall below roof level. Vent-free gas fireplaces are not vented to the outside but are manufactured to meet minimum safety standards for indoor air quality.