What Happens to a Masonary Chimney That Has a Gas Furnance Venting Into it?

Incomplete Combustion

• In addition to venting exhaust gases, chimneys generate draft required for complete combustion. According to the Chimney Safety Institute of America, 10 cubic feet of air can provide enough oxygen to burn 1 cubic foot of natural gas. Cool chimneys can’t generate enough draft, resulting in incomplete combustion and the production of poisonous carbon monoxide, reports the Institute.

Condensation

• Most gas furnaces produce about 1½ gallons of water every hour. This water, a byproduct of combustion, is vented out the chimney. Because of the improved efficiency of modern gas furnaces, much of this water condenses inside the chimney instead of being vented out as steam. The water that condenses is usually acidic because it contains household contaminants like chlorine from bleach. It may also contain coal and oil contaminants if the chimney was ever used to vent an oil or coal furnace. This acid corrodes the mortar and brick of your masonry chimney and can eat through an aluminum flue liner. Condensation problems are even worse in masonry chimneys with oversized flues.

Symptoms

• Discovering crumbling mortar or wet and stained drywall near your chimney could indicate an oversized flue. Other symptoms include crumbling bricks, corroded metal vent pipes, peeling wallpaper or the discovery of white stains on the outside of your masonry chimney. The corroded mortar and brickwork may cause blockages in your flue. You may also discover a crumbly white powder in the vent pipes. This powder is a corrosive salt that forms from flue-gas condensates.

Solutions

• You can reduce problems with your masonry chimney by venting new and efficient gas furnaces into smaller 4- and 6-inch tile flue liners. Have a certified chimney sweep evaluate your system. In addition to installing a liner that’s the correct size, you may be able to rework the connector pipe between the furnace and the chimney to produce a better draft and reduce problems with condensation.