A gas-fired furnace needs oxygen to fuel burner combustion and a means to convey hot, hazardous combustion byproducts safely out of the house. The furnace flue is the pipe that exhausts these gases away from living spaces. Usually a galvanized metal pipe sized to accommodate the furnace's output of British thermal units, the flue connects to the furnace combustion chamber and is typically routed up through the ceiling and attic, terminating at a hooded vent on the exterior of the roof.
How the Flue Works
In a conventional, noncondensing furnace, the convection of hot gases in the flue creates a "chimney effect" that continuously pulls gases up the flue and out of the house. Common byproducts of burning natural gas include nitrogen dioxide, sulfur dioxide, water vapor and carbon monoxide. The last component is the reason an intact, unobstructed furnace flue is critical for safe heating. Carbon monoxide gas is harmful and may be deadly in sufficient concentrations.
Why Good Flues Go Bad
The integrity of a furnace flue can be compromised by leaks at joints between the segments, corrosion due to exposure to water vapor and obstructions caused by bird's nests or other debris. Negative indoor air pressure caused by insufficient furnace air supply or induced by exhaust fans may also reverse the flow in the flue. Called backdrafting, outdoor air is sucked into the flue and dangerous carbon monoxide pulled back into the home. An inspection of the furnace flue as well as verifying proper combustion and indoor air balance should be part of an annual furnace checkup by a heating, ventilation and air conditioning technician.