While a standard-efficiency gas furnace incorporates a single pipe to exhaust dangerous fumes such as carbon monoxide, high-efficiency furnaces with an AFUE (Annual Fuel Utilization Efficiency) of 95% use a two way pipe system. The standard efficiency furnace inducts air required for the combustion process directly from inside the house. However, in the two-pipe system in a high-efficiency unit, a second pipe is utilized to draw fresh outdoor air into the furnace for combustion instead of taking it from inside the house.
Why Choose a Two-Pipe System for High Efficiency Furnaces
While it’s technically possible to operate a high-efficiency furnace with just a single pipe to exhaust combustion emissions, in recent years, it has become preferable to choose a two-pipe system for high efficiency furnaces for the following reasons.
It reduces outdoor air infiltration
When a furnace uses indoor air for the combustion process, it can create a negative pressure condition indoors. Because outdoor air is positive pressure, this differential can cause infiltration of outdoor air into the house through structural leaks. This, in turn, can raise heating costs as the furnace runs longer cycles to compensate for infiltrating cold air. Outdoor air inducted into the furnace through a dedicated pipe helps reduce outdoor air infiltration and supports greater indoor comfort and lower heating costs.
It preserves combustion components
Indoor air typically contains vapor pollutants emitted by cleaners, detergents and aerosol products, as well as volatile organic compounds (VOCs) released by paints, carpets, and other household sources. When burned in the combustion process, residue from these pollutants may coat and deteriorate sensitive furnace parts like flame sensors and igniters. Outdoor air is typically cleaner and free of these pollutants.
It maintains air balance
Most houses have components that already directly exhaust indoor air to the outside, such as bathroom and kitchen vent fans. Water heaters frequently burn indoor air too. By using outdoor air for furnace combustion, this avoids adding another source that removes indoor air, thus helping maintain the indoor/outdoor air balance.