Gas furnaces require an exhaust system. Like other gas-powered appliances, they consume gas. They burn gas – typically natural gas – inside of a heat exchanger to create warm air. Exhaust gases, however, will be created as a byproduct of this process. If you have a gas furnace, it will create exhaust gases during use. An exhaust system will ensure that these exhaust gases are safely routed to the exterior of your home rather than building up inside of your home.
Standard-Efficiency Furnace Exhaust Systems
Different types of gas furnaces use different types of exhaust systems. Standard-efficiency gas furnaces – furnaces with an Annual Fuel Utilization Efficiency (AFUE) rating of below 90 – use a traditional flue pipe. The flue pipe runs from the heat exchanger to the roof.
Exhaust gases will build up inside of the heat exchanger while the gas furnace is running. The pressure difference between the heat exchanger and the flue pipe will force the exhaust gases to enter the flue pipe. From there, the exhaust gases will continue to rise up and through the flue pipe. The flue pipe will typically have an exit point on the roof, which is where the exhaust gases will be released.
High-Efficiency Furnace Exhaust Systems
High-efficiency gas furnaces require an exhaust system as well, but most of them use a different type of exhaust system than their standard-efficiency counterparts.
Also known as condensing furnaces, high-efficiency gas furnaces feature two heat exchangers. They still burn gas inside of an initial heat exchanger. The exhaust gases from this initial heat exchanger, though, are transferred to a second heat exchanger where they burned again.
Exhaust gases typically consist of at least some unburned gas. This is particularly true for standard-efficiency gas furnaces. If you have a standard-efficiency gas furnace, it probably won’t burn all of the gas in the initial heat exchanger. Rather, it will leave behind some unburned gas as part of the exhaust gases.
High-efficiency gas furnaces are designed to squeeze more heat out of the gas. They will burn the gas in the initial heat exchanger, and then they’ll burn it again in a second heat exchanger. After burning the gas in two separate heat exchangers, high-efficiency gas furnaces will produce a liquid byproduct. In other words, they will convert the exhaust gases into liquid. This liquid byproduct will then be flushed through a drainage pipe that runs to your home’s exterior.