Heat exchanger
The metal chamber inside a furnace that transfers burner heat to your home's air without mixing in exhaust gases.
The heat exchanger is the heart of a gas furnace: a set of sealed metal chambers where the burners fire, getting hot while household air blows across the outside of the metal and picks up that heat. Critically, it keeps the combustion byproducts — including carbon monoxide — separated from the air you breathe, venting them out the [flue](/glossary/flue) while only clean heat reaches your ducts. Over years of heating and cooling cycles the metal can fatigue and crack, which lets exhaust leak into the airstream; that's why a cracked heat exchanger is a safety condemnation that usually means furnace replacement rather than repair. It needs adequate [combustion air](/glossary/combustion-air) to burn cleanly, and its lifespan is a big factor in a furnace's overall [service life](/glossary/service-life). Learn more in our [furnace replacement cost](/guides/furnace-replacement-cost) guide.