The heat exchanger is the metal chamber inside a gas furnace where combustion heat is transferred to the room air without mixing the two. Fuel burns inside the heat exchanger, hot flue gases circulate through internal passages, and air being pushed across the outside of those passages absorbs the heat.
The metal wall between the combustion side and the air side is critical: if it cracks, dangerous combustion byproducts (carbon monoxide, nitrogen oxides) can leak into the supply air and into your home.
Any HVAC tech inspecting a gas furnace should check the heat exchanger for cracks each year — typically with a borescope camera or pressure test. Working CO detectors are also essential in any home with a gas furnace.
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