The heat exchanger in a gas furnace is the component that separates the combustion gases from the heated air that enters your living space. Hot combustion gases flow through sealed metal chambers while return air flows over the outside of those chambers, picking up heat without ever contacting the exhaust gases.
A cracked heat exchanger is a serious safety hazard. If the metal develops cracks or holes, carbon monoxide from combustion can mix with the supply air and circulate throughout the home. Carbon monoxide is odorless and colorless, making it undetectable without a CO detector.
HVAC technicians inspect heat exchangers annually using visual inspection, combustion analysis, and sometimes smoke tests or camera inspection. A cracked heat exchanger typically requires furnace replacement, as repair is rarely cost-effective. Every home with a gas furnace should have working carbon monoxide detectors on every level.