What is the most likely cause of a service call reporting a combustion gas odor when the circulating blower activates?

A cracked heat exchanger is a leading cause of a combustion gas odor when the circulating blower activates. The heat exchanger is responsible for transferring heat from the combustion gases to the air circulating through the home. If the heat exchanger is cracked, it can allow combustion gases, which may include carbon monoxide and other harmful substances, to leak into the air stream that is then distributed throughout the living space.

When the blower activates, it enhances the airflow and can draw these gases into the ductwork, making the odor noticeable during operation. This scenario presents a serious safety concern, as exposure to carbon monoxide is hazardous to health. Hence, identifying a cracked heat exchanger during such service calls is critical for the safety and well-being of the occupants.

In contrast, a malfunctioning thermostat primarily affects the performance and timing of system operation but would not directly lead to combustion gas odors. A clogged air filter can restrict airflow but typically does not cause combustion gas leaks. An improperly installed duct system might lead to airflow issues or drafts, but it would not specifically be responsible for combustion gas odors when the blower operates.