What is the most likely cause of a service call when a combustion gas odor is detected upon blower activation?

The detection of a combustion gas odor upon blower activation is commonly associated with a cracked heat exchanger. A heat exchanger is responsible for transferring heat from the combustion process to the air that is circulated throughout the building. If there is a crack, harmful combustion gases, such as carbon monoxide, can escape into the air stream instead of being vented outside the building. This situation poses a significant safety risk, as these gases can be hazardous to occupants.

When the blower activates, it increases the airflow, which can enhance the detection of these escaped gases. A properly functioning heat exchanger should not allow combustion gases to enter the indoor air; therefore, the presence of an odor indicates a likely failure in this component.

In contrast, while a problem in the air circulating or distribution system, a plugged vent or chimney, or a dirty filter may lead to various issues such as decreased efficiency or airflow problems, they are less likely to directly result in the presence of a combustion gas odor in the air. The bedrock concern in this scenario is the integrity of the heat exchanger, making it the most plausible cause of the detected odor.