A bimetal fan control is malfunctioning and not turning off the indoor blower motor. What is a possible cause?

A bimetal fan control operates by using the expansion and contraction of two different metals in response to temperature changes. When functioning correctly, it opens or closes contacts to control the operation of the indoor blower motor based on the heat exchanger temperature. If the fan control contacts are stuck closed, it would prevent the blower motor from turning off, as there’s no interruption in the electrical circuit supplying power to the motor.

This malfunction could be caused by various factors such as wear and tear, failure of the bimetallic strip due to overheating, or a mechanical failure that prevents the contacts from opening. Recognizing that this specific failure leads to the continuous operation of the blower motor is critical in diagnosing issues in forced-air heating systems.

The other options, while they may cause other issues or symptoms in the heating system, do not directly lead to the constant operation of the blower motor as described. For instance, a malfunctioning thermostat could fail to signal the fan control properly, but it wouldn't cause the contacts of the fan control to remain closed. A defective blower motor would typically result in no operation, and a blocked furnace filter could restrict airflow but would not inherently cause the fan control to malfunction in such a manner.