Gas Heat ESCO Practice Exam

How does a bi-metal fan switch operate the blower motor on a gas furnace?

1. Current flow/Current off

2. Pressure increase/Pressure decrease

3. Temperature on/Temperature off

4. Voltage increase/Voltage decrease

The correct answer is: Current flow/Current off

The operation of a bi-metal fan switch in a gas furnace is primarily based on the temperature change within the furnace. As the furnace heats up, the bi-metallic element within the switch responds to this temperature change, causing it to bend or flex. When the temperature exceeds a certain threshold, it completes the circuit, allowing current to flow to the blower motor, which then activates and starts moving air throughout the space. The correct choice reflects the relationship between temperature and the operation of the fan switch. When the temperature is high enough, it signifies that the blower should turn on ("temperature on"), and when it cools down, the switch will open, stopping the current flow to the motor ("temperature off"). This ensures efficient operation of the heating system, managing when the blower motor should run based on the heating needs of the furnace. In this context, the other options don’t accurately describe how the bi-metal fan switch operates. For example, pressure changes are not a factor in straightforward fan operation; instead, the system relies mainly on temperature changes to control the switching mechanism. Similarly, while voltage changes might affect motor operation, they aren't the primary function of a bi-metal switch; it's fundamentally about temperature regulation to activate the blower motor.