In a gas-fired warm air furnace, the furnace fan should be controlled by:

• A. Furnace bonnet temperature
• B. The high-limit safety control opens
• C. Starts at the same time that the burner comes on
• D. The room thermostat

Answer: (A)

The thing being tested is how airflow is timed in a gas-fired warm air furnace. The blower is driven by a temperature-activated switch tied to the heat exchanger area, not directly by the thermostat. A furnace bonnet (or blower) temperature switch monitors how hot the heat exchanger is and turns the blower on when the heat exchanger has warmed enough to deliver comfortable air. It also helps shut things down safely if the temperature gets too high. The high-limit safety control is a separate safety device that can shut off the burner if the cabinet overheats, not the normal driver of blower operation. The room thermostat simply requests heat; it doesn’t directly control when the blower starts. So, using the bonnet temperature to control the fan ensures the system starts blowing warm air only after adequate heat has been produced, providing better comfort and protecting equipment.