What control prevents a duct heater from operating unless there is adequate supply air?

The fan interlock is a critical safety feature used in duct heaters that ensures the heater does not operate unless there is sufficient airflow present. This is important because without adequate supply air, the heater could overheat and cause damage or create a fire hazard. The fan interlock effectively prevents the heating element from turning on until the fan is running properly and air is flowing through the ductwork. This mechanism helps maintain the safe and efficient operation of the heating system.

Other controls, such as temperature sensors, pressure switches, and airflow sensors, may monitor different aspects of the system, but the specific role of the fan interlock is to directly relate the operation of the heater to the presence of airflow, making it the most appropriate choice for this scenario.