Which component is used to sense air flow to control an HVAC system?

Which component is used to sense air flow to control an HVAC system?

• A. Sail switch
• B. Thermostat
• C. Humidistat
• D. Damper actuator

Answer: (A)

Sensing air flow to control an HVAC system relies on a device that directly responds to moving air. A sail switch uses a small vane placed in the duct; when air is flowing, the vane is pushed and closes a switch, signaling the control system that there’s adequate airflow and it’s safe to start the burner or blower. If airflow is missing, the switch stays open and stops the system to prevent unsafe conditions like overheating or gas buildup. The thermostat measures room temperature to decide when to heat or cool, not how much air is moving. A humidistat controls humidity levels, not airflow. A damper actuator moves a damper based on control signals but doesn’t sense the actual air movement by itself.

Sensing air flow to control an HVAC system relies on a device that directly responds to moving air. A sail switch uses a small vane placed in the duct; when air is flowing, the vane is pushed and closes a switch, signaling the control system that there’s adequate airflow and it’s safe to start the burner or blower. If airflow is missing, the switch stays open and stops the system to prevent unsafe conditions like overheating or gas buildup. The thermostat measures room temperature to decide when to heat or cool, not how much air is moving. A humidistat controls humidity levels, not airflow. A damper actuator moves a damper based on control signals but doesn’t sense the actual air movement by itself.