Which function does the airflow switch at the burner fan housing primarily serve?

• A. Controls the main gas valve
• B. Ensures the primary air fan is functioning
• C. Shuts down the boiler on low water
• D. Monitors flame sensor

Answer: (B)

The airflow switch is a safety check that confirms there is adequate air moving through the burner circuit before gas ignition proceeds. It monitors the draft created by the primary air fan, acting as a pressure- or flow-detection device. When the fan starts and proper airflow is present, the switch closes and tells the control system it’s safe to open the gas valve and light the burner. If there isn’t enough air (blocked vent, faulty fan, etc.), the switch stays open, and the burner won’t ignite, preventing gas buildup. This is about verifying the primary air supply, not about sensing the flame or low-water conditions.