During burner light-off purge, the ______ switch proves that the forced draft damper is fully open and purge air flow is adequate.

• A. Low fire limit
• B. Flame safeguard
• C. Pressure switch
• D. High fire limit

Answer: (D)

During burner light-off purge, the system must confirm that purge air is actually flowing and the damper is open before ignition can occur. The device that does this is the draft (or pressure) switch. It senses the pressure difference created by the forced-draft damper and confirms there’s enough air movement through the furnace. When the switch sees the required draft condition, it signals the control to proceed with ignition. If the draft condition isn’t met, the control blocks light-off to prevent ignition in an area with potentially flammable vapors.

Other switches serve different safety roles: a flame safeguard checks for flame once ignition has begun, and limit switches monitor furnace temperature to prevent overheating. Those do not verify purge air flow or damper position, so they don’t fulfill this particular function.