"vent position" would be just switching the burner itself off, as the blower is on if the engine is on (direct mechanical drive). That main 3-pos switch controls the engine. The burner is operated on a separate circuit, which has safety sensor interlocks for high temperature, airflow, and presence of flame. Lack of any of those will cutoff the power to the ignition coil and fuel solenoid for the burner itself.It doesn't have a vent position? To vent the burner chamber? Is the flame controller a photo electric device?