The vacuum switch is located inside the mechanical area of the appliance and will have a black or reddish-orange hose attached. This switch closes the feed circuit when negative pressure above .05″ WC is present in the firebox. This pressure switch is a safety device to shut off the feed motor if the exhaust or the heat exchanger system is excessively dirty, the venting is obstructed, or if the firebox door is not fully closed and properly sealed.
This is the newest style switch and replaces all previous vacuum switches.