Lennox 63W40 2-Stage Pressure Switch 0.20/0.40

The Lennox 63W40 is a 2-stage pressure switch, an essentialpart of many Lennox gas furnaces and some other compatible HVAC systems. Just picture a small air pressure checkpoint for your furnace. This switch checks the air pressure in the flue, the pipe that carries the combustion products outside the home. It contains two internal switches, each set to trigger at specific air pressure levels, 0.20" and 0.40" WC. When your furnace is on, the burner lights up and hot gases pass through the heat exchanger where the air inside is warmed. Hence this heated air has to rise naturally and go out through the flue.