The Lochinvar & A.O. Smith 100208384 Pressure Switch 0.85" WC is a genuine OEM component designed to monitor and verify proper air pressure within compatible gas-fired HVAC and water heating systems. It works by detecting the required 0.85-inch water column pressure and signaling the control board to allow safe burner ignition only when proper venting and airflow conditions are met. https://www.partshnc.com/085wc-pressure-switch-lochinvar-ao-smith-100208384