Furnace Air Pressure Switch

A furnace pressure switch is a safety and control device that monitors air pressure (often negative or slight vacuum) in the furnace’s venting / flue / inducer system to ensure proper airflow before allowing ignition. If the airflow (draft) is insufficient — due to blocked venting, faulty inducer motor, or leaks in tubing — the pressure switch will prevent the furnace from igniting or will shut it down mid-cycle. It is typically mounted near the inducer motor or in the blower compartment. It connects via small flexible tubing to sense pressure (vacuum or slight positive pressure) created by the inducer blower. Its internal mechanism (diaphragm, spring, contacts) responds to changes in pressure, opening or closing electrical contacts in the furnace control circuit. In many furnace systems, the control board uses the switch’s state (open/closed) as a gating condition: the blower must achieve the required pressure before ignition is permitted. (If the pressure switch does not close, the furnace will lock out for safety.) How to Choose Best HVAC Systems +3 A. O. Smith +3 Aire Serv +3 Because this part is in the safety chain, it must be correctly rated and matched to the furnace’s required pressure setpoints (i.e. the specific “close on vacuum” value). Thus, the pressure switch helps prevent combustion gases (e.g. CO) from being forced into the living space when venting is blocked or compromised.