Pressure switches are safety and control devices that watch a pressure and open or close a circuit when it crosses a setpoint. In refrigeration and air conditioning, they protect compressors from dangerous head pressure, loss of charge and freeze-ups, or cycle condenser fans to keep head pressure stable. In gas furnaces, an air pressure switch proves the draft inducer is moving combustion gases out of the appliance before the gas valve can open.
This category covers high pressure and low pressure refrigerant switches, fan cycling switches, manual reset switches and furnace draft-inducer air proving switches rated in inches of water column (inches W.C.). Supco and other universal lines offer switches with fixed and adjustable setpoints in common configurations to cover many OEM applications.
These switches fail electrically, mechanically or from blocked sensing lines. Just as often, a switch is doing its job and reporting a real system problem. That distinction is the difference between a correct repair and a callback.
Refrigerant pressure switches use a diaphragm or bellows connected to the system, usually through a Schrader fitting with a depressor, a brazed connection or a capillary tube. When system pressure pushes against the diaphragm past a set point, the switch contacts change state. High pressure switches typically open on rising pressure to stop the compressor before head pressure reaches dangerous levels. Low pressure switches typically open on falling pressure to protect against loss of charge, a frozen coil or other low suction conditions. Each switch has a cut-out and a cut-in point; the difference between them is the differential.
Fan cycling switches control condenser fans based on head pressure. In low ambient conditions, cycling the fan off when pressure falls and back on when it rises keeps head pressure high enough for proper metering device operation. They usually close on rising pressure.
Manual reset switches open on a fault and stay open until someone pushes the reset button. They are used where an automatic restart could repeat a dangerous condition, such as a high pressure trip on a commercial system. A manual reset trip is a signal to find the cause before restoring operation.
On induced draft furnaces, a pressure switch connects by a rubber hose to the draft inducer housing or the collector box. When the inducer runs, it creates negative pressure, measured in inches of water column. If the pressure reaches the switch's setpoint, its contacts close and tell the control board that venting is proven. The board then proceeds with ignition. If the vent is blocked, the inducer fails or the hose is plugged, the switch stays open and the furnace will not light.
For a furnace, measure the actual draft with a manometer at the switch port and compare it with the switch's rating, usually printed on the label. If the draft is too low, the problem is venting, the inducer, a blocked heat exchanger or condensate drainage, not the switch.
| Switch type | Typical action | What it protects or controls | Key specs |
|---|---|---|---|
| High pressure | Opens on rise | Compressor and system from excess head pressure | Cut-out, cut-in, reset type, connection |
| Low pressure | Opens on fall | Compressor from loss of charge or low suction | Cut-out, cut-in, reset type, connection |
| Fan cycling | Closes on rise | Head pressure in low ambient conditions | Cut-in, cut-out, electrical rating |
| Manual reset | Opens and latches | Systems that must not restart automatically after a fault | Trip point, reset button access |
| Furnace air proving | Closes on negative pressure | Proves venting before ignition | Setpoint in inches W.C., ports, contacts |
HVAC365 carries Supco pressure switches because Supco has been serving the HVAC and refrigeration trade since 1945 and offers universal and direct replacement switches for many applications. Product pages list setpoints, connection type and cross-reference data so you can confirm fit before you order, and these items ship in 3-5 business days.
The switch is often fine. Common causes are a blocked vent or intake, a weak inducer motor, a plugged hose or port, or condensate backing up in the collector box. Measure the draft with a manometer and compare it with the switch rating.
No. A lower setpoint switch may close when venting is inadequate, which can allow combustion gases to spill into the home. Always use the OEM-specified rating.
A high pressure switch protects against excess head pressure and typically opens on rising pressure. A low pressure switch protects against low suction pressure and typically opens on falling pressure.
It turns the condenser fan on and off based on head pressure, keeping pressure high enough in cold weather for the system to operate correctly.
If the switch is mounted on a Schrader fitting with a depressor, it may be replaceable with minimal loss, but any refrigerant handling still requires an EPA 608 certified technician. Brazed and capillary switches require recovery.
Higher outdoor temperatures raise head pressure. Dirty condenser coils, a weak condenser fan motor, overcharge or restricted airflow push it over the trip point. Address those before replacing the switch.
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