Pressure Switches: High, Low, and Air-Proving (Component Reference)

A field reference on the pressure switches techs meet — refrigerant high-pressure and low-pressure cut-outs and the furnace air-proving (draft) switch — covering cut-in/cut-out, manual vs auto reset, and how to test each.

What it is

A pressure switch is a safety/control device that opens or closes an electrical contact based on a pressure. Three kinds matter in HVAC:

High-pressure switch (HPS) — on the refrigerant high side. Opens (cuts out) if head pressure climbs dangerously high, stopping the compressor before it damages itself or pops a relief.
Low-pressure switch (LPS) — on the refrigerant low side. Opens if suction pressure drops too low (loss of charge, restriction, low load), protecting the compressor from running on no refrigerant. Sometimes doubles as a loss-of-charge switch or a freeze/low-ambient cut-out.
Air-proving (draft) pressure switch — on a gas furnace, senses the negative pressure the draft inducer creates. It must close to prove the inducer is moving combustion air before the board will allow ignition. (Float/condensate switches are covered separately.)

This article is about identifying each, their cut-in/cut-out behavior, reset type, and how to test them.

How it works

Each switch has a diaphragm or bellows exposed to the pressure (refrigerant or air) on one side and a calibrated spring on the other. When pressure moves the diaphragm far enough against the spring, it flips a snap-action contact.

A high-pressure switch is wired normally closed and opens at its cut-out setpoint (high head). It closes again (cut-in) once pressure falls by a built-in differential. Some are auto-reset, some manual-reset.
A low-pressure switch is wired normally closed and opens when suction falls to its cut-out, closing again when suction rises past the cut-in. Usually auto-reset on AC; can have an anti-short-cycle delay.
An air-proving switch on a furnace is normally OPEN at rest and closes when the inducer pulls enough negative pressure. The board checks: switch open before the inducer runs (good — proves it's not stuck closed), then closed after the inducer starts (good — proves draft). If it doesn't close, no ignition.

The gap between cut-out and cut-in is the differential — it keeps the switch from chattering right at the setpoint.

In the field

Identify the switch and read its setpoint. Refrigerant switches are usually stamped with their cut-out/cut-in pressures and the refrigerant. Furnace air-proving switches are stamped with a pressure like "-0.50 in. w.c." and connect to the inducer/vent via a rubber tube.

Test a refrigerant pressure switch: with the system off and the switch isolated, ohm across it — a normally-closed safety should read closed (continuity) at normal pressure. To prove it actually trips, you either watch it operate against gauge pressure during a real fault, or jumper it temporarily ONLY to confirm the switch (not the rest of the circuit) is the open device — then put it back. A switch that's open with pressures normal is either tripped (and latched, if manual reset) or failed.

Test a furnace air-proving switch: with the inducer OFF, the switch should read open. Start the inducer (or call for heat) and the switch should close within a second or two. If it stays open with the inducer clearly running, suspect a blocked/cracked vent, a kinked or disconnected sensing tube, a plugged switch port, water in the tube, or a bad switch. You can confirm the switch itself by reading the actual negative pressure at the tube with a manometer and comparing to the switch's setpoint — if the pressure is there but the switch won't close, the switch is bad.

Manual-reset switches have a button. If it's tripped, find out WHY before you reset — a high-pressure switch that tripped on a dirty condenser will just trip again, and resetting without fixing the cause masks a real fault.

Normal values & targets

R-410A high-pressure cut-out: commonly around 600–650 psig cut-out, cutting back in roughly 400–420 psig (differential of a couple hundred psi). Per the equipment.
R-22 high-pressure cut-out: commonly around 400–425 psig cut-out. Per the equipment.
Low-pressure cut-out: varies widely — a loss-of-charge switch might cut out around 5–25 psig and cut back in higher; some are set for the refrigerant's near-freezing saturation. Always per the equipment/refrigerant.
Furnace air-proving switch: a negative-pressure setpoint, often in the range of about -0.40 to -1.20 in. w.c. depending on the furnace; stamped on the switch.
Reset type: high-pressure switches are often manual-reset (a serious fault); low-pressure and air-proving switches are usually auto-reset.
Normal contact state: refrigerant safety switches are normally closed (open on fault); furnace air-proving switches are normally open (close on proof of draft).

Common faults & what they mean

Compressor cuts out on high pressure → dirty condenser, failed condenser fan, overcharge, or non-condensables — fix the cause; the switch is doing its job. A switch that trips with normal head pressure is bad.
Compressor cuts out on low pressure / loss of charge → low charge, restriction, low airflow, or a stuck metering device. Again the switch is reporting a real problem unless pressures are normal.
Furnace won't ignite, locks out on pressure-switch fault → switch not closing: blocked vent/intake, water in the tube or trap, kinked/off sensing tube, plugged inducer port, cracked heat exchanger affecting draft, or a bad switch. Check the easy stuff (tube, blockage, condensate) before the switch.
Furnace pressure-switch "stuck closed" fault at rest → switch contacts welded closed or sensing tube issue; the board sees it closed before the inducer runs and won't proceed.
Switch chatters / short-cycles right at setpoint → marginal differential, borderline pressure, or a weak switch.

Tech tips & gotchas

A tripped safety switch is almost never the actual problem — it's the messenger. A high-pressure trip means something made head pressure too high; a low-pressure trip means something made suction too low. Fix the cause. Don't replace the switch and walk away, and never jumper a safety to keep a unit running.

On furnaces, the air-proving switch gets blamed constantly when the real fault is upstream: a plugged condensate trap, water in the sensing tube, a critter-blocked vent, or a disconnected hose. Always check the tube and for blockage/water first — the switch is usually fine.

Reading the actual pressure at the switch (refrigerant gauge or a manometer on the furnace tube) and comparing to the setpoint is the definitive test. If the pressure is in range and the switch is in the wrong state, the switch is bad. If the pressure is out of range, the switch is right.

Manual-reset means "this was serious." Find and correct the cause before pushing the button, or you're just deferring the trip.

Match the replacement's setpoints, refrigerant, reset type, and connection. A switch with the wrong cut-out pressure either nuisance-trips or fails to protect.

Safety / code notes

Pressure switches are safety devices — never jumper one out to make a unit run, even "just to test the rest of the system," beyond a momentary isolated continuity check that you immediately reverse. The high-pressure switch backs up the system's relief protection; defeating it risks a rupture. On furnaces, the air-proving switch is what prevents firing without combustion airflow — bypassing it risks unvented combustion and CO. Match replacement switches to the equipment's listed setpoints and reset type. Recover refrigerant before opening a sealed system to replace a refrigerant switch (EPA Section 608).

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