Short definition
A pressure switch is a spring-loaded electrical switch that turns a well pump or booster pump on at the cut-in (lower) pressure and off at the cut-out (upper) pressure. It’s the control device that makes a private well behave like city water — running only when the pressure tank needs refilling. Square D Pumptrol is the dominant residential brand in Washington.
What it is
The pressure switch is wired in series with the pump motor’s line voltage. A diaphragm inside the switch senses water pressure on the system side; a spring presses against the diaphragm to set the trip points. Two adjustment nuts inside the cover:
- Long-spring nut. Sets the cut-in pressure while preserving the differential between cut-in and cut-out. Turn clockwise to raise both cut-in and cut-out by the same amount.
- Short-spring nut. Sets the differential — the spread between cut-in and cut-out. Used carefully; changing the differential without re-pre-charging the pressure tank causes short-cycling.
Common WA settings:
- 30/50 psi — cut-in 30, cut-out 50, 20 psi differential. The default residential setting.
- 40/60 psi — cut-in 40, cut-out 60, same 20 psi differential. Common when the default 30/50 feels weak at upper-floor fixtures.
The 20 psi differential is the design assumption of most pressure tanks and pump motors. Narrower differentials cause short-cycling; wider differentials cause noticeably variable fixture pressure.
The switch mounts directly on a 1/4-inch or 1/2-inch tap on the pressure-tank tee (the manifold of fittings that connects the pump discharge, pressure tank, and supply line).
Why it matters to a homeowner
If you’re on a private well, the pressure switch is one of the components most likely to fail in routine ways:
- Pump won’t turn on. Often pitted contacts on the switch — the contact surfaces have arced over years and welded shut or refused to make. A multimeter test confirms; replacement is a $25–$50 part and 30 minutes of work with the breaker off.
- Pump won’t turn off. Same failure mode, opposite direction. Diaphragm has failed in some cases.
- Pump short-cycles. The pressure-switch differential is mis-set, or the pressure tank has lost its pre-charge, or the bladder has failed. Diagnose the tank pre-charge first (using the Schrader valve), then look at the switch differential.
Annual emergency-prep audit on a private-well property: visually inspect the switch, exercise it by drawing water until the pump cycles, verify the cut-in and cut-out values match the system design.
Common variants and what it isn’t
- Pressure switch (electrical) vs. pressure-reducing valve (mechanical). Pressure switch starts and stops a pump on city water or well water; PRV regulates pressure on city water.
- Mechanical / spring-action vs. electronic transducer. Residential wells use mechanical switches almost exclusively. Commercial systems use electronic transducers feeding programmable pump controllers.
- Standard 1/4-inch tap vs. 1/2-inch tap mounting. Sizing must match the pressure-tank tee.
Common failure modes
- Pitted contacts from years of arcing — pump won’t start, won’t stop, or operates erratically.
- Diaphragm failure inside the switch — erratic on/off cycling.
- Frozen switch in an uninsulated WA well house during a cold snap. Moisture inside the switch case freezes and locks operation.
- Mis-set differential — short-cycling.
Washington note
WA private wells (Olympic Peninsula, Mason County, Whatcom, Kitsap, Jefferson, Snohomish, San Juans, east-of-Cascades counties) overwhelmingly use Square D Pumptrol-style mechanical pressure switches at 30/50 or 40/60 psi. Frozen switches during November–January cold snaps are a recurring problem on uninsulated well houses and pump houses east of the Cascades; insulation and a small in-shed heater on a thermostat prevent the failure mode. Annual fall pre-cold inspection plus a spare switch in the cabinet is cheap insurance.