A well pump that starts and stops every few seconds while a tap is running is short cycling, and it is not a quirk to live with. Electric motors are damaged by starting, not by running — inrush current is several times running current, and it heats the windings each time. A pump that would last fifteen years cycling normally can fail within a year cycling every ten seconds.
In the overwhelming majority of cases the cause is the pressure tank, and the tank costs a fraction of the pump.
What the tank is for
The pump does not run whenever a tap opens. It fills a tank containing a compressed air cushion — either behind a bladder or above the water in older tanks — and that cushion pushes water out to the house between pump cycles.
The volume delivered between cut-in and cut-out is the draw-down. It is what keeps the pump from starting every time somebody washes their hands.
When the air cushion is gone, the tank holds only water. Water does not compress, so pressure collapses the instant a tap opens, and the pump starts immediately. The tank becomes a length of pipe with a big label on it.
The one-minute test
Tap the tank. Knuckles down the side, top to bottom. A healthy tank sounds hollow at the top and dull at the bottom, with a clear change where the water level sits. A waterlogged tank sounds dull all the way up.
Then check the air valve. There is a Schrader valve — like a tyre valve — on the top of the tank. With the pump off and a tap open to relieve pressure, press it:
- Air comes out → the bladder is intact. The pre-charge may simply be wrong.
- Water comes out → the bladder has failed. Replace the tank; there is no repair.
That test settles the question, and both parts of it take a minute.
The pre-charge rule everybody gets wrong
The tank's air pre-charge must be set 2 psi below the pump's cut-in pressure, measured with the tank empty of water and the system depressurised.
For a 30/50 switch — cut in at 30, cut out at 50 — the pre-charge is 28 psi. For a 40/60 switch it is 38 psi.
Two errors are common:
- Measuring with water in the tank. The reading is meaningless. Turn off the pump, open the lowest tap, drain it, then measure.
- Matching the pre-charge to cut-out instead of cut-in. A tank pre-charged to 48 psi on a 30/50 system delivers almost no draw-down and cycles constantly, with a perfectly good bladder.
New tanks ship at a standard charge — often 40 psi — that matches nobody's system. Setting it correctly at installation is a five-minute job that is skipped more often than not.
When it is not the tank
A leaking check valve or foot valve. Water drains back down the well between cycles, so pressure falls with no water used. The clue is a pump that cycles when nothing is running.
A waterlogged captive-air tank in an old galvanised system. Older tanks without a bladder absorb air into the water over time and need periodic recharging, not replacement. If the tank has no bladder, this is expected maintenance.
A pressure switch with a narrow differential. If cut-in and cut-out are close together — say 45/55 — draw-down is small no matter how good the tank is. A 20 psi differential is normal.
A clogged screen or a failing pump that cannot reach cut-out pressure, so the switch never opens. Here the pump runs continuously rather than cycling, which is a different symptom pointing at a different problem.
A tank that is simply too small for the household. The rule of thumb is a draw-down of at least one gallon per gallon-per-minute of pump output, so a 10 GPM pump wants at least 10 gallons of draw-down — which needs a tank rated well above 10 gallons, because draw-down is only a fraction of tank volume.
Do not wait
Short cycling is one of the few plumbing faults where delay has a direct and predictable cost. Every cycle is another start, and the pump is at the bottom of the well. Replacing a pressure tank is a morning's work at ground level; replacing a submersible pump means pulling several hundred feet of pipe and wire out of a hole.
The cheap part fails first and takes the expensive part with it if it is left alone.