When I installed the new pressure switch about a year ago it was set a bit low with the pump turning on at 25 psi and off at 50 psi (I was aiming for 30/60). I just went out there and the pressure gauge was at 80 psi!

I turned the top pressure sensitivity on the gauge shut off at 65 psi, but when I cycle the pump the water pressure drops slowly (as expected) to about 60 psi, then I hear a rush of water and the pressure suddenly dramatically drops to about 40 psi and the pump kicks back on and pressure is restored.

These systems aren't that complicated but I'm at a loss as to what would cause that sudden drop of pressure. Is there a backflow valve that I'm unaware of that may have been damaged or something?

  • Sounds as starting the pump is allowing pressure to escape back down tbe well until the pump builds it up again. Is there a check valve in that line, or is it a valve that's being opened and closed? If the former it may not be working, if the latter it may be opening before the pump has gotten up to speed...
    – keshlam
    Jan 29 at 0:00

1 Answer 1


Your pressure tank is evidently set incorrectly, or the bladder (or diaphragm) has failed.

That should not have caused the pressure switch setting to change, though.

With no pressure on the water side (pump off, system drained) the pressure tank should be set to 2-3 PSI below the cut in pressure (so, for 30 as you claim to have set the switch, 27-28 PSI. If it's now set at 40, 37-38 PSI.)

If the pressure tank won't hold pressure, or water comes out the air valve, the bladder or diaphragm has failed.

The sudden drop in pressure happens when the water being supplied by the pressure tank runs out. The idea of setting the tank bladder pressure 2-3 PSI below the cut-in is to give the pump a chance to get started before the tank runs out of stored water.

You can also check your tank for "drawdown volume" by letting the system pump to shut off, and while using no water elsewhere, draw off water into a graduated bucket or jugs just until the pump stats, and compare that to the published chart for your tank and the pressure at cut-out & pressure at cut-in. If the volume is significantly less than the chart indicates, that's another indicator that the bladder has failed.

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