I have a borehole that is connected to a pressure sensor and an expansion vessel. A pipe runs up a hill to a large holding tank, which has a float valve. I have connected the pressure switch to switch the pump off when pressure exceeds 70PSI.

With the valve to the holding tank off, it works as expected: pressure increases as the expansion tank fills until it reaches about 70PSI, at which point the pump switches off until the pressure is released.

When connected to the holding tank up the hill, however, it behaves differently: pressure rises to 60PSI, and stays there until the tank is full. Then, pressure instead drops to 40PSI, and the pump's power factor drops from 1.0 to 0.85. The pump does not switch off because the pressure limit is not reached.

What could cause this behaviour and how can I remedy it?

1 Answer 1


I found the source of the issue - posting it here for anyone encountering a similar issue.

The float valve was faulty, and still allowed a limited amount of water to flow when closed. The pressure I was seeing during operation was the combined pressure of the head and the pressure required to force water past the damaged float valve.

What I thought was the float valve closing was in fact the well running dry, with the corresponding decrease in pressure and change in power factor.

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