Pump Cycling on a Pressure-Tank System
A driller installs a 1 hp submersible pump in a 120 ft deep well serving a house with a 40-gallon bladder pressure tank. The pressure switch is set 30/50 psi. The homeowner reports the pump short-cycling every few seconds and the water pressure surging. The driller finds the tank's air charge is low, the pump delivers about 12 gpm, and the tank bladder appears intact. Static water level is 50 ft and the pumping level at 12 gpm is 70 ft. The driller has a tire gauge, air compressor, and replacement switch on the truck. The client wants the problem fixed today without replacing the pump.
1. Diagnose the short-cycling and explain the role of the tank charge.
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Short-cycling every few seconds with a 40-gallon tank and 12 gpm pump is the classic signature of a waterlogged or undercharged tank. With a 30/50 psi switch, the tank precharge should be about 28 psi - 2 psi below the cut-in - so the bladder has room to accept water before the pump starts. If the precharge is low, the tank fills almost immediately, pressure hits 50 psi, the pump shuts off, and a small draw drops it back to 30 psi within seconds. The driller should check the precharge with the tire gauge at the air valve with the system drained, restore it to 28 psi, and re-test. If the bladder were ruptured, water would come from the air valve; the report says it is intact, so recharging is the likely fix.
2. If recharging the tank does not stop cycling, what else should be checked, and how does the pump's 12 gpm output factor in?
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If the precharge is correct and cycling persists, the driller should check the pressure switch settings and differential, the tank's actual acceptance volume, and for a stuck check valve or a leak on the house side that bleeds pressure back. A 12 gpm pump with a 40-gallon tank is a mismatch for a 20 psi differential: the drawdown volume between 30 and 50 psi is small relative to pump output, so even a healthy tank cycles quickly. The fix may be a larger tank, a higher differential switch, or a cycle-stop valve. The driller should verify the pumping level of 70 ft is stable and that the pump is not losing prime, then size the tank so the run time meets the pump manufacturer's minimum cycle time.