What Pressure Should Be in Your Well Tank?

For many bladder or diaphragm well tanks controlled by a conventional pressure switch, air precharge should be 2 psi below the pump’s cut-in pressure, measured with the tank empty of water. That means 28 psi for a 30/50 system or 38 psi for a 40/60 system. Both Amtrol’s Well-X-Trol instructions and Pentair’s Pro-Source PSC manual specify this relationship.

But that number is not the normal reading on the plumbing pressure gauge. Before changing anything, distinguish air precharge from operating water pressure—and identify your tank and controller.

Two pressure readings, two different jobs

Operating pressure is what the gauge on the tank piping shows. In a conventional 40/60 system, the pump starts at approximately 40 psi—cut-in—and stops at approximately 60 psi—cut-out. Pressure normally moves between those limits as water is used and the pump refills the tank.

Air precharge is the pressure of the trapped air cushion before water enters the tank. It is checked at the tank’s tire-style air valve with no water remaining in the tank.

The pump pushes water into the tank and compresses that air cushion. When a faucet opens, the compressed air pushes water back out. The water delivered between pump cycles is called drawdown. Amtrol illustrates this operating cycle.

Consequently, an air-valve reading taken while the tank contains pressurized water does not establish its precharge. A 38 psi precharge and a 60 psi plumbing-gauge reading can both be correct—under different test conditions.

Common precharge settings

Use this table only where your equipment instructions specify 2 psi below cut-in:

Pump starts / stops	Empty-tank air precharge
20/40 psi	18 psi
30/50 psi	28 psi
40/60 psi	38 psi

These combinations appear in Pentair’s precharge chart. Base the target on the verified cut-in pressure, not merely a switch label or the tank’s factory air charge.

Constant-pressure systems can require a different setting. For example, Franklin Electric’s European manual for the SubDrive75/100/150 controllers calls for precharge at 70% of the pressure-sensor setting—not 2 psi below it. That is an example from that manual, not a rule for every SubDrive version or variable-speed system. Follow the instructions for your exact controller and tank. See our guide to constant-pressure well-pump systems for the differences between control methods.

This table also does not describe servicing a traditional air-over-water tank without a separating bladder or diaphragm.

How precharge is checked

First read the tank’s service instructions. Do not automatically add air to a tank that has begun short-cycling. Amtrol restricts checking and adjusting precharge to qualified professionals. Its manual warns not to add pressure after loss of air, or adjust precharge when there is visible corrosion or reduced pump-cycle time. It also warns that repressurizing a corroded, damaged or weakened tank can cause rupture or explosion. Read its safety requirements before service.

Where the equipment instructions permit the check, the measurement sequence is:

1. Record the operating pressures first. With covers closed, observe the plumbing gauge as ordinary water use starts the pump and refilling stops it. Do not open the pressure-switch cover to obtain these readings.
2. Disconnect electrical power to the pump before draining or servicing the tank. Keep electrical equipment dry and prevent unexpected restart.
3. Open the designated drain or a nearby faucet connected to the tank. Drain the tank completely and confirm that water-side pressure is zero. Do not assume draining an isolated section of plumbing has emptied the tank.
4. Measure at the air valve with an accurate tire-pressure gauge. Compare the result with the model-specific target, then replace the valve cap.

The power-off, complete-drain and air-gauge steps are specified in Pentair’s tank-check procedure. A qualified servicer should decide whether adjustment is appropriate and verify operation afterward.

What an incorrect reading means

The air-cushion mechanism explains why precharge matters: too much precharge can let the tank empty before pressure reaches the pump’s start setting, leaving a brief delivery gap. Too little precharge can reduce useful drawdown because more water remains in the tank at cut-in. These are consequences of the tank’s compression-and-expansion cycle, not diagnoses you can make from the plumbing gauge alone.

A pump starting repeatedly after only a small water draw warrants inspection, but it does not prove that adding air is the repair. Use our pressure-tank troubleshooting guide to separate tank symptoms from other system faults.

Finally, precharge is not a water-pressure booster setting. Raising it alone does not raise the pressure-switch limits. If the tank gauge behaves normally but fixture pressure is weak, investigate the delivery system rather than inflating the tank; start with how to increase water pressure from a well.