Well Pressure Tank: How It Works, Sizing and Air Pressure

A well pressure tank holds a reserve of water under air pressure, so the pump can fill it in one longer run and then stay off while you draw water from the tank. Size it so the pump runs at least one minute every time it starts: tank volume equals pump gpm times minimum run time, divided by the drawdown multiplier for your pressure switch setting. Set the air precharge 2 psi below the switch’s cut-in pressure, measured with the tank empty of water.

How a diaphragm or bladder tank works

Inside a modern well tank, a diaphragm or bladder separates a pocket of compressed air from the water. Amtrol’s Well-X-Trol instructions describe the cycle in two steps:

1. As the pump fills the tank, the water compresses the air above the diaphragm. Pressure rises until the pressure switch reaches its cut-out setting and turns the pump off.
2. When you open a tap, the compressed air pushes water out of the tank. Pressure falls until it reaches the cut-in setting and the pump starts again.

The water delivered between the pump shutting off and starting again is the drawdown. Amtrol notes that a larger tank gives more drawdown, so the pump runs less often, which saves energy and extends pump life.

Older galvanized tanks with no diaphragm keep air and water in direct contact. The water slowly absorbs the air, and the tank eventually becomes waterlogged unless air is replenished. Diaphragm tanks avoid this because the air never touches the water.

Tank volume is not drawdown

The number on the label (20, 44, 86 gallons) is the tank’s total volume. Most of it is air and reserve. The usable water per cycle is a fraction set by the switch pressures, following Boyle’s law. Pentair publishes drawdown multipliers for its Wellmate tanks:

Multipliers: Pentair Wellmate data sheet (approximate). Gallon columns are the tank volume multiplied by the multiplier.

The higher your pressure setting, the smaller the share of the tank you can use. Raising a 30/50 system to 40/60 for better shower pressure shrinks drawdown by about 14% (0.25 versus 0.29) and shortens every pump run.

How to size a well pressure tank

The formula

Pentair’s sizing formula is:

Tank size (gal) = pump gpm x minimum run time (min) / drawdown multiplier

Pentair recommends at least a 1-minute run time, and Franklin’s AIM manual gives the same 1-minute minimum for submersible motors so they can dissipate the heat built up by starting current.

Worked examples

For a 10 gpm pump and a 1-minute minimum run:

• At 20/40 psi: 10 x 1 / 0.35 = 28.6 gal total tank volume
• At 30/50 psi: 10 x 1 / 0.29 = 34.5 gal
• At 40/60 psi: 10 x 1 / 0.25 = 40.0 gal

Pentair’s guidance is to choose the closest size, usually the next larger one. Pentair’s own example: a 1 gpm pump at 20/40 needs 2.86 gal, so the 4.6 gal WM02, with 1.7 gal of drawdown, is the right pick.

If you want longer runs than the minimum (some installers size for 2 minutes), double the run time in the formula and the tank size doubles with it.

Why run time matters

Every start heats the motor and wears the starting components. Franklin’s AIM manual caps single-phase submersibles at 300 starts per 24 hours for motors up to 0.75 hp and 100 starts for 1 to 5.5 hp, and it warns that excessive cycling shortens the life of control components and can damage motor splines and bearings. An undersized or waterlogged tank is the usual reason a pump blows past those limits. If you are matching a tank to a new pump, the well pumps guide covers how to size the pump itself, and our submersible pump roundup lists rated flow by model.

Setting and checking the air precharge

The precharge is the air pressure in the empty tank. Amtrol and Goulds both set it at 2 psi below the pressure switch’s cut-in. Amtrol’s table:

Source: Amtrol well tank FAQ; Amtrol Well-X-Trol instructions (example: 38 psig for a 40/60 switch).

To check it, following the Amtrol and Goulds instructions:

1. Turn off power to the pump.
2. Open a faucet and drain the tank until no water flows. The reading only means something with the tank empty of water.
3. Remove the cap on the air valve and read the pressure with a tire gauge.
4. Release air, or add air, to reach cut-in minus 2 psi. Make adjustments only at ambient temperature.
5. Replace the valve cap. Goulds notes the cap is the primary seal on its tanks.

Do not check it constantly: Goulds says you lose 1–2 psi every time you put a gauge on the valve.

Amtrol’s instructions add a limit: adjust precharge on a new tank, or on one whose integrity has been confirmed. If a tank in service has lost air, or shows corrosion, Amtrol says not to re-pressurize it, because the air loss can mean internal corrosion, and adding pressure can rupture the tank. Amtrol also states that only licensed professionals should check or recharge tank precharge. See well pressure tank replacement for what to do instead.

Where the tank goes

• Close to the pressure switch. Amtrol says to install the tank as close as possible to the switch, to cut the effect of friction loss and elevation differences. If the tank sits at the end of a long pipe run, relocate the switch with it or adjust the setting. Goulds lists a jet pump more than 5 ft from the tank as a cause of excessive cycling.
• With a relief valve. Amtrol rates Well-X-Trol tanks to a maximum working pressure of 150 psig and calls for a relief valve set to open at 125 psig or more, sized to discharge the pump’s full capacity.
• Within temperature limits. Amtrol says not to expose the tank to freezing or to temperatures over 130°F, and not to install it in direct sunlight. Pentair limits Wellmate tanks to 90°F and 100 psi.
• Where a leak won’t do damage. Amtrol says to fit a drip pan connected to a drain if leaking could damage property.

Signs the tank is the problem

Amtrol lists rapid cycling of the pump and low system pressure as the symptoms of a tank that needs replacement. Franklin’s AIM manual names a waterlogged tank as a cause of a motor that starts too often, alongside a stuck-open check valve and a leak. If you have water coming out of the air valve or a precharge that keeps dropping, go to well pressure tank replacement. If the pump is not running at all, start with well pump not working.