Well pressure tanks: how they work, the settings and the air charge

A well pump is built to run in long stretches, not to start every time someone rinses a glass. The pressure tank is what makes that possible. It stores pumped water under pressure, so the house has water while the pump is off and a reserve for moments of heavy use¹.

How it works

Inside the tank, water pushes against a pocket of air. Air compresses and water doesn’t, so the squeezed air acts as a spring that pushes water out into the pipes when a tap opens¹.

A pressure switch watches the pressure and runs the pump¹:

• When pressure falls to the low setting (the cut-in), generally between 20 and 40 psi, the pump starts.
• When it rises to the high setting (the cut-out), generally between 40 and 60 psi, the pump stops.

The common pairs are 20/40, 30/50 and 40/60 psi³.

Three kinds of tank

The Private Well Class, a program run by the University of Illinois and the Rural Community Assistance Partnership, describes three¹:

• Bladder tanks. The usual choice today. A flexible bladder keeps the air and the water permanently apart.
• Standard air-over-water tanks. The original design: one tank where water compresses the air directly. They are rarely installed now.
• Standard tanks with a wafer or float. An older tank fitted with a flexible barrier to keep air and water separate.

Homes with a variable-speed pump are different again: those systems use a tank of only a few gallons¹.

How much water the tank really holds

Less than the number on the label, because part of the tank is always air. The useful figure is the drawdown: the water that comes out between one pump start and the next.

Tank size	20/40 psi	30/50 psi	40/60 psi
19 gallons	6.9 gal	5.8 gal	5.0 gal
35 gallons	12.7 gal	10.7 gal	9.3 gal
50 gallons	18.3 gal	15.5 gal	13.4 gal
85 gallons	30.0 gal	26.0 gal	22.0 gal

Source: Pentair / Sta-Rite PRO-Source Plus owner's manual, Chart II. Other brands differ; the pattern is what matters.

Two things follow from the chart³. A tank delivers roughly a quarter to a third of its nominal size, and raising the pressure setting reduces what you get per cycle. A bigger tank means fewer pump starts, which is why wells that produce only a few gallons a minute need a large one¹.

The air charge

A pre-charged tank leaves the factory with air in it, and the installer adjusts that charge to 2 psi below the pump’s cut-in pressure, measured with no water in the tank³.

Pressure switch	Air charge, tank empty
20/40 psi	18 psi
30/50 psi	28 psi
40/60 psi	38 psi

Source: Pentair / Sta-Rite PRO-Source Plus owner's manual, Chart I. Check your own tank's manual.

The Private Well Class puts it the same way, “just a few PSI under” the pressure at which the pump starts, and suggests checking it about every six months².

How the check is done

The manufacturer’s procedure is short³:

1. Shut off electric power to the pump.
2. Open a faucet near the tank and drain it completely. A reading taken with water in the tank is wrong.
3. Measure the pressure at the air valve on top of the tank with an ordinary tire gauge.
4. If it is more than 2 psi below the cut-in setting, add air with a compressor or a portable air tank.
5. Brush soapy water around the air valve. Steady bubbling means the valve is leaking.
6. Put the valve cap back on and tighten it. A missing cap can let air escape and eventually waterlog the tank.

When the tank is waterlogged

Air slowly dissolves into the water in a standard tank, so the cushion shrinks and air has to be added¹. A bladder tank loses its cushion when the air leaks out or the bladder fails. Either way the result is the same: with too little air, the pump switches on and off far more often, which wastes energy and is hard on the pump¹.

To test for a failed bladder, the manufacturer’s method is to cut the power, drain the tank, release the air and tip the tank: if water runs out of the air valve, the water cell is leaking and has to be replaced³. In practice that is usually a job for whoever replaces the tank.

If your pump is starting every few seconds or minutes, see why a well pump keeps turning on and off.

Looking after it

• Check the air charge about every six months².
• Don’t let the tank or pipes freeze³.
• Watch for sediment. Sand wears pumps and piping². If grit is collecting, read our guide to cloudy or sandy well water.
• Book an annual inspection. The National Ground Water Association recommends a professional well inspection every year, including a flow test and a check of the pressure tank and pressure switch⁴.