Well Pressure Tank: How It Works, Settings, Sizing and Replacement
The pressure tank lets your well pump rest between uses. When it fails, the pump short-cycles and wears out early, so it's worth checking first and setting correctly.
How a well pressure tank works
- The pump pushes water into the tank, compressing an air cushion above a rubber bladder or diaphragm.
- At the switch's cut-out pressure (e.g. 50 PSI), the pressure switch turns the pump off.
- When you open a tap, the compressed air pushes water out. That usable volume is the drawdown.
- At the cut-in pressure (e.g. 30 PSI), the switch turns the pump back on.
Without enough air, the tank can't store water, and the pump starts and stops every few seconds ("short-cycling"). HomeServe notes this accelerates pump wear.
Correct air pressure (precharge)
| Pressure switch setting | Tank precharge |
|---|---|
| 20/40 | 18 PSI |
| 30/50 | 28 PSI |
| 40/60 | 38 PSI |
| 50/70 | 48 PSI |
AMTROL's rule is 2 PSI below cut-in. To check or adjust it:
- Turn off power to the pump at the breaker.
- Open a faucet until water pressure drops to 0 on the gauge.
- Check the air valve (on top or the side of the tank) with a tire gauge.
- Add air with a compressor or let some out to reach cut-in minus 2 PSI.
- Close the faucet and restore power.
What size pressure tank do you need?
Pentair's WELLMATE sizing rule: required drawdown = pump flow (GPM) × minimum run time, with at least 1 minute of run time. Then pick a tank whose drawdown at your switch setting meets that, using the manufacturer's chart. Drawdown is much smaller than the tank's total size.
This is the drawdown the tank must deliver, not the tank's total size. Check the tank maker's drawdown chart for your pressure switch setting (e.g. 30/50 or 40/60).
Signs your pressure tank is failing
- Rapid cycling: the pump clicks on and off every few seconds (AMTROL)
- Low or fluctuating pressure at the taps (AMTROL)
- Low or no air at the air valve when drained
- Water comes out of the air valve: the bladder has failed
- Sputtering faucets. HomeServe links this to a leaking bladder, or to a pump drawing air from a low well
Replacing a pressure tank
- Cost drivers: tank size, whether the pressure switch, gauge and tank tee are replaced too, plumbing changes, and local labor. Ask for an itemized quote.
- Re-set the precharge on the new tank before filling it. Tanks ship with a factory charge that may not match your switch.
- A tank swap is usually a much smaller job than a pump replacement. Compare with well pump replacement cost.
- After any work on the water system, consider disinfecting and testing for bacteria.
Frequently asked questions
What should the air pressure be in my well pressure tank?
AMTROL says to set the tank precharge 2 PSI below your pressure switch’s cut-in pressure: about 28 PSI for a 30/50 switch, 38 PSI for 40/60 and 48 PSI for 50/70. Check it with the pump off and the system drained of water pressure.
How does a well pressure tank work?
A rubber bladder or diaphragm separates water from a cushion of compressed air. When the pump runs, water squeezes the air until the switch’s cut-out pressure. When you open a tap, the air pushes water out until pressure drops to cut-in and the pump restarts. This stops the pump starting every time you use water.
How do I know if my pressure tank is bad?
AMTROL lists rapid pump cycling and low system pressure as the signs. Turn off the pump, drain the pressure, and check the air valve. Low or no air (or water coming out of the air valve) usually means the bladder has failed and the tank needs replacing.
What size pressure tank do I need for my well?
Size it by drawdown, not by total tank size. Pentair’s WELLMATE guidance is pump flow (GPM) × minimum run time, with at least 1 minute of run time. A 10 GPM pump needs at least 10 gallons of drawdown at your switch setting. Use the manufacturer’s drawdown chart.
How do I drain a well pressure tank?
AMTROL’s winterizing steps: disconnect power to the well pump, then open a drain or faucet until the pressure gauge reads 0 PSI, and drain the tank and nearby piping.
How much does it cost to replace a well pressure tank?
It depends on tank size, whether the pressure switch and fittings are replaced too, and local labor. Published estimates online vary widely and many are unreliable, so get two or three itemized quotes. A tank swap is usually a much smaller job than replacing the pump.
Sources
- Well tank FAQs (precharge, winterizing, replacement signs), AMTROL
- WELLMATE in-line pressure tank datasheet (sizing: pump GPM × minimum run time), Pentair
- How do you adjust the settings on a 9013FSG switch?, Schneider Electric (Square D)
- 7 Signs Your Well Pump Is Not Well, HomeServe
- Well Pump Cost and Installation Guide (updated March 2026), This Old House
Sources checked October 24, 2026. Found an error? Tell us and we will fix it.