Pressure Tank Precharge: What to Set It To and Why
Pressure tank precharge should be set 2 psi below the pump's cut-in pressure. AMTROL's installation instructions put it in one line: "Using an air gauge, adjust the precharge to 2 psig below the cut-in (LO) setting." On a 40/60 switch that means 38 psi.
What the precharge does
The precharge is the air pressure in the tank with no water in it. When the pump fills the tank, that air is compressed; when you open a tap, the air pushes the water out. Set the precharge close to the cut-in and the air starts working the moment the pump stops, so almost every gallon between cut-out and cut-in comes from the tank.
The numbers on a 40/60 switch
By Boyle's law the usable share between 40 and 60 psi is (precharge + 14.7) × (1 ÷ 54.7 − 1 ÷ 74.7):
| Precharge | Usable share | From a 45.2-gal tank |
|---|---|---|
| 38 psi (cut-in − 2) | 25.8% | 11.7 gal |
| 28 psi | 20.9% | 9.4 gal |
| 20 psi | 17.0% | 7.7 gal |
A tank left at a low factory precharge on a 40/60 switch gives a third less water per cycle, and the pump starts about half again as often. Over a year of short cycles that is the difference the precharge makes.
Common settings
| Pressure switch | Precharge |
|---|---|
| 20/40 | 18 psi |
| 30/50 | 28 psi |
| 40/60 | 38 psi |
| 50/70 | 48 psi |
Doing it
Turn the pump off at the breaker and open a tap until the water pressure falls to zero; the precharge is measured with no water pressure on the tank. Read the air valve with a tire gauge and add or let out air to reach cut-in minus 2. Check it once a year: a falling precharge with a waterlogged-sounding tank is a sign the diaphragm or bladder has failed. If you raise the pressure switch, raise the precharge with it — and recheck the tank size, because a higher switch setting needs a larger tank.
Set it 2 psi under cut-in, check it yearly, and the pressure tank precharge will give you all the drawdown the tank was sized for. The calculator shows the drawdown for any setting.
Sources
- AMTROL, Well-X1 installation and operating instructions — precharge 2 psig below cut-in.
- Usable shares computed with Boyle's law; the method reproduces Goulds' published pressure factors. See the methodology.
Questions
What should the precharge be on a 40/60 pressure switch?
38 psi — 2 psi below the 40 psi cut-in, per AMTROL's instruction. A 30/50 switch takes 28 psi and a 20/40 switch 18 psi.
What happens if the precharge is too low?
Less of the tank is usable. On a 40/60 switch, a 20 psi precharge leaves 17.0% of the tank usable instead of 25.8%, so the pump cycles about half as often again.
What if the precharge is above cut-in?
Then the air holds the water out of the tank until pressure rises past the precharge, and the tank gives almost no drawdown before the pump restarts.