What size pressure tank for a 5 GPM pump at 30/50 psi?
| Tank sizenominal size on the label | 20gal |
|---|---|
| Usable drawdown31% of the tank at 30/50 psi — the rest is air | 6.2gal |
| Drawdown needed5 GPM × 1 minute minimum run | 5gal |
| Run time per cycleunder one minute and the motor never sheds its start heat | 1.2min |
| One size down (20 gal)still acceptable, with less margin | 1.2min |
| Pre-charge pressure2 psi below cut-in, measured with the tank empty and the pump off | 28psi |
Notes
- A 20 gallon tank does not hold 20 gallons of water. Most of it is compressed air. At 30/50 psi you get 6.2 gallons of usable drawdown, and that is the only number that matters when sizing.
- Higher pressure gives you LESS drawdown, not more. 30/50 psi yields 31% of the tank against 31% at 30/50. Turning the pressure switch up for better showers shrinks the water buffer and makes the pump start more often — the opposite of what most people expect when they make that change.
- The one-minute rule is about the motor, not the water. A submersible pump has to run at least a minute per start to shed the heat of starting. Pumps die from cycle count, not run hours. A 20 gallon tank still clears a minute here, with less margin.
- Set the pre-charge to 28 psi, with the tank empty. Two psi below cut-in, measured at the schrader valve with the pump off and a tap open. Wrong pre-charge is the most common failure in a well system: too high and the tank waterlogs, too low and drawdown collapses. Check it yearly — it is a five-minute job with a tyre gauge.
- Constant-pressure (variable speed) pumps change this arithmetic completely: they modulate instead of cycling, so they need only a small tank. If the system has one, size by the manufacturer's table rather than by the rule above.
A 5 GPM pump at 30/50 psi
20 gallon tank — giving 6.2 gallons of usable drawdown and 1.2 minutes of run time per cycle.
The tank does not hold what the label says
Most of a pressure tank is compressed air. The water you actually get between pump starts is called drawdown, and at 30/50 psi it is only 6.2 gallons of a 20 gallon tank.
Drawdown is the number to size by. Tank volume is the number on the box.
Higher pressure gives you LESS drawdown
This is the part that catches everyone.
| Switch | Usable share of tank |
|---|---|
| 30/50 | ~31% |
| 40/60 | ~26% |
| 50/70 | ~22% |
Turning the switch up for better showers shrinks the buffer and makes the pump start more often — the opposite of what most people expect when they make that change.
The one-minute rule is about the motor
A submersible pump has to run at least a minute per start to shed the heat of starting.
Pumps die from cycle count, not run hours. Undersizing the tank saves a hundred dollars once and costs a pump.
Pre-charge is the annual five-minute job
Set it to 28 psi — two below cut-in — measured at the schrader valve with the pump off and a tap open until the tank is empty.
Wrong pre-charge is the most common failure in a well system. Too high and the tank waterlogs; too low and drawdown collapses. A tyre gauge checks it.
Constant-pressure pumps change all of this
A variable-speed pump modulates instead of cycling, so it needs only a small tank. If the system has one, size by the manufacturer's table rather than the rule above.