Size the pressure tank so your pump runs at least one minute per start. That takes about one gallon of drawdown per gallon per minute of pump output. Tanks deliver roughly 30% of their nominal size as drawdown, so a 10 gpm pump needs about a 30–40 gallon tank, and a 15 gpm pump needs about 50–60 gallons or more.
Water Well Drilling Weatherford resizes a lot of pressure tanks that were bought to match the old one rather than the pump. What size pressure tank do I need? has a simple answer once you know your pump’s flow rate.
The one-minute rule
Submersible motor makers call for the motor to run at least one minute every time it starts, so it can shed the heat of the starting current. The pressure tank is what makes that possible. It stores enough water that the pump can run a full minute before the house pulls the pressure down again. Tanks that are too small cause short cycling. Short cycling burns out switches and motors.
Sizing chart
About one gallon of drawdown per gpm gives a one-minute run. Tanks deliver roughly 30% of their nominal size as drawdown:
| Pump output | Drawdown needed | Minimum tank (nominal) | Common tank size |
|---|---|---|---|
| 5 gpm | ~5 gal | ~17 gal | 20 gal |
| 7 gpm | ~7 gal | ~23 gal | 32 gal |
| 10 gpm | ~10 gal | ~33 gal | 32–44 gal |
| 12 gpm | ~12 gal | ~40 gal | 44 gal |
| 15 gpm | ~15 gal | ~50 gal | 62 gal |
| 20 gpm | ~20 gal | ~67 gal | 86 gal |
Two smaller tanks piped together add up the same way as one large tank.
Finding your pump’s flow
Check the pump’s model on your State Well Report or the service record. A tech can also time how long the pump takes to raise the tank from cut-in to cut-out while no water is being used, which gives the delivered gallons per minute. Parker County wells vary. A shallow Peaster well and a deep Aledo well with the same horsepower pump can deliver very different flows.
Setting it up right
- Pre-charge 2 psi below cut-in with the system drained: 28 psi on a 30/50 switch, 38 psi on 40/60.
- Check the pre-charge yearly. Bladder tanks lose a little air over time.
- Keep the tank out of direct sun in an unvented well house. Heat shortens bladder life.
Common mistakes
- Replacing a failed tank with the same undersized tank.
- Raising the pressure switch setting without resetting the tank pre-charge.
- Adding irrigation to a system sized for a house.
See signs of a bad pressure tank, pressure tank replacement cost and well pressure tank replacement.
Bottom line
Find your pump's flow in gallons per minute, divide by about 0.3, and round up to the next standard tank size. Set the pre-charge 2 psi below the cut-in pressure. When in doubt, go bigger: an oversized tank costs a little more, while an undersized one costs you a pump.
Questions about your own well? Call (817) 382-3840. A person answers 24/7.
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