1. How many gpm do you need?
Gallons per minute (gpm) is how fast the pump must deliver water when several things run at once. Two rough starting points installers use:
- Fixture count: count the water outlets (sinks, toilets, showers and tubs, dishwasher, washer, hose bibs) and allow roughly 1 gpm per fixture.
- Bedroom rules: many charts land somewhere around 6–12 gpm for a typical 3-bedroom, 2-bath house. Treat any such chart as a ballpark, not a promise.
Example. A house with 2 toilets, 3 bathroom/kitchen sinks, 2 showers, a dishwasher, a washer and 2 hose bibs has 11 outlets. The one-gpm-per-fixture rule says about 11 gpm; in practice not everything runs at once, so many people would be comfortable with 8–10 gpm here. The pressure tank covers short peaks.
Irrigation adds on top. Add the flow of the sprinkler zone (or zones) that will run while the house is also using water. If one zone needs 8 gpm, the example house above is now planning for roughly 16–19 gpm.
Check the well, too. A pump that outruns the well’s recovery rate will pump it down and run dry. Your driller’s well report is the number that matters.
2. Total dynamic head (TDH)
Head is pressure expressed as feet of water. A pump’s curve shows how many gpm it delivers at a given total head, so you need your job’s total:
TDH = vertical lift + pressure in feet + friction loss
- Vertical lift: from the pumping water level (where the water sits while the pump runs, not the resting level) up to the pressure tank or house, including any height above the wellhead.
- Pressure in feet: psi × 2.31. Use the pressure switch’s cut-out (the higher number), since the pump has to reach it.
- Friction loss: look it up in a friction-loss table for your pipe material and size at your flow. Longer, thinner pipe loses more.
Worked example (made-up numbers). Pumping water level 120 ft, tank 5 ft above the wellhead: vertical lift = 125 ft.
Switch set 40/60 psi: 60 × 2.31 = 138.6 ft, call it 139 ft.
Suppose the friction table gives 4 ft per 100 ft for your pipe and flow, and you have 200 ft of pipe: 8 ft.
TDH ≈ 125 + 139 + 8 = 272 ft. You would look for a pump whose curve delivers your target gpm at about 272 ft, with some margin.
3. Pressure tank drawdown
Drawdown is the water you actually get out of the tank between the pump turning on (cut-in) and off (cut-out). It is only a fraction of the tank’s total volume, and it shrinks as the pressure range rises. That is why two “44-gallon” tanks can differ, and why a stated drawdown is only meaningful at a stated pressure setting.
Why it matters: bigger drawdown means the pump runs longer and starts less often. Frequent short cycling wears the pump and the switch. Many manufacturers ask for roughly at least one minute of run time per cycle as general guidance, so the drawdown you want is about your pump’s gpm × 1 minute.
| Switch setting | Approx. drawdown as share of tank volume* |
|---|---|
| 30/50 psi | about 30% |
| 40/60 psi | about 26% |
| 50/70 psi | about 23% |
*Ideal-gas estimate with the correct precharge; real tanks differ. Use the manufacturer’s own drawdown table.
Example. A 10 gpm pump at 40/60 psi wants roughly 10 gallons of drawdown for a one-minute run. At about 26%, that points to a tank with around 40 gallons or more of total volume. Compare tanks by their listed drawdown at your setting, not by total gallons.
Precharge: the air pressure in the tank should be about 2 psi below the cut-in pressure (38 psi for a 40/60 switch). Check it with the pump switched off at the breaker and the tank drained of water pressure, using a tire gauge on the air valve. Disconnect power first.
4. Shallow vs deep wells
A pump above ground can only pull water up with suction, and atmospheric pressure limits how far. In practice about 25 ft of lift is the working limit for a shallow-well pump (less at altitude, with long pipe runs, or with warm water).
- Shallow-well jet pumps sit at the wellhead or in the house, with one pipe into the well.
- Deep-well jets put an ejector down in the well so the pump can serve deeper water, using two pipes. They get less efficient the deeper they work.
- Submersibles sit in the water and push it up, so depth is limited by pump head and motor, not suction. They are the usual choice for deeper wells.
5. 2-wire vs 3-wire submersibles
- 2-wire: the motor’s starting parts are inside the motor, so only two power wires (plus ground) run down the well and no control box is needed. Simple, but typically offered in smaller sizes.
- 3-wire: starting parts live in an above-ground control box, with three power wires (plus ground) down the well. The box can be serviced without pulling the pump, and 3-wire motors go to larger sizes.
A 3-wire motor needs the matching control box for its horsepower, voltage and brand specification — see the Controls catalog. Three-phase motors use different starters and are not covered by this rule of thumb.
6. Constant pressure vs conventional
- Conventional: a pressure switch and a tank. Pressure swings between cut-in and cut-out (often 20 psi apart). Simple and lower cost.
- Constant pressure: a variable-speed drive adjusts the motor speed to hold one pressure, usually with a much smaller tank. Steadier showers, softer starts, and often smoother on the electrical supply, at higher cost and with a drive that has to match the motor.
Browse both in the Constant-pressure systems and Pressure tanks catalogs.