Well Pump Types and Sizing: Submersible vs Jet, GPM and Head

The depth to the water in your well picks the pump type. If the water level, plus friction losses in the suction pipe, is within 25 feet of the pump, a shallow-well jet pump sitting above ground can pull it up. Deeper than that, you need a deep-well jet with an ejector down the well, or a submersible pump hung below the water line. Once the type is settled, you size the pump by two numbers: the flow your house needs at peak, in gallons per minute, and the total dynamic head it has to push against, in feet.

Why 25 feet is the dividing line

A jet pump at the surface works by suction, and suction has a hard ceiling. Goulds’ jet pump manual gives a maximum suction lift of 25 ft for shallow-well systems, and that 25 ft counts the actual vertical lift plus friction loss in the suction piping. A well whose water sits 22 ft down with a long, undersized suction line can already be past the limit.

A submersible sits below the water and pushes rather than pulls, so suction lift never comes into it. That is why water deeper than about 25 ft calls for either a submersible or a deep-well jet with its ejector down the well.

The three pump types

Submersible pumps

The motor and pump are one long assembly lowered on drop pipe into the casing, below the lowest water level the well reaches while pumping. Only the pressure switch, the tank and, on some motors, a control box are above ground. Franklin’s AIM manual requires at least one positive-sealing check valve; if the pump has no built-in check valve, a line check valve goes in the discharge within 25 ft of the pump and below the drawdown level. Swing-type check valves are never to be used with submersibles.

The trade-off is access: when a submersible fails, the whole string has to be pulled out of the well. That is the single biggest reason well pump replacement cost varies so much.

Shallow-well jet pumps

The pump and its built-in jet (ejector) sit above ground, usually beside the pressure tank, with a suction pipe down to the water and a foot valve on the end. They are cheap to reach and service, but they must be primed with water before they will pump, and an air leak anywhere on the suction side can stop them making pressure. Goulds lists “pump not primed” and “air leak in suction pipe” among the causes of little or no water. See our shallow well pump picks.

Convertible and deep-well jet pumps

A convertible jet pump can run a shallow well with a bolt-on shallow-well adapter, or a deep well (over 25 ft) with a jet assembly installed down in the well on twin pipes or a packer. Deep-well jets get past the suction limit by sending some of the pumped water back down to drive the ejector. They keep the motor above ground, at the cost of more piping in the well and a jet assembly that has to be pulled if it plugs.

Pump types compared

Sources: Goulds IM102R11; Franklin Electric AIM manual.

2-wire vs 3-wire submersibles

Single-phase submersible motors come in two layouts, and you need to know which one you have before troubleshooting or ordering parts.

• 2-wire motors carry their starting components inside the motor. Franklin’s 2-wire motors use a solid-state switch that drops out the start winding as the motor comes up to speed. There is no control box at the surface; the pressure switch feeds the motor directly.
• 3-wire motors require a control box above ground. Franklin’s manual lists what is inside: starting capacitors, a starting relay and, in some sizes, overload protectors, running capacitors and contactors. The box and motor are a matched pair. Horsepower and voltage must match, and running a 3-wire motor without the correct box can cause motor failure.

The practical difference: a 3-wire system gives you a serviceable box at the surface (a failed capacitor or relay can be replaced without pulling the pump), while a 2-wire system has fewer parts but nothing to fix above ground if its starting circuit fails.

Sizing a pump: flow and total dynamic head

Flow, in gallons per minute

The Nebraska DWEE sizing worksheet uses two quick estimates. First, count outlets (each tub or shower, toilet, sink, dishwasher, washer and hose bib) and allow 1 gpm per outlet. Second, estimate peak demand as the number of outlets likely to run at the same time, multiplied by 3 gpm.

Example: a house with 2 showers, 2 toilets, 2 lavatories, a kitchen sink, a dishwasher, a washing machine and 2 hose bibs has 11 outlets, or 11 gpm by the outlet count. If 3 outlets typically run at once, peak demand is 9 gpm. A pump rated around 10 gpm at the required head covers it.

The pump must not out-pump the well. The worksheet asks for the well’s recovery rate, and Franklin’s troubleshooting table lists a pump exceeding well capacity as a cause of continuous running. If the well yields less than the house’s peak demand, the fix is storage, not a bigger pump.

Head, in feet

Total dynamic head (TDH) is what the pump works against: the vertical elevation from the pumping water level to the tank, plus friction loss in the pipe and fittings, plus the system pressure converted to feet at 2.31 ft per psi (the conversion used in the Nebraska worksheet).

Worked example: pumping level 120 ft below the tank, a 40/60 psi pressure switch, and 10 ft of friction loss (an assumed figure; take the real one from a friction-loss chart for your pipe size and flow).

• Pressure: 60 psi x 2.31 = 138.6 ft
• TDH: 120 + 10 + 138.6 = 268.6 ft, call it 270 ft

You then pick a pump whose published performance curve delivers about 10 gpm at 270 ft. Our submersible well pump roundup lists the rated flow and head data that manufacturers publish.

The tank and switch are part of the pump system

Franklin’s AIM manual says to run motors at least one minute per start to shed the heat from starting current, and it caps starts per 24 hours: 300 for single-phase motors up to 0.75 hp and 100 for 1 to 5.5 hp single-phase motors. The pressure tank makes those limits achievable, and Pentair’s sizing formula (pump gpm x minimum run time, divided by the drawdown multiplier for your switch setting) ties tank size directly to pump size. Goulds ships its jet pumps with 30–50 psi switch settings. The pressure tank guide works through the sizing math for each common setting.

Where to go next

• Well pressure tanks: how they work and how to size one
• Well pressure tank replacement
• Well pump replacement cost
• Best submersible well pumps
• Best shallow well pumps
• Well pump not working: a troubleshooting order