Choosing a Well Pump: Sizing and Pressure

Choosing a Well Pump: Sizing and Pressure | ERS Shop

The depth to your water decides the pump. A drilled borehole needs a 4-inch submersible pump lowered into the well itself, while a shallow dug well with the water surface within suction reach can be served by a surface-mounted self-priming jet pump. Choose on that basis, size the pump on head and flow rather than wattage, and set the pressure side up properly, and the two most common well pump complaints, weak pressure and constant cutting out, largely take care of themselves.

This guide covers private wells supplying a house, yard or parlour: which type of pump the well needs, how to read the two figures that matter on a pump's data plate, what the pressure tank and switch are actually doing, and what to check when a pump keeps stopping.

SUBMERSIBLE OR JET: THE WELL DECIDES

A surface pump has to pull water up to itself, and physics caps that: Pedrollo rates its JSW jet pumps to 9 metres of manometric suction. No surface pump gets around the limit by being more powerful.

That is what a 4-inch submersible borehole pump is for. It sits below the water level and pushes rather than pulls, so depth stops being the constraint: the Pedrollo 4SR spans flows up to 350 litres per minute and heads up to 576 metres across the range, on clean water with a modest sand tolerance (150–200 g/m³ depending on model).

Before you compare anything: 4-inch borehole pumps are commonly sold as a pump end only. A working unit is pump end plus a separate submersible motor, such as the Franklin single-phase borehole motors, plus a starter box matched to the motor rating . Check what a listing includes.

SIZING: HEAD AND FLOW, NOT KILOWATTS

Two figures size a pump, and motor power is neither of them.

Total head adds up three things:

• the vertical distance from the water level in the well to the highest outlet
• the pressure you want at the tap (roughly 10 metres of head per 1 bar)
• friction losses in the pipe run

So a pump lifting from 30 metres down, asked for 3 bar in the house, needs around 60 metres of head before friction is counted.

Flow is what runs at once: a shower while the washing machine fills and a yard tap runs, not the single busiest outlet.

Both matter together because flow falls as head rises. Pedrollo's JSWm 1A delivers 48 metres of head at no flow but 17 metres at its maximum 60 litres per minute; neither headline figure describes your system. Read the curve at your head and flow, and if your duty point falls near the end of a pump's curve, step up a model.

THE PRESSURE SIDE: TANK, SWITCH OR CONTROLLER

The pump makes the pressure; the tank and switch decide when it runs. A pressure tank holds a cushion of water under air pressure, so small draws come from the tank instead of starting the pump. The switch starts the pump at cut-in and stops it at cut-out.

Two settings do most of the work, both from Global Water Solutions, whose PressureWave tanks ERS stocks:

• Pre-charge: 0.2 bar below the pump's cut-in pressure (for systems with up to 2 bar between cut-in and cut-out)
• Tank size: at least 1 litre of volume per litre-per-minute of pump flow, for fewer starts and longer runs

The alternative is an electronic controller. The Presflo Multi starts and stops the pump on flow demand, with dry-running protection and a restart pressure adjustable from 1 to 5 bar. For shallow wells and storage tanks there are ready-assembled booster sets, such as the JSWm 1A on a 20 L tank.

WHY A PUMP KEEPS CUTTING OUT

Rapid start-stop at every tap points to the tank, not the motor: pre-charge lost or set wrong, or a tank too small, so the pump answers every draw itself. Check against the two figures above.

Cuts out and stays out when the well is worked hard may be dry-running protection doing its job. The fix is the water supply or the pump's depth in the well, not the controller.

Overload trips are hand-reset for a reason: repeated trips are a specification question, worth checking against the pump curve before it becomes a motor repair.

FAQ

How deep can a surface jet pump draw from?

Pedrollo rates the JSW self-priming jet pumps to a manometric suction head of 9 metres. Deeper than that, no surface pump will do it, and the well needs a submersible.

What pre-charge should a pressure tank have?

Global Water Solutions specifies 0.2 bar below the pump's cut-in pressure, for systems with up to 2 bar between cut-in and cut-out. Check it with the system drained of pressure.

What size pressure tank do I need?

The manufacturer's guideline is at least 1 litre of tank volume per litre per minute of pump flow. Bigger means fewer starts and a longer motor life, not more pressure.

The order of decisions, then: establish the water level and well type, which picks submersible or jet; work out total head and simultaneous flow, which picks the model off its curve; then set up the tank or controller so the pump starts as seldom as possible. The full range of borehole and surface pumps, tanks and controls is in Power & Pumping and if you have the well depth, water level and outlets to hand, a specification question asked before ordering is quicker than a return afterwards.