The Water Pump Handbook.

Most pumps that fail to impress were the wrong pump for the job, not a bad pump. The flow printed on the box is a maximum, measured with clean cold water and no hose. A real installation adds lift, hose length, fittings and dirty water, and the flow drops accordingly.

This guide shows you how to work out what you need before you buy, how to read a pump curve, and how to tell the pump types apart, so the pump you order online does what you expect on the day.

Replacing a pump: photograph the data plate first. Model, voltage and how long it lasted tell us most of what we need. Buying new: work out the volume to move, the flow you need, the vertical lift from water level to the end of the pipe, and the horizontal run with its elbows and valves.

Then the liquid: clean, silty, sewage or chemical, and how hot. The largest solid it must pass sets the free passage you need. Finally the power source: 230 V domestic, 110 V site, 400 V three-phase, 12 V, or petrol and diesel for outdoors only. If the suction lift would be more than about 6 m, you need a submersible, not a surface pump.

Flow runs along the bottom, head up the side. The right-hand end of the line is maximum flow at no head; the left-hand end is shut-off head, where water stops coming out. Your installation sits somewhere in between: that is the duty point.

Aim for the middle third of the curve. Off the right-hand end the motor is overloaded; off the left-hand end it is wasting energy and wearing the seal. Leave headroom too, because hose scales up and impellers wear.

Submersibles sit in the water, need no priming and only have a discharge hose. Surface pumps sit beside the water, need an inlet and an outlet, and must be primed. Engine pumps are surface pumps with a petrol or diesel motor, for outdoors and off-grid work.

Within those, the impeller decides what the pump can swallow: closed impellers for clean water and pressure, semi-open for grit, vortex for soft solids, single-channel for sewage, and cutters when rags are involved.

Manual pumps run as soon as they have power and need someone to switch them off. Automatic pumps start and stop on a level control: pendant floats on a lead, tube-guided floats for narrow sumps, arm floats, and probe sensors for very low levels.

The sump has to give the float room to travel, otherwise the pump short-cycles. Most motors allow about 25 starts an hour. Pressure switches and demand pumps start when a tap opens. Thermal protection is a backstop, not a feature to rely on.

kW on the plate is output; input is higher. The amp figure is running current, and starting current is four to six times that, which is what a generator or breaker has to cope with. IP ratings: the first digit is solids, the second liquids; IP68 means fully submersible. Duty S1 means continuous.

Inlet and outlet sizes given in inches are threaded; in millimetres they come with a hose tail. Suction lift is 10 m in theory and about 6 m in practice for water, less for fuels and oils.

Free passage, cavitation, shut-off head, vortex impeller, oil lifter, lip seal, S1 duty, inrush: the words on a data sheet, each explained in a sentence or two, with a link to the article that goes deeper.

The full A to Z lives on its own page so each term can be linked from the specification tables on our product pages.

Runs but no water, tripping the RCD, lost flow, will not start, short cycling, noisy, overheating, will not prime, float not working. Nine symptoms, each with the two-minute checks, the less common causes and when to stop.

If you are still stuck, send us the details by form and we will email you the next steps. Troubleshooting and warranty are handled by email so we can look at your photos and check the pump properly.