Almost every disappointing pump purchase comes from reading one number and ignoring the other. Flow rate says how much water the pump moves. Head says how high it can push it. A pump can be excellent at one and useless at the other, and the specification sheet will look impressive either way.
Flow and head
Flow rate — cubic metres per hour — is the headline figure, and it is measured under ideal conditions: no lift, short hose, clean water. Head — metres — is how high the pump can raise water at all.
The two work against each other. The higher you pump, the less volume you get; at maximum head the flow approaches zero. So the honest question is not «how many cubic metres», but «how many cubic metres at my actual height and hose length». A supplier who cannot answer that is quoting a brochure, not sizing a pump.
Hose length matters as well as height: friction inside a long or narrow hose consumes head just as lifting does. A long horizontal run can cost more than a short vertical one.
Suction height is limited by physics
A surface pump does not pull water up — atmospheric pressure pushes it up into the vacuum the pump creates. That sets a hard ceiling of roughly seven to eight metres in practice, and no model, price or brand overcomes it. If the water sits deeper than that, a surface pump is the wrong tool and a submersible one is the answer.
Note the difference: suction height is limited, discharge height is not. A pump can lift water tens of metres upwards from where it stands, provided the source is close enough below it.
Clean, semi-trash or trash water
The three classes are different machines, not different settings:
- Clean water — wells, tanks, irrigation. Tight tolerances, high efficiency, intolerant of solids.
- Semi-trash — water with sand and small particles: flooded basements, construction pits.
- Trash — larger solids, sludge, debris. Wider passages, coarser impeller, lower efficiency by design.
Running a clean-water pump on dirty water is the fastest way to destroy it: the impeller and seals are not built for abrasion, and the failure is usually not covered by warranty.
Port diameter
The inlet and outlet size sets the practical ceiling on flow. Fitting a narrower hose than the port throttles the pump; a wider hose does not increase output beyond what the pump can pass. Match the hose to the port, and buy them as a set rather than separately.
Engine
Petrol pumps are lighter, cheaper and start easily — right for occasional and mobile work. Diesel is for long continuous duty, where fuel economy and engine life outweigh purchase price. The logic is the same as for generators: estimate running hours before comparing prices.
Common mistakes
- Choosing on flow rate alone, without stating the lift and hose length.
- Expecting a surface pump to draw water from deeper than physics allows.
- Using a clean-water pump on construction or flood water.
- Ignoring the suction strainer — one blockage and the pump runs dry.
- Running dry at all: most pumps are cooled and lubricated by the water they move.
Tell us the lift, the distance and what the water contains — not just the volume — and we will specify a pump that delivers it. We supply, deliver and service the equipment we sell.