Pumps: Surface

Pumps: how to choose the right model based on flow rate and discharge head

A pump is chosen based on three technical figures, not the look of the housing: flow rate expressed in m³/h or L/min, total dynamic head (TDH) expressed in metres, and motor power in watts. A submersible pump for a 4-inch well that lifts water from 40 metres deep has nothing in common with a surface pump that drains a pool at 1.5 metres. Confusing the two uses is the most common mistake among buyers who choose on price alone.

Surface pump, submersible pump, sump pump: three families, three uses

A surface pump is installed outside the water and draws it up through a hose submerged in the liquid, with a suction lift limited to around 7-8 theoretical metres (often 6 metres in practice due to head losses). It suits watering, filling tanks or transferring clean water over short distances. A submersible pump, on the other hand, goes directly down into the well, borehole or buried tank: it pushes the water upward instead of drawing it in, which lets it handle much greater discharge heights, sometimes 60 to 80 metres for domestic borehole models. A sump pump or drainage pump, often fitted with an automatic float switch, triggers on its own once the water level reaches a set threshold and is used to clear basement seepage or a flooded cellar after a storm.

Flow rate and head: the two figures to cross-check before buying

The flow rate shown on a product sheet almost always reflects maximum performance, measured at zero head. Under real conditions, as soon as water needs to rise a few metres, flow drops according to the pump’s performance curve, available in the manufacturer’s technical datasheet. For a 500 m² garden watered by sprinklers, a usable flow of 3,000 to 4,000 L/h at 20 metres of head is a common benchmark. To empty a cellar quickly during flooding, it’s better to prioritise a high raw flow rate, even if that means sacrificing discharge height, since the water is usually discharged just a few metres away.

Priming: why some pumps refuse to start

A standard surface pump needs to be primed: the pump body and suction hose must be filled with water before starting, otherwise trapped air prevents the vacuum needed for suction from forming. So-called self-priming models include a chamber that retains a little water after shutdown, which simplifies repeated start-ups, typically for garden use where the pump is switched on and off several times a day. A submersible pump doesn’t have this issue since it’s permanently immersed in the liquid it pumps.

• Surface pump: suction up to 7-8 m theoretical, ideal for watering and short-distance clean water transfer
• Submersible pump: discharge possible over 60-80 m, suited to deep wells and boreholes
• Sump pump: automatic float switch, prioritises raw flow for fast drainage
• Wastewater lift pump: handles solid particles up to 10-35 mm depending on the model, for dirty or muddy water

Materials and durability: clean water, dirty water, hot water

A cast-iron pump body withstands thermal shocks better and lasts longer than a technical plastic body, but it’s heavier and costs more upfront. For water containing sand or fine particles, typical of a shallow well or a poorly filtered rainwater harvesting tank, a stainless steel impeller or vortex design limits premature wear on internal components. A standard clean-water pump should never handle wastewater containing solids: beyond the passage diameter stated by the manufacturer (often 5 to 10 mm on domestic models), the risk of clogging or motor failure becomes real within the first few weeks of use.

Electrical power and running costs

A standard garden pump draws between 600 and 1,100 watts, while a domestic borehole submersible pump can reach 1,500 watts or more depending on the required depth. Over a daily 30-minute use for summer watering, the consumption difference between a 750 W and a 1,100 W model remains marginal on an annual bill, but it becomes significant for a sump pump running continuously during a prolonged rainy spell. Checking for a suitable starting capacitor and built-in thermal protection prevents unexpected shutdowns in case of motor overheating.

Routine maintenance to extend pump lifespan

Rinsing the pump body after each use with dirty water limits deposit build-up on the impeller. Before winter, fully draining an outdoor surface pump prevents the body from cracking due to residual water freezing. For a submersible pump, an annual check of the power cable and seal reduces the risk of water ingress, the most common cause of motor failure on this type of equipment.

What discharge height should I choose for a 15-metre deep well?

A submersible pump capable of 30 to 40 metres of head amply covers this need, as hose head losses and the outlet height at ground level must be added to the well’s simple depth.

Surface pump or submersible pump for automatic irrigation?

A surface pump is enough as long as the water source (tank, shallow well) is within 7 metres of suction lift; beyond that, or for a borehole, a submersible pump becomes necessary.

How do I know if a sump pump can handle solid debris?

The free passage diameter, given in mm on the technical datasheet, specifies the maximum particle size tolerated; below that threshold, clogging is likely from the very first uses.

Why does a self-priming pump cost more than a standard model?

It includes a water retention chamber and an extra air-venting system that simplify frequent restarts, which justifies a higher manufacturing cost compared with a manually primed pump.

What power should I choose for a sump pump in case of a flooded cellar?

Prioritise a high raw flow rate, generally above 8,000 L/h, rather than a high discharge height, since drainage is usually only over a few metres to the outside.