How a submersible pump works

According to the theory of evolution, as the complexity of primitive aquatic organisms developed over time, they gained the ability to leave the primordial soup for dry land in the blind pursuit of survival.

If we look at how pond pumps have evolved in the pond keeping hobby over the last decades, we have seen the reverse happen, where technological advancement has enabled the pump to leave dry land for the benefits of the pond.

Consequently, the vast majority of pumps in use in our hobby today are submersible, rather than external or surface-mounted.

The submersible pump has been instrumental in making successful pond keeping accessible to any would-be pond or koi keeper. A reliable pump is essential for successful pond keeping because of its inseparable relationship with a pond’s bio-filter – the fish’s life support system. Pump design has kept pace with our demands for efficiency, robustness, performance and reliability. This has enabled us to quite literally fit a pump and forget about it, expecting it to pump semi-solids in all extremes of temperature, every day of the year. We expect the life-support machine of our pond’s own life-support system to function faultlessly well past its three-year guarantee (which is now standard) with the reliability of many pumps serving well beyond their guarantee.

So how has the submersible pump revolutionised pond keeping?

Ease of installation. Submersible pumps represent plug-and-play technology for the pond. There external counterparts had to be protected from all moisture, requiring forethought as to how you proposed to plumb it in. They required priming before they would pump water, involving a bucket of water, spanner, non-return-valve (and usually an extra pair of hands!). Alternatively, they could be installed below the water level (but still outside the pond) so that the water pressure would prime the pump automatically. Even still, this method would require some creative plumbing, possibly through a pond or filter wall. At least using this method, you would not have to re-prime the pump every time you turn it back on after servicing.

Yet with the submersible pump, the minimum plumbing skills and expertise required for installing as submersible pump is to know the diameter of the discharge pipe, so we can buy and attach the correct diameter flexible hose) using nothing more than a jubilee clip. A submersible pump does not require priming, and provides an instant flow of water at the flick of a switch.


When submersible pumps became really mainstream into the trade, I remember there was a degree of customer cynicism and resistance against dropping a 240V pump directly into a pond full of water (and expensive fish). In fact, at the time, there was a thriving alternative market for low-voltage pumps that represented less of a risk should water and electricity come into contact. However, as a consequence of their rugged design, submersible pumps have had and continue to have an excellent safety record, so much so that low voltage pumps now only occupy a tiny part of the market. The pump casing which houses the electric motor (which in turn drives the impeller) is filled with a liquid epoxy resin which floods and submerges all exposed electrical parts. This liquid epoxy then soon sets, and the guts of the motor is sealed completely with the pump housing. Even though the pump will unavoidably involve moving parts, in a magnetically-driven pump the only moving part is the impeller itself, which is housed and located outside the motor casing, lubricated and cooled by the pond water.

How a pond pump works.

There are essentially three parts to a submersible pond pump.

The motor

The impeller

The volute

These three components can vary in design, shape, power and in the materials that are used to manufacture them. Once assembled they form a centrifugal pump.

What is the centrifugal pump?

You will be familiar with centrifugal force if you remember how you felt the last time you experienced the forces on a playground roundabout. The spinning action causes the water in a pump (or you and me on a roundabout) to be thrown outwards. But that is only part of the story as regards a pump. Water enters the pump at the centre of the rotating impeller (sometimes referred to as the eye) and gains energy as it moves to the outer diameter of the impeller. The pond water is forced out of the pump by the energy it obtains from the rotating impeller. Simultaneously, because water is being pushed away from the eye by the centrifugal force, the pressure is at its lowest at the eye causing water to be drawn into the volute from the pond.

The motor. The motor in a typical submersible pond pump consists of electrical parts that are safely separated from the moving of mechanical part (shaft and impeller). Being ‘solid-state’ with no moving parts, the motor uses electromagnetic fields to drive the impeller.

Submersible pond pumps can be divided into two groups, depending on how the motor causes the impeller to rotate.

Synchronous. This involves a permanent magnet that is attached to the impeller. The motor causes an outer electromagnetic field to drive the inner permanent magnet that is mounted to the impeller in a circular motion, causing the impeller to rotate and pump water. A useful way of determining whether your pump is driven by a synchronous motor is to test whether the impeller is fixed to a permanent magnet. If it is, then it is highly likely that the pump is synchronous. They are called synchronous motors because they use an inner and outer magnet ring with an equal number or size of magnets in each ring, with the inner impeller rotating in unison with the outer magnetic field.

Asynchronous. This kind of pump only uses a permanent magnet in the motor, with the impeller that is attached to an iron core with either aluminium or copper bars covered with a corrosion-resistant material. The rotating magnetic field induced by the electromagnetic motor makes the impeller itself into an electro magnet that follows the magnetic field and spins accordingly. They are called asynchronous because the impeller will spin at a different speed to the inducted magnetic field.

These magnetic drive pumps do not employ a moving process shaft seal (that would be found in say an external pump preventing the pond water from entering the electrics). Consequently, these sealless submersible pumps are made safe by a stationary physical barrier (the plastic casing and resin) that lies between the impeller and the motor, preventing any water ingress.

The impeller. The impeller end design must not be underestimated as a highly influential feature that will affect a pump’s performance. Their size, proximity to each other and shape (straight or curved) will all combine to produce different pump performances. The solids-handling pumps will have larger, widely spaced impeller vanes, whereas high-pressure pumps will tend to have closely spaced vanes.

The volute. Sometimes also called the diffuser, this encases the impeller and will also affect a pond’s performance by how it marries with an impeller. The tighter the fit, the higher the pressure that pump will be able to produce, but the more likely it will be to clog. The volute also determines the diameter of the discharge pipe, which in turn will set the diameter of pipework used around your pond.

Pump performance and efficiency.

Pump performance is routinely measured as a combination of two factors; flow rate and pressure (head). Every pump will have a performance curve showing how its output (flow rate) changes when it is required to pump water to different heights. There is a trade-off between flow rate and head, with flow rate decreasing as head increases. When choosing a pump, you should consider both the volume of your pond and filter system, and the vertical distance above your pond water’s surface required to pump i.e. to a waterfall.

Friction losses. In addition to taking into account the negative effect head will have on the performance of a pump, attention should also be shown to friction loss. Whenever water flows through pipework, fittings, valves, elbows and even straight connectors, it will encounter differing degrees of resistance and therefore friction. When studying at pump’s performance curve, remember that these will not take into account friction loss. There are mind bending formulae and calculations that can be applied to different fittings and pipe work to work out the friction loss. But rather than introduce these figures and equations here, it is worth noting that you should make additional allowances for friction loss when considering which pump to buy.

A good pump selection would be one whose flow rate is sufficient for your pond’s volume (turnover the pond every two hours) at approximately half that of your pump’s head capacity. This will give you a little extra power should you need it as a result of unexpected friction loss or inappropriate selection of the diameter of pipe work.


Pumps are required to run continuously to maintain a mature and healthy bio-filter. Just as you might compare the fuel consumption of two comparable cars, it makes financial sense to compare the running costs of two comparable pumps. If you run your pump non-stop throughout the year, that is a total of 8760 hours’ work and electricity consumption that must be paid for.

If you have a choice between pumps of comparable performance, assess their power consumption (in Watts) and make the necessary calculation; assuming they offer the same guarantees.

Eg Pump A 4000GPH @ 8′ head 500W

Pump B 4000GPH @ 8′ head 600W

Assuming 1 kilowatt-hour of electricity costs approximately 10p, then across the duration of the year, pump B will cost 100W x 8760 hours = 876Kwh x10p = 87.60 more than pump A.

Assuming both pumps will run for three years, then it would be financially prudent purchase pump A even if it cost 260 more (representing a saving of 3 x 87.60) than pump B.

Electromagnets and motors. An electromagnet starts with an electricity source and a wire. As the electrons flow from the positive terminal through the wire to the negative terminal, a small magnetic field is generated in the wire. This small magnetic field is the basis of an electromagnet and is true for any wire carrying electricity. You can increase the power of the magnetic field by wrapping the wire around a coil; the greater the number of turns the stronger the electromagnet. A pump motor uses magnets to create the movement we see in a pump’s impeller. Opposite poles attract, like poles repel. Inside an electric motor, these attracting and repelling forces create rotational motion. The fixed magnet within the pump body and the ever-switching electromagnet created by the action of the electricity on the coils of wire cause a rapidly-changing attraction and repulsion forces resulting in the impeller (which is itself either a magnet or an inducted electromagnet) turning.

Kill blanketweed and string algae.