A submersible pump does its job out of sight, sitting down in the water it is moving. Drop one into a flooded cellar, a sump, a borehole or a tank, switch it on, and water simply disappears up the hose. The principle behind almost every submersible pump is straightforward, and understanding it makes choosing the right pump far easier. At the centre of it all is one spinning part: the impeller.
This guide explains what an impeller is, how it creates the pressure that moves water, and why its size and arrangement decide whether a pump is built to shift a lot of water or to push water high. Get that idea straight and terms like flow, head and multistage stop being abstract jargon.
The heart of the pump: the impeller
Almost all the pumps we sell are centrifugal pumps. The name is a slight misnomer, as we will see, but the idea behind them is simple: they use a spinning impeller to move water. An impeller is a disc with vanes on it, fixed to the motor shaft. When the motor runs, the impeller spins at high speed inside a casing, and everything the pump does follows from that.
The vanes are the key. As they spin, they push against the water and accelerate it, driving it from the centre of the impeller out to the rim, and it is the size and shape of those vanes that determine how much water the pump moves and how hard it can push.
How an impeller creates pressure
It is worth being precise here, because the usual shorthand, that "centrifugal force throws the water outwards", is not really what happens. There is no outward force acting on the water. What the pump actually does is use the spinning vanes to create a difference in pressure, and water, like any fluid, always flows from higher pressure to lower pressure. Here is the sequence:
Think of stirring a cup of tea. As you spin the spoon, the tea is pushed out towards the sides of the mug, causing the liquid level to dip slightly in the centre. This creates a lower-pressure area in the middle, and the surrounding tea flows back in to fill the gap. A centrifugal pump works in much the same way: the spinning impeller pushes water away from its centre, creating a low-pressure region that allows more water to flow in, while the pump casing directs the moving water out as a pressurised flow.
So nothing is sucked, and there is no real outward force. The impeller adds energy to the water and creates a pressure difference, and the water flows, as it always does, from high pressure to low. This also explains a practical quirk between pump types. Because the flow into the eye depends on outside pressure pushing water in, a surface pump sitting above the water can only draw it upwards so far before the available pressure is used up, around 7 to 8 metres in practice. A submersible pump avoids the problem entirely: it sits down in the water, so water is already at its intake and the impeller can get straight to work.
Size matters: how a bigger impeller changes things
The single most useful thing to understand is this: the size of an impeller ultimately determines how much water a pump can move and how high it can push it. Change the impeller and you change the whole character of the pump.
A small impeller in a compact pump is perfect for domestic jobs. The Stream SPP2-4 puddle pump, one of our best sellers, weighs around 3 kg and delivers about 70 litres a minute. That is ideal for clearing a flooded patio, draining a hot tub or emptying a water butt, where a bigger pump would be overkill.
Step up to a large, wide impeller and the flow rate climbs dramatically. The APP HD-15 is the most powerful 230V pump we sell, moving an enormous 830 litres a minute. That wide impeller is built to shift large volumes fast, which is exactly what you want for site drainage or a serious flood, but it would be far too much for a small domestic puddle.
So a wider impeller generally means more flow. But flow is only half the story, and this is where a lot of buyers go wrong. Moving a lot of water and pushing water high are two different demands, and they call for two different impeller designs.
The below image compares an APP HD-15 Pump Impeller with a Tsurumi LB480. The HD-15 max Flow Rate is 830 litres/min where as the LB480's is 225 litres/min. Think of the impeller as the pump's paddle wheel. The HD-15 has a much larger paddle wheel with wider channels, so every rotation moves significantly more water. That is why the HD-15 can achieve flow rates approaching 830 litres per minute, compared with 225 litres per minute for the smaller Tsurumi LB480.
Flow versus head, in one comparison
The clearest way to see this is to compare two pumps with the same size motor. Take two 500 watt pumps. One delivers 225 litres a minute but can only push to about 11 metres high. The other delivers just 46 litres a minute but pushes all the way to 60 metres. Same power going in, completely different results coming out, and the only real difference is the impellers.
That vertical pushing height is called head, and it is simply a measure of pressure. As a rule of thumb, 10 metres of head is roughly 1 bar of pressure. A pump tends to be built for one or the other: high flow at low head, or low flow at high head. Our guides to pump curves and how to read a pump curve show how every pump publishes this trade-off on a chart, so you can see exactly what it will deliver at the head of your job.
Stacking impellers: how a slim pump pushes water high
So how does that second 500 watt pump reach 60 metres when the first manages only 11? The answer is the trick at the heart of every borehole and high pressure pump: stacked impellers.
Instead of one large impeller, a multistage pump places several smaller impellers one on top of another along the same shaft. Each impeller is called a stage. Water passes up through the first impeller, which raises its pressure a little, then straight into the second, which raises it again, and so on up the stack. Each stage adds pressure without adding flow, so by the time the water leaves the top impeller it is under enormous pressure.
This is exactly why well and borehole pumps are long and slim. A borehole is a narrow hole drilled deep into the ground, and a wide pump simply would not fit. By stacking narrow impellers vertically, the pump fits down the hole and still generates the high pressure needed to lift water many tens of metres to the surface. The more stages, the more head: tall pumps with more impellers reach higher. The Acuasub range, for example, is built for high head and can reach around 100 metres depending on the model, while a wider, fewer-stage pump like the Acuabig trades some of that pressure for higher flow through a larger outlet. A standard wide single impeller pump gives you the opposite: lots of flow, but far less pressure.
"Multistage" just means stacked impellers
This same idea appears on the surface too, and it catches people out because of the name. If you see a surface pump described as a multistage pump, that is simply telling you it has stacked impellers, the same principle working above ground rather than down a borehole. The Speroni RSX multistage pump, for instance, comes in 3, 4, 5 and 6 stage versions: each extra impeller adds pressure, so the 6 stage model reaches around 60 metres of head while the flow stays roughly the same across the range. That makes multistage pumps the natural choice for sprinklers, irrigation and pressure boosting, where you need pressure more than volume.
So whether it is called a borehole pump, a high head pump or a multistage pump, they are all doing the same thing: stacking impellers in series to build pressure.
Why a pump has to be matched to its job
All of this has a consequence that surprises a lot of people: no single pump can do everything, and that is down to physics, not poor design.
Why submersibles need to be under water
The pressure an impeller can build depends on the density of what it is moving, and water is around 800 times denser than air. Spin an impeller in air and the pressure difference it sets up is tiny, nowhere near enough to draw water up to it. That is why most submersible pumps are designed to sit fully immersed: surrounded by water, the impeller is always working on water, and many also rely on that surrounding water to cool the motor, which is why running one dry can quickly damage it.
Is my pump leaking? Probably not
The same physics explains priming. A surface pump sitting above the water starts with its casing and suction pipe full of air, and until that air is replaced with water the impeller has nothing dense enough to pump. This is why many surface pumps, including engine-driven pumps, need to be primed, meaning their casing is filled with water, before they will draw, and why a foot valve that keeps the pipe full between runs makes starting far more reliable. A self-priming pump is simply one built to clear that air itself, within limits.
Where the impeller sits decides what the pump can handle
The position and design of the impeller, the very thing that makes a pump good at one job, is what makes it poor at another.
A puddle pump has its intake set extremely low so it can pump a floor almost dry, down to 1 or 2 mm. To do that, the gap between the impeller and the base has to be tiny, which means it can only pass very small particles. Ask it to handle solids and that same low, fine intake simply blocks. A borehole pump has the same weakness for a related reason: its stacked impellers run to fine tolerances to build pressure, so grit and debris would jam or wear them, and they need clean water.
A pump built to handle solids, such as a sewage pump, solves this with large openings, a vortex impeller that lets solids pass without really touching them, or a cutter that chops them up first. But those big clearances cost pressure and efficiency, and the intake usually sits up off the floor, so a solids handler cannot also pump down to the last millimetre or push to a high head.
You can have two of three, never all three
Put those together and the conflict is clear. Pumping right down to the floor needs a tiny intake gap, which cannot pass solids. Handling solids needs large clearances, which lose pressure and cannot pump low. Building high pressure needs tight, multi-stage impellers, which cannot pass solids. You can usually have two of these at once, but never all three, and no amount of money or engineering gets around it, because it is the physics of the impeller, not the quality of the pump.
We once explained exactly this to a customer who wanted a single pump that would drain to floor level, pass solids and deliver high pressure, all at the same time. When we told him physics would not allow it, he said he would call another supplier and see what they had. He could not find one there either, because it does not exist. Knowing which two of the three matter most for your job is the real skill, and it is exactly the kind of thing a quick chat with us will sort out in minutes.
Why this matters when you choose a pump
Every pump is a balance between flow and head, set by its impeller. Before you buy, the two questions that matter most are how much water you need to move, and how high or how far you need to push it:
- Lots of water, not very high? You want a wide single impeller built for flow, such as the high flow submersible drainage pumps.
- Water lifted from deep down or pushed under pressure? You want stacked impellers: a borehole pump or a multistage pump.
- A modest domestic job? A compact pump like the Stream SPP2-4 has exactly the right size impeller, and a bigger one would only cost more and wear faster.
If you are not sure which side of that balance your job sits on, our Pump Finder will point you to the right type, the Water Pump Performance Calculator helps you work out the head your pipe run adds, and our guide to surface pump vs submersible pump covers where each type belongs. Or simply call the team on 0115 987 0358 and we will talk it through in plain English.
Frequently asked questions
What is an impeller in a pump?
An impeller is the spinning disc with curved vanes that moves the water. Driven by the motor, its vanes do work on the water and accelerate it outwards, which leaves a region of low pressure at the centre, so the higher pressure water around the pump is pushed in to replace it. Its size and shape decide how much water the pump moves and how high it can push it.
How does a submersible pump create pressure?
The spinning vanes accelerate the water and drive it out to the rim of the impeller, leaving a low pressure region at the centre that the surrounding higher pressure water is pushed into. The fast moving water then enters the volute, the widening casing, where it slows down and that speed is converted into pressure (Bernoulli's principle) before leaving the outlet. Water flows because of the pressure difference the impeller creates, not because it is sucked.
What is the difference between flow and head?
Flow is how much water a pump moves, measured in litres per minute or per hour. Head is how high it can push that water, measured in metres, and it is a direct measure of pressure (about 10 metres of head equals 1 bar). Most pumps are built for one or the other: high flow at low head, or low flow at high head.
What does a multistage pump mean?
A multistage pump has several impellers stacked one above another along the same shaft. Each stage adds pressure without adding flow, so the pump can reach a high head. Borehole pumps use this so a slim pump can lift water from deep down, and surface multistage pumps use it for sprinklers, irrigation and pressure boosting.
Why are borehole pumps long and thin?
Because they have to fit inside a narrow drilled borehole, and because they need many stacked impellers to generate enough pressure to lift water a long way up. Stacking narrow impellers vertically keeps the pump slim while building the high head a deep well needs.
Do I need a high flow or a high head pump?
It depends on the job. To move large volumes a short distance, such as draining a flood or a site, you want a high flow pump with a wide impeller. To lift water from depth or push it under pressure, such as from a borehole or to a sprinkler system, you want a high head, stacked impeller pump. If you are unsure, use our Pump Finder or call the team.
Why do submersible pumps need to be in water?
An impeller builds far less pressure in air than in water, which is around 800 times denser, so out of the water it cannot draw water up to itself effectively. Most submersible pumps also rely on the surrounding water to cool the motor, so running one dry can quickly damage it. It is also why many surface pumps, including engine-driven pumps, must be primed (their casing filled with water) before they will draw.
Can one pump drain to floor level, handle solids and give high pressure?
No, and that is down to physics, not pump quality. Pumping right down to the floor needs a tiny intake gap that cannot pass solids. Handling solids needs large clearances that lose pressure. High pressure needs tight, multi-stage impellers that cannot pass solids. You can have two of these in one pump, but never all three at once.
