Quick answer: You can read most of what a pump does from the outside. One connection means a submersible pump, its inlet is the strainer that sits in the water; two connections, an inlet port and an outlet port, mean a surface mounted pump that stays dry and pulls through a suction hose. Then look closer. Small holes or slots in the strainer mean a drainage pump for clean or lightly dirty water. A pump standing on legs with a big open inlet is a sewage pump built to pass solids. A flat rubber or plastic base with the inlet at floor level is a puddle pump that pumps down to a millimetre or two. A tall thin body is a borehole pump that gives pressure, not volume, and a short wide body is a drainage pump that gives volume, not pressure. The outlet size tells you the flow class, and the float tells you how big a sump it needs. Then check the data plate to confirm.
Every week someone sends us a photo of a pump and asks what it is for, whether it will do a new job, or what to replace it with. Often the data plate is corroded or missing. The good news is that pump manufacturers are consistent: the job a pump is built for shows in its ports, its strainer, its base, its shape, its outlet and its float. Here is how we read a pump before we have read a single number.
1. First: submersible or surface mounted?
The quickest tell of all is where the water goes in. A submersible pump has one connection, the outlet. Its inlet is the strainer or the base, which sits in the water, and the whole pump, motor included, is sealed and goes under. A surface pump has two connections, an inlet port and an outlet port, usually at opposite ends of the casing or at right angles to each other, with a filler plug on top. It sits on feet or in a frame outside the water and draws through a suction hose. Petrol and diesel pumps are always surface pumps, and so are the jet, peripheral and multistage booster pumps such as the Speroni ranges, and every swimming pool pump.
Other giveaways: a submersible has a long rubber cable and a carrying handle but no fan grille, because the water cools it. A surface pump has a motor with cooling fins or a fan cowl, often a pressure switch or controller, and must never be submerged or left in a flooding plant room. Which to choose is covered in surface pump versus submersible pump; the short version is that a submersible goes where the water is and pushes, while a surface pump stays dry and pulls, and pulling is limited by physics to about 7 or 8 metres of lift.
2. The strainer: what the holes tell you
The strainer is the first thing to look at, because its holes are the pump's free passage. Nothing bigger than the hole can reach the impeller, so the manufacturer sizes the holes to match what the impeller can safely pass. Our guide to pump solids handling explains the numbers; the strainer shows you them.
Lots of small holes or narrow slots
A drainage or sump pump for clean or lightly dirty water. The holes are a few millimetres across, so the pump will pass grit, silt and small soft solids but nothing larger. The Tsurumi OM5 and 50PNI2, the AMA Drainer 301, the Arven Mizar and Regal and the APP BPS all look like this, with free passages between 5 and 10 mm. If the water will carry anything bigger, this is the wrong pump, and a stone jammed in the strainer is the usual reason one of these stops.
Large holes, a wide cage or an open base
A dirty water or site drainage pump. The strainer is there to keep out stones and rags rather than grit, and the impeller behind it is built to take sand. The Tsurumi LB and HS, the EVAK Drainox and the APP HD-15 are the common examples, passing 6 to 10 mm solids with a wear resistant impeller to survive what gets through.
No strainer at all, and the pump stands on legs
A sewage or solids handling pump. The legs lift the inlet off the chamber floor so that the whole mouth of the volute is open and large soft solids can be drawn straight in. There is no strainer because a strainer would defeat the point. Look inside and you will usually see a recessed vortex impeller, pulled back out of the flow so solids pass without touching it. The EVAK Hippo, APP BCV and APP SV all stand on legs for this reason, and pass 27 to 80 mm solids. If a pump looks like this, it is for foul water and dirty water with solids, not for clean water or for pumping a floor dry.
3. The base: flat and rubbery means a puddle pump
Turn the pump over. If the base is a flat rubber or plastic plate with the water inlet in the side or the underside right at floor level, you are holding a puddle pump. A normal submersible pump draws water in through a strainer some way up its body, which is why it leaves a few centimetres behind. A puddle pump draws from the very bottom so it can pump down to 1 or 2 mm, and the soft base lets it sit directly on tiles, screed or a lift pit floor without marking them. The Tsurumi LSC, EVAK Residox, Stream SPK530 and SPP2 and the EGO 500 LS all have this look. The trade-off is solids: a puddle pump passes 2 to 9 mm at most, so it is a flood and floor pump, not a sewage pump. Our guide to what a puddle pump is covers where they earn their keep.
One more tell: many puddle pumps have no float at all. Either they are manual, because they are used while someone is there, or they use an electronic sensor or probes mounted low on the body, because a conventional float cannot work in 20 mm of water.
4. The shape: tall and thin gives pressure, short and wide gives volume
This one surprises people. Two pumps with similar motors can look nothing like each other, and the shape tells you which way the designer has gone.
A tall, thin pump about 100 mm across and half a metre or more long is a borehole pump. Inside it is a stack of small closed impellers, each adding pressure to the one before. It will push water a long way up, but not much of it. The Umbra Pompe Eurojet is a good example: 0.55 kW, 100 mm in diameter, 46 litres a minute, but up to 60 metres of head.
A short, wide pump with a single big impeller is a drainage pump. It moves a lot of water but will not push it very high. The Tsurumi HS2.4S has a 0.4 kW motor, moves 205 litres a minute and reaches 12 metres. Same order of motor, four times the water, a fifth of the head. The physics behind this is in high flow versus high head, and it is why kW tells you so little about performance.
So if a pump is tall and narrow, it belongs down a borehole or in a well, and it will be no use emptying a flooded cellar quickly. If it is squat and wide, it is for moving volume at modest lift, and it will not feed a sprinkler system on a hillside.
5. The outlet: size and position
The outlet size is a quick guide to the flow class. A 25 mm (1 inch) outlet usually means a small puddle or sump pump moving up to about 200 litres a minute. A 32 mm (1¼ inch) outlet is typical of domestic sump pumps. A 50 mm (2 inch) outlet means a serious drainage or sewage pump, typically 200 to 500 litres a minute, and an 80 mm (3 inch) outlet means high flow, 500 litres a minute and more, such as the APP HD-15, the Tsurumi NK or the larger BCV models. The outlet size is also the hose size you will need, which matters more than most people expect; see how hose length and diameter change pump performance.
Position matters too. A top outlet means the pumped water travels up past the motor and cools it, which is how pumps like the Arven Mizar and the Tsurumi Family-12 keep cool with the water level low. A side outlet on a cast iron body is the sewage pump pattern, where the volute is wide to pass solids and the motor is cooled by being submerged.
6. The float: how big a sump it needs
The automatic control tells you the kind of sump or chamber the pump was designed for.
- A pendant float on a cable needs room to swing, so the pump wants a sump at least 300 mm across. The cable length can be adjusted to change the on and off levels. The Tsurumi HS and LB automatics and the EVAK Hippo automatics use one.
- A vertical tube or rod float rises and falls inside a guide, so the pump will work in a narrow chamber, but the switching levels are fixed. The Tsurumi POMA and the APP BPS100EA are built this way, and it is the first thing we suggest for narrow sumps.
- Probes or a floor sensor with no float mean a puddle pump designed to start and stop in a few millimetres of water, such as the LSC1.4S Automatic, the Residox automatics and the SPK530A.
- No float and no sensor means a manual pump. It runs while it is plugged in, and it is usually the better choice for a pump that is deployed by hand in a flood.
Our guide to automatic pumps and their switches goes through each type in detail.
7. Engine pumps: look at the inlet and the strainer
Petrol pumps follow the same logic. A clean water or dirty water engine pump such as the Honda WB20 or WB30 has a fine mesh suction strainer on the end of the hose and passes solids of about 6 mm; it is for flood water, irrigation and transfer. A trash or semi-trash pump such as the Tsurumi TDS has a bigger inlet, a coarser strainer and a wider volute, and passes 20 mm solids; it is for site water with debris. A diaphragm pump such as the Tsurumi TD has no impeller at all, which is why it will pass sludge and 30 mm solids that would stall a centrifugal pump. If you are not sure which you are looking at, the strainer basket gives it away: little holes for clean water, big holes for trash.
Two more clues on engine pumps. A filler plug on top of the pump casing means it must be filled with water before the first start, even though it is self-priming. And the suction hose length is limited by physics, not by the pump: no surface pump can draw water up more than about 7 or 8 metres, because it is atmospheric pressure that pushes the water up the hose, and in practice you should keep the lift to around 6 metres. We explain this in pumping water from a river or stream.
| What you see | What it means | Typical free passage | Examples from our range |
|---|---|---|---|
| Two ports, a filler plug and a fan cooled motor | Surface mounted pump, installed dry, pulls through a suction hose | Clean water, or as rated | Speroni CM, CA(M), RSX; Honda WX10, WB20, WB30; pool pumps |
| One connection, sealed body, long rubber cable | Submersible pump, goes in the water and pushes | See the rows below | Every pump in the rows below |
| Small holes or slots in the strainer | Drainage or sump pump, clean or lightly dirty water | 5 to 10 mm | Tsurumi OM5, 50PNI2; AMA Drainer 301; Arven Mizar, Regal; APP BPS |
| Large holes or an open cage, heavy casing | Site drainage pump for sandy water | 6 to 10 mm | Tsurumi LB, HS, NK; EVAK Drainox; APP HD-15 |
| No strainer, pump on legs | Sewage or solids handling pump | 27 to 80 mm | EVAK Hippo; APP BCV, SV; Speroni SXS |
| Flat rubber or plastic base, inlet at floor level | Puddle pump, pumps to 1 or 2 mm | 2 to 9 mm | Tsurumi LSC; EVAK Residox; Stream SPK530, SPP2; EGO 500 LS |
| Tall and thin, about 100 to 130 mm across | Borehole or well pump, high head and low flow | Clean water only | Umbra Pompe Eurojet, Acuasub, Acuatec, Acuabig |
| Short and wide, single big impeller | Drainage pump, high flow and low head | 5 to 10 mm | Tsurumi HS; APP HD-15 |
| Agitator under the impeller | Slurry pump for settled silt and sand | 10 mm plus | Tsurumi HSD2.55S; EVAK Trenchman, Agivort |
| Engine with fine strainer basket | Clean or dirty water engine pump | 5 to 6 mm | Honda WX10, WB20, WB30; Tsurumi TE |
| Engine with coarse strainer and big inlet | Trash or diaphragm pump | 20 to 30 mm | Tsurumi TDS, TD |
What this means when you are buying
Match the look to the job before you look at the numbers. If the pump has to stay dry on the bank or in a plant room, you want two ports and a surface pump; if it can go in the water, a submersible. If the water is clean and needs to go high, you want the tall thin pump. If a floor needs to be dry, you want the flat base. If there are solids, you want the legs. If the chamber is narrow, you want the tube float. Only then compare flow and head on the curve, at the head you will actually be pumping to, as explained in how to read a pump curve.
Reading the pump also stops the most common mistake we see, which is buying a dirty water pump with a strainer halfway up its body to clear a flooded floor, then wondering why it leaves 30 mm of water behind. The pump is not faulty. It was never built for that job.
Then confirm it on the data plate
If you would rather we read it for you, use the Pump Finder, or browse all pumps, where every product page lists the strainer, free passage, outlet and float type.
Order online, or by phone. Order by 2pm for next working day dispatch.
