Tag Archives: faq

What is a submersible pump float switch and which type do I need?

When researching which submersible pump to buy, you will inevitably see pumps which are available with numerous types of float switch, here we’ll describe their operation and two of the most common types.

Mac-3 Pendant type Float SwitchA float switch is a mechanism used to detect the level of water around a submersible pump, allowing the pump to operate automatically – when the water reaches a certain pre-set depth the float switch will trigger the pump to switch on and start pumping. When the water level drops below another pre-set level, the pump will then be turned off automatically and stops pumping.

This automatic operation ensures that the pump will never “run-dry”. The electric motor in a submersible pump is normally water cooled – the action of pumping the water through the pump body keeps the electric motor within normal operating temperatures – when there is no water present, the pump could run-dry and overheat, cause damage to the motor.

Operation of a pendant type float switch in a sump

The submersible pumps we stock can be supplied with a float switch for automatic operation, or without a float switch for manual operation.

For most of our pump range, there are two options for the type of float switch: a free-moving float attached to the pump via an electrical cable, or a tube float which is normally mounted on the side of the submersible pump.

Free-Moving Pendant type Float Switch

JS-250 Submerisble Pump with Mac-3 Pendant Float SwitchThe standard option is the free-moving pendant float – these are generally manufactured from tough polypropylene plastic with a micro-switch mechanism inside. When there is sufficient depth of water the air-filled bulb will float upwards, triggering the micro-switch to power up the pump. When the water has been pumped away and the level decreases, the float will drop, switching the pump off again. This type of float switch generally requires the submersible pump to be installed in a tank or sump and needs approximately 300 mm of space around the pump to operate properly. This ensures the float doesn’t get snagged or tangled on the sump or tank walls.

Tube-type Magnetic Float Switch

RS-150 Submersible Pump with Agma Magnetic Tube Float SwitchThe second option is the tube-type, or magnetic float switch. This is close-coupled to the pump body and has a float which slides vertically inside a tube – when the float is at the top of the tube, a micro-switch turns the pump on and when the float drops to the bottom, the pump is switched off. This type of float switch is ideal for installing pumps is space-limited areas, where a standard float switch wouldn’t have room to operate.


When purchasing a submersible pump keep in mind the intended use: a manual submersible pump will require you to switch it on and off when required. A submersible pump with a tube type float is ideal for automatic operation in small sumps, and the standard bulb, free-moving pendant float switch is suitable for most other applications requiring an automatic operation.

How to Keep Your Cellar or Basement Dry

One of our most frequently asked questions is “what’s best pump to deal with cellars/basements that periodically get flooded out?” Typically, the flooding occurs after heavy rainfall, if the water table is high or if you live in close proximity to a river/lake. The solution is a submersible pump.

Submersible Pumps

Pedrollo TOP Submersible PumpFor this type of application, we would normally recommend a submersible pump – a small electric pump, which can be completely (or partially) submerged underwater. The pump sucks the water through a strainer in its base and pumps it from the outlet (normally located on the top of the pump) through a hose to a suitable discharge point (the nearest drain or similar).

For ease of use, most submersible pumps are available with a float switch (it’s the round, black and orange part of the pump in the photo) – this a similar to a ballcock in a toilet cistern – when water is present it floats up and turns the pump on, when the water has drained away the float drops down and turns the pump off. This allows for automatic operation, meaning the user doesn’t have to manually turn the pump on and off when the water rises. However, submersible pumps with float switches work best when installed in a sump (essentially a hole dug in the floor) and need to be of a suitable size to allow the float switch to move freely (typically around 400 mm x 400 mm x 400 mm). They will usually only drain water down to a level of a few inches.

Puddle Pumps

RS-Pumps RSD-400 Submersible Puddle PumpIf you prefer not to dig a sump, we can offer an alternative – a puddle pump. These low-level drainage pumps are engineered to pump water from flat surfaces down to around 1 mm deep (hence the name!), and are ideal for cellars, basements, car parks and flat roofs.

Due to their low-level performance, puddle pumps are not supplied with a float switch and as such are normally manually operated.

For fixed installations, such as keeping your cellar/basement dry, we would recommend using an automatic control panel which uses a sensor to detect the presence of water and operate the pump.

For other applications, such as drainage of persistent nuisance water from areas such as flat roofs and car-parks  puddle pumps are best manually operated and can be used like a vacuum cleaner to “hoover” up the water – you can even sweep the water into the pump to help it drain away!

Our Submersible Pump Recommendation

Our most popular submersible pump for domestic drainage is the Pedrollo TOP range, these lightweight yet robust pumps are available in a number of different sizes, each offering improved performance (the bigger the electric motor, the faster is will drain the water away).

Pedrollo also offer a puddle pump, the TOP Floor which drains down to 2-3mm, but our most popular model is the JS-Pumps RSD-400. This stainless steel-bodied pump provides excellent performance and superior low-level drainage.

Which Pump Hose Do I Need?

Layflat Hose

Layflat HoseLayflat is lightweight and convenient hose which rolls flat when not in use.

It is quick to deploy and ideally suited for use as short runs (say 50 m) of delivery hose. The pressure of the water in the hose holds it open and therefore may cause greater friction losses than rigid pipe of the same internal diameter.

Can be used across roads and will accept light traffic driving over it (although pin-hole leaks may occur). For a better solution, see our Road Ramp.

It is not suitable for use as suction hose.

Available in various inside diameters such as 25 mm (1″), 32 mm (1¼”), 38 mm (1½”), 51 mm (2″), 76 mm (3″), 102 mm (4″) & 152 mm (6″). Sizes 51 mm (2”) and up are available with bauer compatible couplings.

Rigid Plastic Polypipe

Polypipe Rigid Delivery HoseA rigid yet lightweight hose ideally suited for use on the delivery side of the pump.

Polypipe is ideally suited to longer runs or semi-permanent installations.

It is supplied in 3”, 4”, 6” and 8” internal diameters in 6 m (20 ft) “sticks” with bauer compatible quick release couplings.

Wire Armoured Hose

Wire Armoured Rubber Suction & Delivery HoseA very durable wire reinforced rubber hose fitted with Bauer couplings.

Wire Armoured Hose is ideal for use on the inlet side of the pump as it won’t collapse under suction, although can be used as delivery hose as well.

Wire Armoured Hose is available in 3”, 4”, 6” and 8” internal diameters in 3m (10 ft) or 6 m (20 ft) lengths.

Green Medium Duty Hose (“Kanaflex”)

Green Medium Duty Suction and Delivery HoseThis is a general purpose, lightweight PVC spiral/corrugated hose ideal for use as suction and delivery hose.

It is flexible, hardwearing and cost-effective making it a suitable alternative to Wire Armoured Hose for smaller pumps and lighter applications.

Green Medium Duty Suction and Delivery Hose is available in a range of inside diameters such as 25 mm (1″), 32 mm (1¼”), 38 mm (1½”), 51 mm (2″), 76 mm (3″), 102 mm (4″) & 152 mm (6″) and in lengths up to 30 m (100 ft). It is available as hose, or as an assembly with bauer-compatible couplings.

What is the difference between a Puddle Pump and a Submersible Pump?

A submersible pump is usually a small electric pump used for drainage duties. As the name implies, the whole pump can be submerged under water – in most cases this is necessary to keep the electric motor cool and prevent overheating.

Submersible Pumps

Submersible pumps can be manually or automatically operated. Manual pumps require the user to switch the pump on when required and off when the water has drained away (dry-running of a submersible pump will normally cause damage to its internals). Automatically operated pumps have a float switch (similar to a ball-cock in a toilet cistern) which switches the pump on and off at pre-set water levels. Submersible pumps with float switches usually require a certain amount of space around the float for it to operate and these work best in tanks or for drainage of cellars/basements in a sump (essentially a hole dug into the floor where water collects).

Submersible pumps will typically drain water down to a level of approximately 30 mm.

A popular example of a domestic-use submersible pump is the Pedrollo TOP range – you can see in the diagram below a typical installation in a sump, with sufficient space for the float to operate unobstructed.

Pedrollo TOP Submersible Pump installation in a sump

Puddle Pumps / Puddle Suckers

Puddle Pumps (also known as Puddle Suckers) are submersible pumps with the ability to drain down to very low levels (typically as low as  1mm, and so can be fully or partially submerged). They are usually manually operated (no float switch) and are ideal for draining water from flat surfaces such as roofs, car-parks and cellars.

Puddle Pump in Cellar

Automatic operation is an option for some puddle pumps in fixed installations through the use of a capacitive water sensor and control unit.


A Puddle Pump is really just a type of submersible pump with a specifically designed impeller and strainer to allow for low-level drainage applications on flat surfaces.

Our two best-selling puddle pumps are the Pedrollo TOP Floor, which pumps down to 2mm and is ideal for domestic drainage duties, and the RSD-400 Puddle Pump, a higher performance pump which drains down to 1mm and due to its stainless steel construction is suitable for domestic and on-site drainage.

See our full range of submersible pumps.

How to understand a Pump Flow Curve

When considering a new pump, you will invariably come across a graph which shows the pump’s performance characteristics. This will normally be a curve (or straight line) of flow rate against head.

Defining Pump Head

Head, usually given in units of meters or feet, is really just a measure of the pressure a pump can produce. It tells you the maximum vertical height to which a pump can deliver water (and not the horizontal distance or maximum hose length it will pump).

Take a look at the curve below for one of our most popular submersible pumps, the RSD-400 Puddle Pump.
RSD-400 Submersible Puddle Pump Flow Curve
You can see that the maximum delivery head for this pump is 11m when the delivery volume (or flow rate) is 0 (zero) l/m. What does this mean? Quite simply that the pump can lift water to a vertical height of 11m, but will not deliver any flow – the weight of the water will equal the amount of “push” the pump can produce (or more correctly the pressure it can generate).

The pump will need to generate 1 psi of pressure to “push” a column of water vertically up by 2.307 ft, therefore our RSD-400 will produce maximum pressure of:

11 m = 36.01 ft

36.01 ft / 2.307 ft = 15.6 psi

So, what if the height you’re pumping to is less than the maximum delivery head? The “left-over” pressure will be used to actually pump the water. For instance, at just under 7 m head, the pump will deliver approximately 100 l/m of water and with zero head the pump will deliver its maximum flow rate, in this case 180 l/m.

Another way to visualise this is to imagine a pump which has a pressure gauge and valve connected on the outlet. With the pump running and the valve closed, the pressure gauge will display the maximum pressure (or head) it can produce. If you slowly start to open the valve and let the water flow out, the pressure will drop until it reads zero when the valve is fully open (maximum flow rate).

This however, doesn’t tell the whole story – you also have to factor in the friction losses in the delivery hose. Using our on-line friction loss calculator, you can determine the pressure (or head) loss for flow through a length of hose.

Let’s consider a real-world example for a submersible “puddle sucker” pump

A cellar measuring 4 m x  4 m has been flooded to a depth of 300 mm. Using the RSD-400 and a 10 m length of 32 mm layflat hose we shall pump the water out through the coal shoot (a total height of 3 m).

RSD-400 Submersible Puddle Pump Pumping Out Flooded Cellar

Referring back to the flow curve, you can see that a 3 m head would give us roughly 150 l/m of flow at the pump outlet. Using our pressure drop calculator, we can determine that the friction losses in the layflat hose would be equivalent to an extra 2.8 m head, making our total head 5.8 m. Again, referring back to the flow curve this gives approximately 112 l/m of flow.

The volume of water in the cellar is as follows:

4m x 4m x 0.3 m = 4.8 m³ = 4800 litres

Therefore, assuming there is no further ingress of water and at a flow rate of 112 l/m, our pump would take 43 minutes to drain the cellar.

How Maximum Suction Lift Changes with Altitude and Atmospheric Pressure for an End-Suction Pump

MountainThe maximum vertical distance an end-suction pump can “suck” water is determined by two parameters: the altitude at which the pump is situated (and therefore the atmospheric pressure) and the design of the pump.

Since end-suction pumps, don’t actually suck the water up the hose (they create a vacuum and rely on atmospheric pressure to “push” the water up), the maximum suction lift is determined by the pump’s altitude. Generally speaking, the closer to sea level the more suction lift is available and the easier the pump is to prime.

The table below shows the theoretical limit and a more realistic maximum for maximum suction lift.

Altitude (m) Atmospheric Pressure (kg/cm2) Theoretical Max. Suction Lift (m) Realistic Max. Suction Lift (m)
0 (Sea Level) 1.03 10.33 7.52
100 1.02 10.21 7.40
200 1.01 10.09 7.28
300 1.00 9.97 7.16
400 0.99 9.85 7.04
500 0.97 9.73 6.92
600 0.96 9.62 6.81
636 (Kinder Scout) 0.96 9.58 6.76
700 0.95 9.50 6.69
800 0.94 9.39 6.58
900 0.93 9.28 6.46
978 (Scafell Pike) 0.92 9.19 6.38
1000 0.92 9.17 6.35
1344 (Ben Nevis) 0.88 8.79 5.98
2000 0.81 8.11 5.29
3000 0.71 7.15 4.34
4000 0.63 6.29 3.47
4810 (Mont Blanc) 0.57 5.65 2.84
5000 0.55 5.51 2.70

How does an end-suction pump work?

Or, more commonly, “How far up will a pump suck?”

A common question asked on our hire desk is “how far up will a pump suck?”. The simple answer is, at least in theoretical terms, 10.34 m. However, as simple answers go, it’s a little bit wrong (or at least not quite right). The reason is that the 10.34 m limit assumes perfect conditions: that you’re pumping at sea level, that the pump produces a perfect vacuum, that the water is cold and that there are no friction losses in the suction hose. In reality, the actual limit is 7 or 8 m.

Why a theoretical 10.34 m limit on suction lift?

Part of realising of how this limit arises from comes from understanding the fundamentals of how an end-suction pump operates. Firstly, pumps don’t suck – they produce a vacuum and rely on atmospheric pressure (or more simply, the weight of the atmosphere) to “push” the water up the suction hose. The water flows from an area of high pressure to an area of low pressure – a low pressure region (i.e. a vacuum) is created inside the pump and the pressure differential raises or lifts the water up the suction hose.

The 10.34 m limit is the point where the weight of the atmosphere “pushing” the water up the hose equals the weight of water in the hose.

Why a 7 or 8 m real-world limit on suction lift?

Since real-world pumps aren’t capable of producing a perfect vacuum, a more realistic limit of 7 or 8 m can be calculated. This is the point where the weight of water in the hose equals the force from the difference between atmospheric pressure and the less-than-perfect vacuum the pump is producing.

Maximum Suction Lift Explained in Pictures

Confused yet? Take a look at the diagram below – Column A is a hollow tube with a cross-sectional area of 1 cm² (why 1 cm²? It makes the following calculations a bit easier) representing the suction hose. If one end of the tube is open to the atmosphere (analogous to the pump being switched off), and the other end is submerged under water, then there will be no pressure differential and so no force applies. Therefore, the water isn’t “pushed” up the tube.

Maximum Suction Lift for An End-Suction Pump

Column B is also a hollow tube of cross-sectional area of  1 cm². If the top end of the tube is sealed and subjected to a perfect vacuum (analogous to a theoretical super-pump) and the other end is submerged underwater, then a pressure differential will exist and the weight of the atmosphere will “push” the water up the tube. The height the water rises to, and therefore the maximum possible suction lift, can be calculated as follows:

Atmospheric pressure at sea level = 14.7 psi = 1.034 kg/cm² (effectively the weight of the atmosphere acting on every square centimetre of the water’s surface).

Vacuum Pressure inside sealed tube = 0 psi = 0 kg/cm² (a perfect vacuum)

Weight of 1 cm³ of water = 0.001 kg

(Atmospheric Pressure – Vacuum Pressure) / Weight of Water

(1.034 kg/cm² – 0 kg/cm²) / 0.001 kg = 1034 cm = 10.34 m

So this gives a value of 10.34 m for the maximum possible suction lift under perfect conditions. In reality this will never be achieved – even the most efficient pumps aren’t capable of producing a perfect vacuum and then you still need to factor in friction losses in the suction hose, water temperature (the warmer the water, the less the suction lift) and even the altitude (as you move up from sea level the atmospheric pressure decreases, effectively reducing the amount of “push” available).

So, what’s a more realistic figure? Let’s ignore water temperature, friction losses and altitude and concentrate only the how much vacuum a real-word pump can produce. Taking a look at the diagram, Column C is also a hollow tube of cross-sectional area of 1 cm² with a sealed top end subjected to a partial vacuum with the other end submerged underwater. Now since the tube is only subjected to a partial vacuum (just like a real world pump produces), the pressure differential is less, meaning that the water is raised or lifted up the tube to a reduced height.

Performing the same calculation again:

Atmospheric pressure at sea level = 14.7 psi = 1.034 kg/cm²

Partial Vacuum pressure inside tube = 4 psi = 0.2812 kg/cm² (a realistic amount how much of a vacuum a pump can produce)

(1.034 kg/cm² – 0.2812 kg/cm²) / 0.001 kg = 752 cm = 7.52 m

So, in this instance our real-world pump would have a maximum suction lift of 7.52 m, however you would still need to factor in other effects such as friction losses in the suction hose – making the actual value even lower.