How to choose a submersible pump for a well or borehole

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Submersible pump for a well or borehole – why the choice is so important

A submersible pump is the heart of any water supply system that pumps water from a well or borehole. Unlike a surface pump, it operates directly under the water level, which gives it a crucial advantage – it does not have to deal with suction and suction lift, which is physically limited to approximately 7-8 meters theoretical and in practice more like 6-7 meters real depth. A submersible pump, on the other hand, is able to push water from depths of tens of meters, as it works on the principle of overpressure, not underpressure.

Over the years I have been involved in the sale and installation of pumping equipment, I can say that the vast majority of complaints and dissatisfied customers did not arise because the pump was bad, but because it was poorly selected for the specific well or borehole. Wrong diameter, underdimensioned power, a pump without dry-run protection in a sandy well – these are classic scenarios that can be avoided by making the right choice right from the start. This article is intended to give you a comprehensive overview of how to proceed step by step when making a selection.

Basic types of submersible pumps – screw and centrifugal

Before we move on to specific parameters, it is important to understand that submersible pumps are divided into two main groups according to their construction principle:

Screw (helical) pumps – use a rotor in the shape of a screw that rotates in a flexible rubber stator insert. They pump water smoothly, are particularly resistant to sand and impurities in the water and tolerate short dry runs better than classic centrifugal pumps. A typical example is the Submersible screw pump for narrow boreholes STING 2", which thanks to its narrow diameter fits even into boreholes where a standard centrifugal pump simply cannot fit.
Centrifugal (multi-stage) pumps – consist of several impellers (stages) stacked one after another, with each stage adding to the total head. They are suitable for cleaner water, generally have a higher flow rate at lower energy consumption and are the standard for most domestic boreholes with a diameter of 3" and 4". An example is the Submersible pump IBO 3" STM 20, which is specifically designed for boreholes with an internal diameter of 3" or more.

A detailed comparison of both principles, including advantages and disadvantages for specific use, can be found in a separate article titled "Submersible screw pump vs. centrifugal pump – differences and applications". Here we will just summarize that screw pumps are preferred where there is a narrow borehole and/or dirty, sandy water, while centrifugal pumps are the first choice for normal domestic wells and boreholes with decent water quality.

Diameter of the borehole or well – the first and most important criterion

Before you even start considering power, depth or price, you must know the exact internal diameter of your well or borehole. This is a parameter that cannot be bypassed or compensated for – a pump that is wider than the borehole will not physically fit into it, or will get stuck during lowering or later during removal.

In practice, we most often encounter the following diameters:

Boreholes with a diameter of 2" (approx. 50 mm) – typical for older, narrow boreholes or wells with a smaller diameter of the casing. Only a narrow design, such as the mentioned screw pump STING 2", is suitable here. Standard 4" pumps simply cannot fit in.
Boreholes with a diameter of 3" (approx. 75-80 mm) – a very common standard for newer domestic boreholes, where there is enough space for a multi-stage centrifugal pump, for example IBO 3" STM 20.
Boreholes with a diameter of 4" (approx. 100 mm) and more – the most common diameter for larger boreholes with a higher yield, allowing the use of more powerful pumps with a higher flow rate.
Dug wells – here the diameter is usually not a limiting factor (casings are 80-100 cm), the more important factor is the depth of the water level and the water quality (often with sand and impurities).

An important practical note: the diameter of the borehole is not the same as the diameter of the pump that you can just "push in". You should always allow a minimum clearance of 4-6 mm in diameter, as the pump may slightly sway during lowering, and the borehole may not be perfectly straight along its entire length (especially in older drilled wells, there may be a slight narrowing or deformation of the casing). A detailed analysis can be found in the article "What borehole diameter do I need: the difference between 2", 3" and 4" pumps", or in the article "What pump diameter do you need – 2" or 3" borehole?"

Water level depth and total head – the second key criterion

After solving the diameter, it is time to calculate the required total head. This is a parameter that is most often underestimated or calculated incorrectly in practice. The total head of the pump is not just the depth at which the pump hangs below the water level – it is the sum of several partial heads and losses:

Static head – the distance from the water level (at rest, i.e., before pumping starts) to the point of consumption (e.g., a water tank, household water connection, the highest point of withdrawal).
Dynamic level drop – during pumping, the water level in the well drops (so-called depression), and in some low-yield wells, it may drop by several additional meters beyond the static level.
Friction losses in the piping – the longer and narrower the pipe, the greater the loss; approximately 1-2 m is calculated for every 10 m of pipe at standard diameters and flow rates, but in longer runs (over 50-100 m), this can become a significant factor.
Required pressure at the end point – if you want to have, for example, 3 bar at the tap or pressure tank, this corresponds to approximately an additional 30 meters of head that the pump must "push".

Practical example from a customer case: a well with a static water level 8 m below ground level, which drops an additional 2 m during pumping (total dynamic level 10 m), the house is located 40 m horizontally away, and the pump must push water into a pressure tank set at 3.5 bar (approx. 35 m). We also add friction losses of about 3-4 m. The total required pumping head thus comes to approximately 10 + 35 + 4 = 49 m. When selecting a pump, it is always advisable to allow for a reserve of 15-20 %, as the pump's performance gradually decreases over time due to wear and the well may experience a slight reduction in yield.

This topic is so comprehensive that we have dedicated a separate article to it: Choosing the right pump power and pumping head based on well depth, where you will also find an approximate table and a simple formula for calculation.

Yield of the source and sand content in water

Along with the diameter and depth, it is extremely important to know the yield of the well or borehole, i.e., how many liters of water per minute (or per hour) the source is able to supply without the water level dropping to the bottom or below the pump's suction part. If you install a pump with a flow rate of 60 l/min into a well that can only supply 20 l/min, the pump will regularly run dry, leading to overheating and rapid damage to the motor and seals.

In practice, I recommend choosing a pump with a lower flow rate if the yield is unknown and supplementing the system with a water level or pressure protection against dry running, or a float switch. Many modern pumps already have built-in electronics with such protection directly in the control unit, which significantly extends their lifespan.

The second important factor is the sand and mechanical impurity content in the water. This is a typical issue especially with older dug wells or with boreholes that have poorly executed casing and perforation. If you know the water contains sand:

Prefer a positive displacement pump, which can handle sand much better than a centrifugal pump.
Consider installing an Sand separator 1" on suction after the pump (on the discharge side), which captures mechanical impurities before they reach the distribution system, the pressure tank, or household appliances (washing machine, boiler, radiators).
Install the pump at a sufficient height above the bottom of the well (usually at least 1-1.5 m), so it does not suck up sediments from the bottom, but also low enough to avoid air suction.

Motor power, flow rate and electrical connection

When selecting a specific model, you will encounter several key technical parameters:

Rated flow rate (Q) – given in liters per minute or m³/h, it should match the actual household demand. For a typical family home with a four-person household, a peak demand of around 15-25 l/min is usually considered (e.g., simultaneous use of a shower and washing machine), although the average daily consumption is much lower.
Maximum pumping head (H max) – the value at zero flow, serves more as an orientation, the actual operating point of the pump is always somewhere in the middle of its performance curve.
Power consumption and power supply – domestic pumps are mostly 230 V (single-phase), more powerful or deeper applications may require 400 V (three-phase) supply. It is important to check in advance what power supply is available in the location and whether the electrical installation can handle the required power.
Discharge port diameter – most commonly 1" or 1¼", must match the diameter of the selected rising main (PE pipe or stainless steel hose).

A good practice is to look at the performance (Q-H) curve of the pump in the catalog and verify that your operating point (required flow at the calculated pumping head) lies somewhere in the middle, efficient part of the curve – not at its very edge. A pump operated outside the optimal range has lower efficiency, higher consumption, and faster wear.

Material of the pump and resistance to corrosion

Submersible pumps are made from various materials – from plastic parts through stainless steel to cast iron. For long-term submersion in water, especially if it is well water with a higher mineral content or more aggressive composition, I strongly recommend pumps with a stainless steel casing (AISI 304, or the better AISI 316 for more aggressive water). A stainless steel construction significantly extends the lifespan compared to cheaper plastic alternatives, which over time become brittle and crack, especially with fluctuating water temperatures.

In the case of positive displacement pumps, the quality of the rubber stator is also important – higher quality compounds (e.g., NBR rubber) better resist sand wear and have a longer lifespan than cheaper alternatives.

Comparison of specific models in the range

To go beyond theory, let's look at a concrete comparison of two models that we encounter most frequently in practice.

Parameter	STING 2" (positive displacement)	IBO 3" STM 20 (centrifugal)
Suitable borehole diameter	from 2" (approx. 50 mm)	from 3" (approx. 76 mm)
Water type	including sand content	cleaner water
Typical use	narrower, older boreholes	standard household boreholes
Dry running	can tolerate short periods better	more sensitive, requires protection
Maintenance	stator replacement after some time	check bearings, seals

A more detailed comparison of both brands, including price relations and recommendations for specific types of wells, can be found in the article Comparison of STING and IBO pumps – what to choose.

Installation of a submersible pump – what not to forget

The pump selection is only half of the success, the other half is the correct installation. In practice, I most often encounter these mistakes:

The pump is hung too low, practically at the bottom of the well – it sucks in sediment and sand.
A check valve is missing on the discharge, which causes water backflow and water hammer when the pump stops.
Using an unsuitable, too thin or low-quality lifting rope (instead of a steel or stainless steel rope, only the electrical cable itself is used, which can loosen over time).
Poor dimensioning of the electrical cable for long distances, which causes voltage drop and motor overload.
Missing thermal and current protection (circuit breaker, or possibly a separate control box with dry-run protection).

The complete step-by-step installation procedure, including recommendations for cable fastening, lowering the pump into the well and the first start-up, can be found in the separate article "Installation of a submersible pump into a well or narrow borehole."

Most common faults and how to prevent them

Even a high-quality pump can fail if it is not operated or maintained correctly. Among the most common faults we encounter in practice are:

Motor overheating during dry-run – the most common cause of complete pump failure; it is solved by installing a float switch or electronic protection.
Clogging of impellers or stator with sand – it is manifested by a drop in performance; the solution is a sand filter and re-evaluation of the pump's position in the borehole.
Wear of the rubber stator in rod pumps – it is a common consumable part that can be replaced without the need to buy a new pump.
Cable damage during lowering or lifting – it occurs mainly due to inconsiderate handling or if the cable is not properly fixed to the lifting rope.
Blocking of the check valve – causes constant pump switching (so-called "cycling"), which shortens its lifespan.

A detailed analysis of faults, including instructions on how to diagnose and repair some of them yourself, can be found in the article "Most common faults of submersible pumps and how to solve them."

Submersible vs. surface pump – when to choose which

We also encounter customers who are considering between a submersible and a surface pump. In general, if the water level is more than 6-7 meters below ground level, a surface pump (even with an ejector) will no longer function reliably or will have significantly reduced performance. For deeper wells and boreholes, a submersible pump is practically the only realistic option. On the other hand, for shallow wells (up to 5-6 m), a surface pump can be a cheaper and more easily accessible alternative in terms of service, as it does not need to be pulled out of the well. This topic is discussed in detail in the article "Submersible vs. surface pump for water from a well: what is more cost-effective."

Practical scenarios from everyday practice

Scenario 1 – old dug well with sand: The customer had a well with a water level of 5 m, but the water contained a noticeable amount of fine sand after every heavy rain. We recommended a rod pump due to its resistance to abrasive particles and added a sand filter to the discharge, which significantly extended the lifespan of the pressure tank and household appliances.

Scenario 2 – deep narrow 2" borehole: A family house had an older 2" diameter borehole, into which a standard 3" or 4" pump simply would not fit. The only solution was a 2" rod pump of the STING type, which is specifically designed for such narrow applications.

Scenario 3 – new 3" borehole with good yield: A new construction with a borehole yielding over 40 l/min and a water level depth of 12 m. Here we chose the centrifugal pump IBO 3" STM 20, which offers a good balance of performance and consumption for standard operation of a family house, including garden irrigation.

Common questions (FAQ)

What is the difference between static and dynamic water level?

The static water level is the water level in the well or borehole in a resting state, i.e., when no pumping has occurred for a long time. The dynamic water level is the level to which the water drops during active pumping – the difference between them is called depression and depends on the source yield. When selecting a pump, it is always necessary to calculate with the dynamic water level, not the static one, because it is precisely during operation that the level is lowest.

Can I use a 4" pump in a 3" borehole?

No, the pump must always be smaller in diameter than the internal diameter of the borehole, never larger or the same. A 3" or smaller (e.g., 2") pump is required for a 3" borehole, and it is also necessary to allow for a few millimeters of clearance due to possible irregularities in the casing.

How often is it necessary to lift and inspect the pump?

If there are no problems and the water is clean, regular disassembly is not necessary, and the pump can operate reliably for several years without intervention. However, we recommend paying attention to signs such as pressure drop, increased noise or frequent pump switching – these are indicators that it is appropriate to lift and inspect the pump.

Do I need a pressure tank for a submersible pump?

Yes, a pressure (membrane) tank is a standard part of a household water supply system. It ensures smooth water pressure in the distribution system without constant pump switching for each minor water draw, which significantly extends its lifespan.

How can I determine the yield of my well or borehole?

An approximate yield can be estimated by a pumping test – you let the pump run and observe how quickly the water level drops, or whether the level stabilizes at a certain level after some time (this is a sign of equilibrium between inflow and withdrawal). More accurate data is provided by a hydrogeological assessment, which should be part of the documentation for the borehole if it was professionally drilled.

Can a submersible pump be used in a wider borehole, for example 4"?

Yes, a smaller diameter pump can be used in a wider borehole without any problems. The limitation works only in one direction – the pump must not be wider than the borehole. A submersible pump in a wider borehole is a fully valid option, especially in cases where the water contains an increased sand content, regardless of the casing diameter.

Summary – How to proceed step by step when choosing a pump

To conclude, let’s summarize the entire process into clear steps we recommend following for every submersible pump selection:

Determine the exact internal diameter of the well or borehole (2", 3", 4" or larger).
Measure or have the static water level measured, and if possible, also the dynamic water level.
Calculate the total required pumping head, including friction losses and the required pressure in the system.
Determine the approximate yield of the water source to ensure the pump is neither undersized nor oversized.
Check the water quality – if it contains sand, prefer a multistage pump and add a sand separator filter.
Select the material (stainless steel) and construction type suitable for your conditions.
Ensure proper installation, including a check valve, lifting rope, and dry-run protection.

If you are unsure about any of these parameters, we recommend consulting a specialist before purchasing – an incorrectly selected pump is one of the most common and yet easiest to avoid causes of problems in a household water supply system. A properly selected and installed pump, on the other hand, can reliably serve for ten or more years with no major interventions.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.