Low pressure from a borehole pump is one of the most common complaints from owners of private water supply systems, and it is also one of the most misdiagnosed. The pressure at the tap is the end result of several interacting components: the pump itself, the pressure vessel, the pressure switch, the pipework between the borehole and the property, and the borehole's own water yield.
When pressure falls below acceptable levels, the cause could lie in any one of these components, or in a combination of them, and identifying the right cause before spending money on a replacement pump is the most important step in resolving the problem efficiently.
This guide works through each of the main causes of borehole pump pressure loss, how to identify which is responsible for the symptoms you are experiencing, and what the resolution looks like in each case.
How Pressure Is Generated in a Borehole System
Understanding how a borehole pump system generates and maintains pressure makes the diagnostic process considerably more logical. A submersible borehole pump pushes water up through the rising main to the property, where it passes through a pressure vessel before entering the household plumbing. The pressure vessel contains a diaphragm or bladder with a pre charged air side: as the pump fills the water side of the vessel, the air charge compresses, storing pressure energy. When a tap is opened, the stored pressure drives water to the outlet without the pump needing to start immediately. The pump starts again when vessel pressure drops to the cut-in point on the pressure switch, and stops when pressure reaches the cut out point.
When this system works correctly, pressure at the tap is consistent and the pump cycles at a sensible frequency. When something in this chain is not performing as it should, low pressure, erratic pressure, or rapid pump cycling are the typical symptoms.
Waterlogged or Failed Pressure Vessel
The most common cause of low pressure and rapid pump cycling in a borehole system is a waterlogged pressure vessel. This occurs when the bladder or diaphragm inside the pressure vessel fails, allowing water to fill the air chamber. Without a functioning air charge, the vessel cannot store useful pressure energy. The pump starts almost immediately every time a tap is opened, runs briefly, and stops as pressure reaches the cut-out point, only to restart again within seconds when the pressure drops. The pressure at the tap feels low and inconsistent because there is no stored pressure buffer to maintain flow between pump starts.
Diagnosing a waterlogged pressure vessel is straightforward: with the pump switched off and the system depressurised, check the Schrader valve on the air side of the vessel. If water squirts out rather than air, the bladder has failed and the vessel needs replacing. If air escapes but at low pressure, the air charge may simply need topping up to the manufacturer's specified pre-charge pressure. A functioning pressure vessel is fundamental to correct system performance, and replacing a failed vessel is typically less expensive than investigating the pump as a first response.
Incorrect Pressure Switch Settings
The pressure switch controls the cut in and cut out pressures at which the pump starts and stops. If the pressure switch settings have drifted, been incorrectly adjusted, or the switch itself is faulty, the system will either fail to reach an adequate delivery pressure or cut out before pressure has been properly restored. A pressure switch set with a cut out pressure that is lower than intended will result in a system that feels low-pressured even with a fully functioning pump and vessel.
Pressure switch settings should be checked against the manufacturer's recommended values for the system and adjusted if they have drifted outside the correct range. The switch mechanism should also be inspected for contact corrosion or contamination, which can cause the switch to operate inconsistently or to stick in a position that prevents normal pressure cycling.
Pump Wear and Impeller Degradation
If the pressure vessel and pressure switch are functioning correctly but pressure is still lower than it should be, the pump itself may be the cause. Submersible borehole pumps rely on close-running tolerances between the impeller and the pump diffuser to generate pressure efficiently. As these tolerances increase through wear, the pump's ability to generate pressure at a given flow rate deteriorates. A pump that once delivered 4 bar at the surface may now be delivering 2.5 bar under the same conditions, not because anything has suddenly failed but because gradual wear has progressively reduced its pressure-generating capability.
Worn impellers are most commonly caused by running in water with suspended sand or grit content, running the pump in dry or near dry conditions, or simply age and accumulated operating hours. A pump that is reaching end of service life on the performance curve will show reduced pressure at all flow rates compared to its rated specification. The borehole and well pump range covers replacement submersible pumps across 3 inch, 4 inch, and larger casing sizes to suit all standard UK domestic and agricultural borehole installations.
Air Ingestion in the Rising Main
Air entering the system through a leak in the rising main or at the pump head connection causes a loss of consistent pressure delivery. Air pockets in the column of water above the pump compress under pump pressure rather than transmitting that pressure forward efficiently, resulting in pulsing flow and inconsistent pressure at the tap. If the pressure at the tap is erratic or pulsing rather than simply low and steady, air ingestion is a likely contributor.
Rising main leaks below the water table are particularly insidious because the leak point is inaccessible without pulling the pump, and the symptom of air ingestion may not appear immediately if the leak is small. Any borehole system that begins showing air related symptoms after previously performing well should have the rising main inspected, starting with any accessible connections at the pump head and surface manifold before considering a full pull of the pump string.
Reduced Borehole Yield
In some cases, low pressure is a symptom not of the pump system itself but of the borehole's reduced ability to supply water at the rate the pump is drawing. If the borehole yield has declined, due to seasonal water table variation, aquifer depletion, or progressive borehole development over years, the pump will begin to draw down the water level in the borehole faster than the aquifer can replenish it. As the water level drops towards the pump intake, suction conditions deteriorate, the pump operates against a higher effective head than it was specified for, and delivered pressure falls.
A yield test, comparing current borehole recovery rate against the original drilling records, is the appropriate diagnostic step where reduced borehole yield is suspected. If yield has genuinely declined, lowering the pump setting depth or reducing the pump's operating flow rate to match the available yield are the primary solutions.
Frequently Asked Questions
How do I know if my pressure vessel has failed?
Switch off the pump, depressurise the system, and press the Schrader valve on the air port of the pressure vessel; if water flows out rather than air, the internal bladder has failed and the vessel needs replacing.
Can a pressure switch cause low pressure?
Yes; a pressure switch with a cut-out setting lower than intended will cause the pump to stop before adequate pressure is reached, and a faulty or corroded switch may operate inconsistently, preventing the system from maintaining stable pressure.
How long should a borehole pump maintain pressure between cycles?
A correctly sized and functioning pressure vessel should sustain adequate delivery pressure for at least one to two minutes of normal household use before the pump needs to restart; very short cycling intervals of only a few seconds indicate a pressure vessel problem.
Is low pressure always a sign the pump needs replacing?
No; the majority of low pressure problems in borehole systems are caused by a failed pressure vessel, incorrect pressure switch settings, or air ingestion rather than pump failure, and investigating these first is considerably less expensive than an unnecessary pump replacement.
Final Thoughts
Low pressure from a borehole pump is rarely a simple problem with a single cause, but working through the diagnostic process logically, starting with the pressure vessel and pressure switch before investigating the pump itself, resolves the majority of cases without the cost and disruption of an unnecessary pump replacement. When the pump is the genuine cause, whether through impeller wear, age, or incorrect original specification, replacing it with a correctly sized and specified submersible pump from a reputable manufacturer is the most cost-effective long-term solution.
Browse the complete range of borehole and well pump systems and accessories at Henry Pumps to find the right replacement specification for your private water supply installation.