A pump that starts and stops every few seconds, even when no taps are running or demand appears minimal, is one of the clearest signs of a problem in a pressurised water system. This behaviour is called short cycling, and it is one of the most damaging operating patterns a pump can experience over time. Every rapid start puts stress on the motor windings, bearings, and control contactors that a pump running for sustained periods at normal load does not experience. Left unresolved, short cycling significantly shortens pump service life and can lead to premature motor failure.
In most cases the cause is straightforward and the solution is equally clear: a correctly specified and functioning pressure vessel. This guide explains why short cycling happens, how the pressure vessel addresses it, and what to check if a system is cycling more than it should.
What Short Cycling Is and Why It Damages Pumps
A pump in a pressurised water system is controlled by a pressure switch that starts the pump when system pressure drops to a set cut in point and stops it when pressure reaches the cut out point. In a correctly functioning system, the pump runs for a meaningful period each time it starts, building pressure in the system and in the pressure vessel before stopping. The stored pressure in the vessel then sustains delivery pressure while the pump is off, meaning it does not need to start again for every small draw off event.
Short cycling occurs when the pump starts and stops within seconds rather than running for a reasonable period. Each start subjects the motor to inrush current, which is typically five to seven times the running current, generating heat in the windings and mechanical shock in the pump and motor assembly. A pump cycling several times per minute sustains this stress many hundreds of times per hour, accumulating motor winding fatigue, bearing wear, and contactor erosion at a rate that dramatically shortens the system's operational lifespan. What should be a ten to fifteen year motor life can be reduced to two or three years in a severely cycling system.
Why the Pressure Vessel Prevents Short Cycling
The pressure vessel contains an internal bladder or diaphragm that separates the vessel into a water chamber connected to the system and an air chamber pre-charged to a set pressure. When the pump runs, it fills the water side of the vessel and compresses the air charge, storing energy as pressure. When demand arises, the stored pressure in the vessel drives water to the outlet without requiring the pump to start. Only when the vessel pressure drops to the pressure switch cut in point does the pump start again.
The critical factor is the volume of water the vessel can store and release between the cut-in and cut-out pressures of the pressure switch. This is called the drawdown volume, and it determines how long the system can sustain delivery pressure before the pump needs to start. A correctly sized vessel with an adequate drawdown volume means that small demand events, a brief tap run, a toilet filling, a short garden irrigation cycle, are met entirely from the vessel without the pump starting at all. The pump only runs when demand is sustained enough to deplete the vessel's drawdown volume, at which point it runs for a useful period to refill the vessel rather than starting and stopping within seconds.
The Two Most Common Causes of Short Cycling
In most pressurised pump systems, short cycling has one of two root causes:
- A failed or waterlogged pressure vessel bladder: when the internal bladder or diaphragm fails, water fills the air chamber and the vessel loses its ability to store pressure energy. Without a functioning air charge, the vessel provides no drawdown volume and the pump starts immediately every time any demand occurs. This is the most common cause of sudden-onset short cycling in a previously well-functioning system. Confirming the diagnosis is straightforward: with the system depressurised, pressing the Schrader valve on the air port will produce water rather than air if the bladder has failed, and the vessel must be replaced.
- An undersized pressure vessel: a vessel that is too small for the pump output and pressure switch differential provides insufficient drawdown volume to buffer small demand events. The pump depletes the vessel's storage capacity almost immediately after starting and must restart again as soon as the next demand occurs. This typically presents as a system that has always cycled more frequently than it should, rather than as a sudden change in behaviour, and the solution is to replace the vessel with a correctly sized alternative.
How to Confirm the Vessel Is the Problem
Before assuming the pressure vessel is the cause of short cycling, a brief diagnostic check eliminates other possibilities. With the pump running and the system at normal operating pressure, close all outlets and watch whether the pump stops cleanly at the cut out pressure and remains off. If it stops and restarts within seconds with no demand on the system, there is either a leak in the pipework or fittings allowing pressure to bleed down, or the pressure vessel is not maintaining pressure as it should.
Checking for leaks first by inspecting all visible joints, valves, and fittings eliminates the possibility of a leak driven pressure drop before focusing on the vessel. If no leaks are present and the pump is cycling rapidly at rest, the vessel is almost certainly the cause. The pressure vessels range covers CIMM vertical, horizontal, and leg-mounted vessels from 3 to 1,000 litres, providing the full capacity range needed to correctly size a replacement for any system from domestic borehole supply through to large agricultural or commercial installations.
Sizing the Replacement Vessel Correctly
Replacing a failed or undersized vessel with one of the same capacity risks repeating the same problem. The correct approach is to calculate the vessel size required for the system rather than simply matching the existing vessel. The key inputs to the sizing calculation are the pump flow rate in litres per minute, the pressure switch cut in and cut out pressures, and the target maximum number of pump starts per hour.
A commonly used rule of thumb for domestic systems is to allow a minimum vessel volume of approximately ten to fifteen litres per cubic metre per hour of pump output, but the manufacturer's published sizing method, which accounts for the specific pressure switch settings, gives a more accurate result. When in doubt, specify the next size up: a larger vessel than the calculated minimum provides better pump protection and incurs no operational penalty. For borehole pump systems, the borehole pumps range covers the submersible pump options most commonly associated with pressure vessel installations, and for booster systems, our booster pumps range covers surface mounted pressure boosting options.
Setting the Pre Charge Pressure
A new pressure vessel must be set to the correct air pre charge before installation. The pre charge is the pressure of the air in the air chamber before the water side is pressurised, and it is typically set to approximately 0.5 bar below the pressure switch cut-in pressure. An incorrectly set pre charge reduces the usable drawdown volume of the vessel, effectively making it behave as a smaller vessel than it is and partially undermining the cycling protection it is meant to provide. Pre charge is set using a standard tyre pump or compressor via the Schrader valve on the vessel, with the water side completely drained and depressurised.
Frequently Asked Questions
How many times per hour should a pump cycle in a normal domestic system?
A correctly sized system typically allows no more than ten to twenty pump starts per hour under normal domestic demand; a pump cycling more frequently than this is operating outside its intended duty cycle and the cause should be investigated.
Can short cycling damage the pump permanently?
Yes; extended short cycling causes motor winding fatigue from repeated inrush current events, bearing wear from repeated mechanical shock at each start, and contactor erosion in the control panel, all of which accumulate to shorten pump service life significantly.
How do I know if my pressure vessel bladder has failed?
Depressurise the system completely, then press the Schrader valve on the air port of the vessel; if water comes out rather than air, the bladder has ruptured and the vessel must be replaced.
Is it worth repairing a failed pressure vessel or should I replace it?
Pressure vessel bladders are not typically field-repairable; once the bladder has failed the vessel should be replaced, and the replacement should be correctly sized for the system rather than simply matched to the original vessel capacity.
Final Thoughts
Short cycling is a serious and progressive problem that shortens pump life and increases maintenance costs, but in most cases it has a straightforward solution: a correctly specified and functioning pressure vessel. Identifying whether the existing vessel has a failed bladder or is simply undersized for the system, then replacing it with a correctly calculated alternative, resolves the cycling problem and restores the pump to its intended duty cycle.
Browse the full range of pressure vessels and pump system accessories at Henry Pumps to find the right capacity for your system and get expert advice on sizing if needed.