Borehole Pressure Switches and Pressure Tanks Explained

Your borehole pump can be in perfect condition and still deliver frustrating water pressure if the pressure switch or pressure tank is not set up correctly or is starting to fail. These two components work as a pair, and understanding what each one does makes troubleshooting — and talking to an installer — a great deal easier.

What a pressure switch actually does

A pressure switch is a small electro-mechanical device plumbed into the discharge pipework between your pump and the rest of the system. It monitors line pressure continuously and closes or opens an electrical circuit to start or stop the pump based on two set points:

• Cut-in pressure — the lower threshold at which the pump switches on (commonly around 150–200 kPa as a rough guide, though your installer should set this to match your system).
• Cut-out pressure — the upper threshold at which the pump switches off (typically 50–100 kPa above cut-in, as a general reference).

The gap between the two is called the differential. A wider differential means fewer start-stop cycles; a narrower one means the pump responds more quickly to demand. Neither extreme is ideal: too narrow and the pump short-cycles, which generates heat and wear; too wide and your taps dip noticeably between pump starts.

Most residential pressure switches are factory-set and adjustable via a spring-loaded nut inside the cover. Adjustment is straightforward but does require isolating the pump circuit first — not a task to attempt while the system is live.

What the pressure tank does

A pressure tank (also called a bladder tank or expansion vessel) is a sealed steel vessel containing a rubber bladder or diaphragm. One side of the bladder holds air pre-charged to a specific pressure; the other side connects to your water line. When the pump runs, water compresses the air through the bladder. When a tap opens, that stored pressure pushes water out without the pump needing to start immediately.

This buffer does three useful things:

1. Reduces short cycling — the pump does not start every time someone opens a tap briefly.
2. Smooths pressure delivery — taps feel more consistent because pressure does not drop to zero the instant the pump stops.
3. Protects the pump motor — fewer start cycles extend motor and capacitor life.

Tank sizes for domestic borehole systems typically range from 24 litres to 100 litres, though the right size depends on your pump flow rate and household demand. A correctly sized tank is worth discussing with a qualified installer; an undersized tank fails to prevent short cycling even when it is functioning perfectly.

For a broader look at how pump choice affects pressure consistency, the comparison in constant pressure vs standard pumps explains the trade-offs in plain terms.

Signs your pressure switch is failing

Pressure switches are mechanical parts exposed to water pressure fluctuations, and they do wear. Watch for:

• Pump that will not stop — the switch contacts may be welded shut from arcing, or the set point has drifted above system capacity.
• Pump that will not start — contacts corroded open, or the set point has drifted below the actual system pressure.
• Visible corrosion or moisture inside the switch cover — a reliable sign the unit should be replaced rather than adjusted.
• Erratic pressure — the switch chatters between cut-in and cut-out rapidly, often audible as a clicking sound near the pump.

Switch replacement is generally inexpensive compared with other borehole components. Parts are widely available and a competent plumber or borehole technician can swap one in under an hour in most cases. The borehole low pressure troubleshooting guide covers additional causes worth ruling out before assuming the switch is at fault.

If a failing pressure switch or tank is part of a bigger borehole problem, getting a professional assessment saves guesswork. Compare 3 free quotes — vetted installers, no obligation.

Signs your pressure tank is failing

The most common failure mode is a waterlogged tank — the bladder or diaphragm has split, allowing water to fill the air side. You can test for this by tapping the tank: a healthy tank sounds hollow in the upper portion; a waterlogged one sounds solid throughout. A more reliable test is to press the Schrader valve (the tyre-style valve on the tank): if water spurts out instead of air, the bladder has failed.

A waterlogged tank causes:

• Very rapid pump cycling (the pump starts and stops every few seconds under demand).
• Higher electricity consumption from constant motor starts.
• Premature pump and capacitor wear.

Replacing a failed bladder tank is straightforward if the tank has a replaceable bladder; otherwise the whole vessel is swapped out. Pre-charge pressure on the replacement must be set to match your system’s cut-in pressure — typically a few kPa below cut-in — before the tank is filled with water.

Correct pre-charge pressure and why it matters

This is the detail most DIY guides skip. If the air pre-charge in a new tank is set too high, the tank barely accepts any water and provides almost no buffer. Set too low, the pump still short-cycles because the effective drawdown volume is tiny.

The standard recommendation is to set pre-charge to approximately 90% of the cut-in pressure with the system depressurised and the pump off. For example, if your cut-in is 150 kPa, pre-charge the tank to around 135 kPa. Always confirm this with your installer, because actual system pressures vary.

What repair and replacement typically involves

For a pressure switch replacement, a technician will:

1. Isolate the electrical supply and close the isolation valve.
2. Release system pressure safely.
3. Remove the old switch and fit the new one to the same port.
4. Re-pressurise, confirm cut-in and cut-out pressures, and test for leaks.

For a pressure tank, the process adds a step to check and set pre-charge before reconnection.

If your system was installed as part of a complete borehole setup, the borehole drilling overview covers how all the components — casing, pump, pressure system and filtration — relate to one another, which is useful context if you are considering a more comprehensive upgrade.

Unsure whether you need a switch, a tank or a full pump assessment? Get matched with local borehole specialists — no cost, no commitment.

When to call a professional rather than adjust yourself

Some adjustments are genuinely within reach of a confident DIYer — checking pre-charge with a tyre gauge, for instance. But others carry real risk:

• Adjusting a live pressure switch risks electrical shock and should only be done with the circuit isolated and by someone familiar with the equipment.
• Incorrect cut-out settings can cause the pump to run against a closed system, building pressure that exceeds pipe or fitting ratings.
• If your pump is submersible and located down a borehole, any fault diagnosis should account for the full system rather than just the surface components.

When symptoms are ambiguous — pressure that fluctuates but the tank is not waterlogged, or a pump that cycles irregularly — it is worth having a borehole technician check the entire pressure system together rather than replacing components one at a time on speculation.

A pressure switch costs a fraction of a pump motor. Getting the diagnosis right first is nearly always the cheaper outcome.