Equipment
The small steel tank beside the booster pump is not a water store. It is a cushion of compressed air that lets the pump rest between starts. When the cushion goes, the pump starts on every draw and the pressure jumps. This guide explains how the vessel works, what it needs, and how to recognise one that has failed.
September 13, 2026 · 9 min read
Two kinds of tank sit in most villa plant rooms, and they are often confused because both hold water. The large storage tank holds the villa's supply at atmospheric pressure. The small pressure tank, more accurately a pressure vessel, holds very little water but holds it under pressure, against a charge of compressed air. One is about how much water the house has. The other is about how the pump behaves when someone opens a tap.
Cross-section
Top row: the same vessel with the pump off, with the pump charging it, and while water is drawn. Bottom: why a storage tank is a different machine.
A sealed charge of air above the membrane. It compresses as water enters and pushes the water out again.
The rubber membrane that keeps air and water apart, so the air is not slowly absorbed into the water.
The only part connected to the pipework. It fills while the pump runs and empties when a tap is opened.
Where the pre-charge is measured and set, with the vessel isolated and its water side empty.
A single port onto the pump's discharge line, ideally through an isolating valve with a drain.
Little water inside; the air has expanded to fill most of the vessel.
Water is pushed in and the air is squeezed into a smaller space until the switch stops the pump.
A tap opens and the compressed air pushes stored water out, so the pump does not have to start at once.
Open to the atmosphere through a vent, holding volume rather than pressure.
Sealed, small, and only useful for its pressurised buffer.
A modern vessel is split in two by a rubber bladder or diaphragm. Above it is air, charged to a set pressure at the factory or on installation. Below it, or inside the bladder, is the water side, connected to the pipework. As the pump pushes water in, the air is compressed and its pressure rises with the system. When the pump stops and a tap opens, the compressed air pushes the water back out. The membrane matters because air in direct contact with water slowly dissolves into it; older plain steel tanks without one gradually fill with water and lose their cushion.
A fixed-speed pump is controlled by a pressure switch with two settings. At the lower one, the cut-in, the pump starts. At the upper one, the cut-out, it stops. Without a vessel, opening a single tap would drop the pressure below cut-in almost instantly and the pump would start; closing it would push the pressure to cut-out a moment later and the pump would stop. The vessel stores enough water between those two pressures to supply small draws on its own, and to make each run of the pump last long enough to be good for the motor. Fewer, longer runs are the whole point.
A vessel's nominal size is its total internal volume, not the water it delivers. Much of the space is always taken by air, and the water that can be drawn between cut-out and cut-in, the drawdown, is often around a third of the nominal size or less, depending on the pressure band and the pre-charge. That is why a vessel that looks generous on paper can still let a pump start every minute during a shower, and why sizing is done from the drawdown the pump needs, not from the number on the label.
The air side is charged to a pressure set a little below the pump's cut-in. If the charge is too low, the vessel is half full of water before it starts doing anything useful and the drawdown shrinks. If it is too high, the membrane is pushed hard against the water port and the vessel barely takes water at all. The charge also leaks away slowly over the years, even through a sound membrane, which is why it belongs on a maintenance list. It is measured and adjusted only with the pump isolated and the vessel's water side drained, because a reading taken with system pressure on the water side says nothing about the air charge. That is a job for a technician, not something to try on a pressurised system.
A variable-speed booster holds pressure by changing its speed, so it does not need a large vessel to limit starts. It still usually has a small one. The vessel lets the pump stop when demand ends and supplies tiny draws such as a cistern refilling without waking the pump; it also softens pressure spikes when taps close quickly. A vessel that has lost its charge on a variable-speed set shows up as a pump that restarts every so often with nothing open, or never quite stops. The manufacturer specifies the size and pre-charge for the controller, and that is what should be followed.
Several of these also have other causes, such as a leak or a switch setting. Our guide to why a pump keeps cycling separates them by pattern, and our guide to low water pressure covers pressure lost on the way to the taps.
The two look similar and have different fixes. A vessel that is correctly charged but too small for the pump gives short runs that have always been short: the system was built that way. A vessel that has lost its charge, or whose bladder has failed, gives runs that have become shorter over months. If the history is unknown, the pre-charge and the condition of the bladder settle it. A faulty vessel is re-charged or replaced; an undersized one is replaced with the right size, or a second vessel is added alongside.
The vessel belongs on the pump's discharge side, after the check valve and close to the pressure switch, so that the switch senses the pressure the vessel holds rather than a surge at the pump outlet. A short connection through an isolating valve with a drain lets the vessel be serviced or replaced without draining the house. It should stand on a firm base, out of direct sun and with room around the air valve, and its port should never carry the weight of the pipework. In a system with storage, it sits after the booster, never on the storage tank side; how the tanks themselves are laid out is covered in our guide to villa storage tanks.
A larger vessel does give longer pump runs, up to a point. Beyond the size the pump and switch actually need, it adds cost, floor space and weight, holds more water standing still in a warm plant room, and changes nothing about the house's pressure or flow. On a variable-speed set it adds very little at all. The right size follows from the pump's flow, the switch band and how often the motor may start, and it is different for a fixed-speed pump, a variable-speed set and a well pump feeding a house directly. That is why the vessel should be chosen together with the pump, as our guide to choosing a booster pump describes.
If you suspect your pressure tank, send our pump team a photo of the vessel and its label, and tell us how often the pump starts during a shower. For a new villa, the vessel is chosen with the rest of the villa water system.
Equipment
A pump that keeps starting and stopping is usually obeying its controls exactly. The question is what those controls are reacting to: a vessel that has lost its air, a leak, a valve letting water back, a setting, or a tank level. Which one depends on how the system is built and on the pattern of the cycling.
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Equipment
Weak showers are a symptom with half a dozen possible causes between the storage tank and the tap. The fastest way to the right fix is to notice when the pressure is low, where, and whether the tank is full at the time. This guide turns those three observations into a diagnosis.
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