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The bladder tank is the small “buffer” that stops your pump from starting every time a tap is opened. Properly chosen and charged, it steadies the pressure and spares the equipment; badly set, it is one of the first causes of repeated starts. Here is how to choose its size, type and pre-charge pressure.
What is a bladder tank for?
Inside, a membrane (the bladder) separates the water from a cushion of air or gas under pressure. When the pump fills the tank, the cushion is compressed; when you open a tap, it pushes the water out. The tank therefore has several roles:
- storing a small volume of pressurised water for light draw-off, without starting the pump;
- limiting the number of pump starts, and so the wear on the motor and pressure switch;
- absorbing pressure surges (water hammer) in the installation;
- smoothing the pressure at the points of use.
It works with a booster set, a surface pump or a submersible pump fitted with a pressure switch.
In-line, vertical or horizontal tank
| Type | Advantage | What to check |
|---|---|---|
| In-line tank | Compact, fits directly on the pipework, useful to absorb surges. | Small capacity depending on the model: mostly a complement or a small damper. |
| Vertical tank | Takes little floor space, easy to install beside the pump. | Available height, stability and a level floor. |
| Horizontal tank | Low profile, suited to rooms with little headroom; some models take the pump on top. | Floor footprint and type of connection. |
Whatever the type, check on the product page the maximum working pressure (it must exceed the pump’s cut-out pressure), the permitted temperature and whether the membrane suits the intended use, particularly for water meant for consumption. The tank accessories complete the installation.
Size and pre-charge pressure
The larger the tank, the more water you can actually draw between two starts, and the less often the pump starts. That usable volume depends mainly on the gap between the cut-in pressure and the cut-out pressure, on the pump’s flow rate and on the number of starts per hour the manufacturer allows. The usable volume is always well below the stated capacity, as the following example shows.
Worked example (simplified calculation)
A 100 l tank, pre-charged to 1.8 bar, with a pump that starts at 2.0 bar and stops at 3.0 bar. In absolute pressures (atmospheric pressure ≈ 1 bar) and assuming an ideal gas, the air cushion occupies about 93 l at start-up and about 70 l at cut-out. The usable water between the two is therefore about 23 l, less than a quarter of the capacity. Indicative values: for a precise installation, follow the manual and the product page.
A wider setting (higher cut-out pressure) increases the usable volume, but requires a working pressure compatible with the tank and the pump. With a variable speed booster, the tank is generally smaller: follow the manufacturer’s recommendation.
The pre-charge pressure. The basic rule: the tank’s pre-charge pressure, measured with the water drained out of the tank, is about 0.2 bar below the pump’s cut-in pressure. For a start at 2.0 bar, you therefore charge to about 1.8 bar.
- Switch off the pump’s power supply and open a tap to drain the water from the tank.
- Check the pressure at the valve with a suitable gauge (such as a tyre gauge), then correct it with an air pump or compressor.
- Never exceed the maximum pressure marked on the tank.
Check this setting regularly, for example once a year. A pump that starts every few seconds, a pressure that varies widely or water coming out of the air valve may point to an insufficient air cushion or a membrane that needs replacing.
Our 5-step selection method
- Note the cut-in and cut-out pressures set on the pump or pressure switch.
- Note the pump’s flow rate and the number of starts per hour the manufacturer allows.
- Choose the size using the product page, going for the larger size if in doubt.
- Check the tank’s maximum working pressure: it must exceed the pump’s cut-out pressure.
- Choose the type (in-line, vertical, horizontal) according to the space available, then set the pre-charge.
Frequently asked questions
Why does my pump start every few seconds?
Most often the air cushion is insufficient (pre-charge too low, worn membrane) or the tank is too small for the flow. First check the pre-charge pressure with the water drained out of the tank.
Can I fit a bigger tank than the original one?
Yes, provided you respect the maximum working pressure and match the connection. A larger volume reduces the number of starts; check the space available and set the pre-charge.
Do I need a tank with a variable speed booster?
Often yes, but a smaller one: it absorbs surges and covers very light draw-off. Follow the booster manufacturer’s manual.