The pre-charge should sit a little below the pump's start pressure. The right number for the factory setting, how to check and adjust it, and the signs it is low.
Inside a pressure tank there is a rubber bladder surrounded by compressed air. The pressure of that air when the tank has no water in it is called the pre-charge. If this number is wrong, the tank doesn't do its job and the pump starts and stops for no reason. What a tank does and what size you need is in our pressure tank guide; this article is only about the pre-charge.
What the right number is
The pre-charge should be a little below the pump's start pressure, usually about 0.2 bar below. On a HYDROSET controller the start pressure is F2.
| Pump start pressure (F2) | Tank pre-charge |
|---|---|
| 1.5 bar (factory setting) | about 1.3 bar |
| 2 bar | about 1.8 bar |
| 2.5 bar | about 2.3 bar |
Why this number? If the pre-charge is above F2, the tank runs out of water before the pump starts, so it is practically useless. If it is far below, the tank holds little water and the pump starts for every small draw.
You may have read that the pre-charge should be 70% of the set pressure. That rule is for variable-speed boosters that hold a constant pressure. For a pump that a controller switches on and off, the reference is the pump's start pressure.
How to check and adjust it
You need an air pressure gauge (the kind used for car tyres) and a compressor or a hand pump.
- Switch off power to the pump.
- Close the valve between the tank and the pipe if there is one, and drain the tank through its drain valve. If there is no separate valve, open a tap on the lowest floor until the water pressure falls to zero.
- Remove the cap from the air valve on the tank and read the pressure with the gauge.
- If it is low, add air until you reach the number in the table. If it is high, let a little air out.
- Refit the cap, return the valves to how they were and switch the pump back on.
Step 2 matters most. If the tank still holds water under pressure, the number you read is the water pressure, not the pre-charge.
How often
Check the pre-charge once a year. The air inside the tank slowly escapes even when everything looks fine. And whenever the pump starts cycling for no reason, check the pre-charge first.
Signs the pre-charge is low
- The pump starts for every small draw. Why does the pump keep switching on and off?
- Pressure at the taps keeps rising and falling.
- The pump runs noisier and uses more electricity.
- Water comes out of the air valve. That means the bladder has split and adding air won't help; the bladder or the tank has to be replaced.
You can also watch the pressure tank working alongside the pump and the controller in the pump controller simulator; the water level in the tank rises and falls every time the pump starts and stops.