How much air should I have in my pump pressure tank?

We get asked this question all the time and you would think that there would be a simple answer. We really wish there was a simple answer, because the correct answer is neither simple nor brief. The correct air pressure for a pressure tank is 5% less than the cut in pressure setting of the pump, measured with the pump off and the water pressure relieved.

Let’s simplify that a little, and add a bit of theory. The purpose of the pressure tank is to allow the pump system to discharge flows of water less than that of the pump. Let’s say that our pump delivers 50 litres per minute, but the shower only uses 20lpm. Without a pressure tank the pump would need to start and stop (very) rapidly to regulate itself to 20lpm. A pressure tank accepts the extra 30lpm until the pressure builds up to the cut out pressure and the pump switches off. The pressure tank can continue discharging until the cut in pressure is achieved and the pump starts again. Additionally, the pump will not have to start every time someone flushes a toilet or gets a glass of water. So, the larger
the pressure tank, the longer the cycle between starts.

But how much air pressure should I have in the pressure tank? If you measure the air pressure in figure 1 it will show 20psi, and in figure 2 it will show 40 psi. But these are false readings as the air is being compressed by the water. To measure the air pressure, you have to switch off the pump, open a tap to let the water pressure off, and then measure the air pressure. In the example pics, the ideal pressure would be 18psi (5% less than the cut in pressure of 20psi). If the cut in pressure was 30psi, the air pressure should be 27psi, 40psi - 36psi and so on.

When the air pressure is right the pressure tank will still contain a small amount of water when the pump starts, ensuring a smooth flow of water. If the air pressure is too low, the tank will always be too full of water and the pump will cycle more rapidly. If there is too much air the water will discharge completely before the pump starts, causing a brief interuption in water flow and a sudden surge when the pump cuts in. You should check the air pressure very 6 months.