April 1, 2011

How do I set the air pressure in my pressure tank?

The air pressure in the tank should be set according to the low cut-in point of the pressure switch.  Before setting the pressure, it is important to know what the low pressure cut-in is.  There are two ways to determine this.  The first is to find the pressure rating for the switch you installed. This is valid only if the switch has not been adjusted. If the switch was adjusted, the second method should be used, which involves observing the pressure gauge to see at which point the pressure switch turns on the pump.

view of the bottom of a pressure tank

The pressure switch rating is normally written on a tag that is inside the plastic cap covering the switch.  Typically this can be found on the inside of your pressure switch lid.  Before you remove the lid to see the label, you should shut off the power to the switch at the main circuit panel.  To remove the lid, loosen the screw on the top of the pressure switch cap.   Look inside the cover to read the pressure range. The first number is the low pressure  cut-in  pressure.

label inside pressure switch cover

The label on the left is a 30 – 50 pressure switch. Look under the PSI column:  ON/I is the low pressure cut-in pressure, the OFF/O is the high pressure cut-off pressure.

This is the factory setting of the switch when purchased. However, since the switches have adjustment screws, it is possible the switch settings have been changed.

If the switch has been adjusted or you cannot find the rating on the switch, you need to observe the pressure gauge to see when the switch turns on.  You will need power turned on to do this.   Observe  the pressure gauge as you allow water to run in your sink or washing machine.  As the gauge decreases, make a note of where the pressure is when the pump switch activates the pump. This is the low pressure cut-in point. Continue to watch and note the pressure where the pump switch shuts off.  This is the high pressure cut-off point.

Once you have determined the cut-in pressure you can set the tank pressure.  To do so, shut the power to the switch (and pump) off and completely drain the tank of water.  It is important to drain the tank completely before checking and setting the pressure in the tank. If you do not, the pressure will not be set correctly.

There is a valve stem which is usually on the top of the tank under a plastic cap.  With an air pressure gauge, check the air pressure just as you would on a car tire.  It should read 2 to 3 pounds less than the low pressure cut-in pressure.   If the pressure in the tank is not set correctly,  you may then either add or release air as needed.

If you are using a  30-50 pressure switch, the low pressure cut-in point is 30 psi.  Set the tank pressure at 27 or 28 psi.  Once you have the pressure in the tank set correctly, you can turn the pump back on.  Don’t forget to close the boiler valve you opened to drain the tank.