Pressure Tank PSI

Al- · Jun 6, 2024

I am replacing my pressure tank.
Existing tank is a 20gal.
Replacing with a 52gal AO Smith has max 100psi.
I run a Grundfos 22SQE15-220 pump with a CU301 controller. Constant pressure is set at 70psi.
Question is.. what’s the Max pressure air PSI should I precharge this tank to. Grundfos says 70% to maximize performance (see attached)
If any professional can confirm if this 49psi is correct, provide some guidance or what the correct psi should be in this setup, would be greatly appreciated.

Valveman · Jun 7, 2024

First, why are you replacing the pressure and and going with a larger one? Is the pump cycling on and off instead of holding constant pressure and you think the larger tank will help? When a CU301 is working properly a small tank is as good as a large one. The pump will come on when you start using water and stay on until you stop using water, no matter the size of the pressure tank. But if the pump is cycling on and off with 15 PSI between on and off, then the CU301 has failed, which is common. When the CU301 fails the pump goes to cycling and off, so people think they need a larger tank. What is really needed is to replace the CU301 with either a new one or a Cycle Stop Valve and regular 60/80 pressure switch. It is a very common problem for the CU301 to loose connection with the motor, which causes it to revert to cycling on and off. If that is the case, a larger tank won't help. But as for your 70% air charge question, working at 70 PSI would mean needing 49 PSI air in the tank.