Why is a non-return valve required in an accumulator installation?
An accumulator stores water at pressure. In a pumped accumulator installation this pressure is built up by the pump whereas in an unpumped accumulator installation the water in the system is stored at the peak mains pressure.
A non-return valve (NRV) ensures that the built-up pressure in a pumped install and the peak mains pressure in an unpumped install is locked into the system. Without an NRV the system pressure would fluctuate with the mains pressure.
The NRV also prevents water in the system from flowing back into the mains supply.