I have recently had a leaking ball valve (fitted to the mains side of a closed system). As a temporary measure I connected up the flexible filling loop hoping it would at least stop water dripping out. Unfortunately, the pressure in the central heating system gradually started to rise even though the double check valve (heating side) was closed. I removed the filling loop and put a cap on the mains feed. This solved the problems - but how is it possible for mains water to enter the system with the double check valve closed? There is no pressure loss and no evidence of water dripping out of the valve or through the PRV. Is the double check valve also suspect?