monobloc tap problem

[pb21]

I have a relatively new 1yr old monobloc tap that worked perfectly. I have an older type gravity fed system. suddenly we could hear water flowing and the tap had condensation on it. The tap appears to work ie move left hot comes out right cold and up/down to increase decrease flow. If I isolate cold hot comes out ok if I isolate hot then cold comes out ok. with both unisolated it appears that the cold is forcing its way into the hot pipe via the tap this would explain the noise and the condensation on the tap.

I am not sure why as the tap appears to work ok. I have an isolation valve on the hot with the arrow facing the right direction. Is there such thing as an isolating non return valve that will cure this?

Or any other advice would be welcome as its driving me balmy.

 

[kevplumb]

you need check valves fitting

 

[pb21]

does that isolate and only allow in one direction?

 

[kevplumb]

only allows flow in one direction

 

[pb21]

All fixed and working fine, its a mystery as to why it took six months to do this as its been working fine without the check valve BUT nevertheless THANK YOU so much its nice to have hot and cold together again.

regards