ajs heating

>Is it possible that the mains cold water fed into the system

mains water does not feed the system in any way it's a gravity fed system.

>force water back up into the feed and expansion tank.

is it going up to the f/e tank or cws tank ?

if you have a split coil in the cylinder check the other tank if one is filling the other willbe emptying and the ball valve willbe letting in to fill the emptying tank.

which tank is the highest cws or f/e ?


by the sounds of it you have either a shower mixer or a mixer tap that is cold mains and gravity hot and the non return valve is faulty and passing cold to the hot.