My cold water feed tank (gravity system) to my hot water cylinder is overflowing. I have changed the ballcock and it is working fine. There is no leakage from the hot water overflow (into the feeder tank). The only other pipe is the cold water feed to the hot water cylinder. Does anyone know what the problem could be? It is overflowing even when cold water supply switched off and heating off. Can water re-enter the feeder tank from the hot water cylinder? ANy help much appreciated/