thanks gasguru the non return valve is fitted just after gate valve on cws and not near cylinder, cws is 150 gallon as it is in a small hotel, no other mixing valves apart from 3 monobloc mixers with washers replaced, the cylinder is fed from a single 28mm copper feed straight to cylinder, hot water draw off is 22mm with tee around 8 inches from top of cylinder rising up via roof and with expansion pipe bend over roof tiles about 3 ft . ...another way to look at this from top of water level in cws to expansion pipe going out through roof is approx 4/5 ft the immersion heater stat has also been changed over and temp sitting at 50c, so no possible that its boiling over, only when gate valve feed to cylinder is shut down that the water stops via expansion.