I have noticed that the water in the hot water header tank is getting hot. The hot water is entering the tank through the overflow. This only occurs when the central heating and the hot water are on together. The system is pumped, with one pump and two motorised valves. A while ago a plumber told me that this could be cured with an anti surge (?) valve . If this is the case what is it and where should I put it? (be gentle its my first time)