I have a Ravenheat boiler CSI 85 (18 months old). The central heating works OK. The hot water works OK when central heating is on. When central heating is off, hot water does not come on. When the problem started, it used to take several goes of turning the hot water tap on before the boiler would come on. Now, it wouldn't produce any hot water if set to H W mode. Is it something to do with the 3 way valve. Any help please