hi, i am fairly certain that it is your diverter valve diaphram, this is the large brass valve with a 2.5 "dia bit at 0ne end with about 8 screws on the edge of the brass assembly, what happens is the rubber diaph gets holes in it, and then the valve fails to divert from heating mode to hot water mode, depending on how bad the diaph is, it can give very strange results, like some times the hot water is ok then it can go cool then cold, all with in a few minutes.tje vale is located in the boiler, you will see it when you take the front off the boiler.
bob roberts, January 2006
