hi Anthony, i think i have got the answer [the pump]. the pump moves the water round the system at a set temp, when you switch the boiler on to heating, the pump starts up moving the water across the heat exchanger and into the heating system, if the pump is faulty the water still gets hot but will not circulate in the system so the water around the boiler gets very hot and the boiler trip out on over temp, this is a tipical pump failure fault, the hot water then moves into thesystem by graverty replaced by cold water you then re light the boiler and it all starts over again, keep doing this and the whole system would get hot useing the old graverty method ie when they did not fit over temp stats to the sytems.that where the funny noise is coming from [the pump]. now lets look at the hot water sytem, When you turn on the tap to give you hot water, the cold water cominig in keeps the boiler cooler by disipating the heat into the cold water to create your domestic hot water supply so the over temp stat will not sense high temp thus the boiler runs ok on hot water but goes out on heating, you could say that the high temp stat is the problem of which it is ,but its the pump fault that is making the high temp stat do its job.sorry i got it wrong last time i should do more thinking about the sytems, please let me know how you get on with it if you need help fitting one or any more [? ]get in touch.bob. p. s. all things can be solved given the time.
bob roberts
January 2006