I have an Alpha 280E that supplies only tepid water when the CH is off;
very hot with CH on. After examining the boiler I can confirm that the
DHW microswitch is being actuated correctly and the rubber diaphragm in
the DHW assembly is not perished. When hot water taps are opened the
outgoing CH pipes get hotter than the corresponding DHW pipes. Can you
give advice on which component is responsible? The boiler is operating
100% in every other way.

Thanks,
Adam