Don't worry Its just the diaphragm, when you open a h/w tap the pressure from that forces the diaphragm up which makes the micro-switch which then sends the signal to the PCB to goto full gas rate. Hence the reason when you put the heating on you get a slight amount of heat through the h/w tap, it wont be the full amount of heat due to the diaphragm having a small hole in it causing it to kick in then go out.
There is a service kit available for the diverter valve which comes with everything you need to repair the valve. They are under a tenner in price too ;-)
Not something you can do yourself, you need a Gas Man like me ;-)
Graham
April 2013
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