As you open the hot tap, the pin in the centre of this should come out and press against the grey fulcrum arm. This then pivots and makes the microswitch, telling the boiler to fire up. Chances are that the rubber diaphragm inside the brass disc is holed or torn slightly and the pin therefore isn't coming out enough due to the pressure dif being messed up. To test, run the hot tap and pull gently back towards you on the arm where the pin should hit it. If the boiler fires and your hot water is fine, there's your problem.
New diaphragm alone, assuming no greater problem with the valve, is 5 to 10 quid. Easy enough to change - turn off the boiler, turn off the cold to the boiler, drain out by opening hot taps, remove the fulcrum arm, remove the 5 screws around the edge, replace diaphragm, reassemble in reverse order. Job's a good 'un.
Did what you say.
Initially made no difference, then magically seemed to start working & also working much better.
Possibly dicky microswitch, and/or diaphragm as you say.
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1) Turned on hot tap, no hot water as usual.
Saw arm, pin, microswitch, pin was out pushing arm.
Pulled arm gently, didn't move, seemed to be at max position already.
Gently pulled microswitch, which was already operated by system.
Water had been running for 5 minutes, still cold.
Pulled microswitch last fraction backwards/forwards, definitely already operated.
Hands off looking at boiler.
Then maybe 10-20 seconds later, I heard boiler fire up, hot water started to flow. (could indicate dicky microswitch)
2) Tested hot water, by turning on/off number of times.
Hot water flowed ok on each occasion (hasn't done that for months).
3) Turned on heating, checked hot water, which flowed ok.
4) Switched heating off, turned hot water on, hot water did not flow.
Pin was out, checked microswitch by pulling back, it had not been operated. After I pulled microswitch last fraction, which stayed operated, boiler fired up, hot water flowed.
(could indicate faulty diaphragm)
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I then noticed there is scale around the pin, a ring of scale, where it sits in off position.
Underneath seemed slighly damp and there was light scale on microswitch underneath.
I have recently noticed boiler losing pressure over weeks, with drip/damp underneath boiler in that position.
Think there could be small leak/weep from around the diaphragm pin.
Obviously scale around pin should not be there & could be restricting movement slightly.
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Is problem a faulty diaphragm and/or dicky microswitch ??
Can position of microswitch be adjusted slightly ??
