The diverter valve works fine. The diaphragm was recently replaced after bursting and it moves freely.
Things have become a little clearer since I posted.
DHW still does not startup the burner despite the diverter valve moving and the switch operating correctly (tested by meter)...
I ahve a Halstead Ace High boiler.
Recently it has begun to randomly stop working.
When it happened first I decided to drain the system down and refill it.
As soon as I turned the pressure vent knob it fired up.
Thereafter whenever it does it I just 'nudge'the system by turning the vent...
