No hot water when you turn on the tap(Vaillant 242e combi boiler)?

When I turn on the tap for hot water, the boiler does not fire so to overcome this I turned the CH up full to fire the boiler. After some investigation I found that if you make the micro switch manually, on the unit on the right hand side below the pump, the boiler will fire and you have hot water. I was wondering, in the unit I have described is there a diaphragm to operate the micro switch to ignite the boiler. Any help would be gratefull received.