Intermittent hot water on Vaillant VUW 242E

There is an assembly visible if you take the front off he combi, I forget the proper name but there is a valve with a pipe running through it. The valve has a diaphragm in it which pushes up a plunger when you demand hot water. That plunger moves up and 'makes' a red microswitch which fires up the gas and ignition ( I think! ). Normally that valve/diaphragm/plunger assembly moves into position and 'makes' the microswitch within a second or two, but if it is gummed up/knackered, the plunger will edge slooowwwlllyyy up, finally making the microswitch after agaes have past ( i.e. 30 secs plus ).

Can't help you too much more, Ed Sirrett might make more sense of a reply than me, I junked my 242 a couple of years ago. What I have described may be your problem anyhow. I guess the valve I describe is a flow switch on the cold water pipe.

Andy.

I go along with the above. There is some adjustment for the microswitch position but the replacement is about a tenner. It may have needed repositioning after the diverter valve was exchanged.