hi there
I am having problems with the same model ( not your problem though ( bar pressure not holding) so was looking through this interesting site for Vaillant Boilers.
I certainly think you have a diverter problem. We have had 3 replaced in 6 years ( hard water area doesn't help- I suspect some sediment has blocked needle size holes -they are so tiny it doesn't take much to block) also the early models they constructed the component in the wrong materials -engineer told me. hope that helps -Back to my problem. JAB
JAB
January 2009