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