My Vaillant combi boiler has been playing up for a few
weeks now. The first symptom was that often there was
no hot water available whilst the heating was on.
But now it's started staying on (boiler firing, pump
running) even when the timer has turned off. Not often,
but certainly more than once!
I originally thought it was the diverter valve (already
replaced once under warrenty, though sadly it's now
3 1/2 years old) but now I'm not sure..!