I have a similar problem. If the CH is on we have to turn it off to get HW. However, if the CH is on but the boiler is at temperature and not firing, the HW will work.
I've changed the diverter valve which has sorted out other problems of the hot water getting too hot and tripping the limit stat (the CH circuit was interfering with the HW circuit).
I think the problem is with the PCB. There are 2 and one controls the logic of the pump and it's over run. I need to test it but I think a relay is failing to operate the pump when demanding HW in CH mode.
Arbeerich
February 2011
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