Needs servicing, presumably the three port valve isnt being operated properly, probably because the DHW diaphragm has a hole (very common problem). When the diaphragm is replaced make sure all traced of old diaphragm rubber are cleaned off the pusher plate and brass insides or you will be replacing it again within the year. This, along with dirty venturi caused hot water cycling is a very common fault on this model.
Download the manual from Vokera if you need diagrams.
c
January 2011
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