how can i solve a water heating problem on Vokera excell 80 E?

When the boiler starts up with the arm moved do the radiators start to heat up too?. If so suspect three port valve.
There is a sort of spring loaded white plastic valve thing behind the domestic diaphragm, make sure that it is free of debris and working, if turning all the hot taps on full makes the arm move further suspect the white plastic valve thing. It unscrews with a two dog circlip extractor. If the waterflow is low check the strainer on the inlet to the assembly with the DHW diaphragm is not blocked.

c
February 2008