my vokera compact is letting me have hot water for approx 20 secs then runs cold then hot then cold . I have replaced both thermistors (hw +hting ) , the boiler is firing for 5-10 secs then gas goes out and repeats this action .

Can any one help/advise as to how I can fix this problem please !

Terry, July 2012

Thanks for that , I forgot to add its a compact 24.

I'll try to clean the heat exchanger first and see how we go

Cheers

Terry

Terry, July 2012

partially blocked heat exchanger

A1, July 2012

Check the switching for HW demand, Not sure what yours is, Do a search for manual if you do not have one. If the switch if faulty, it could switch off and on. It will be linked to the 3 way valve usually, but some use a flow sensor in a unit on the pipe, One I know of used a flow sensor in a pipe fitting, it used a reed switch, very unreliable.

SD, July 2012