I had a viessmann 100 installed 2year ago was ok for awhile then nothing went right with it BUT trouble renew burner then something else ,water went hot /cold thenHOT, Hot new heat exchanger ok again for awhile then again TROUBLE called in viessmann enginer told me what was wrong the return/flow rubber hoses was breaking up inside blocking up the heat exchanger .He then phoned viessmann about this ,they said it was not the fault Now he is a heating so he would know better well awhile i got new hoses as agood will but I had to pay to have them fitted but the boiler is still not right the person in your service dept told me to power flush it I had this done not long ago also had a adey . Magaclean filter fitted l am fed up with the whole thing .its still under warannty l wish l did not buy it