domestic water temperature flutuates ariston

Welcome to the world of the combi boiler. To work on any gas appliance the person must be a member of a class of persons qualified and competent to do so. Thus when any gas appliance is not working correctly it will require an experienced gas engineer to attend. having had over forty years repairing gas boilers it never fails to surprise me that human nature always wins. The question is do you keep replacing parts until eventually the hot water is running to full temperature and constant. Or cut your losses and pay a tradesman/ woman to repair the boiler. That way you pay once. The alternative is very expensive. You say your boiler has had a plate heat exchanger. What ever the cost was you are starting again from that loss. My advice to customers is get either the boiler manufacturer to attend. Or a gas engineer this is the only advice to give. This answer actually is not just to you but all home owners who think there is a cheap way to a repair. Your example is a typical one. And an expensive lesson. I get customers that tell me what the problem is with their boiler before I attend. How because they have down loaded the service and installation instructions and read the fault code which tells them the problem. No! It shows a possible cause of the fault. This is not the case as the same fault code could be caused by numerous reasons. Yes it is good for the parts industry, as owners of boilers buy lots of parts which are not needed. Then after hundreds of parts fitted which I find a common problem, the hands are up and asking for advice. Hopefully this kind and well meant advice will be taken in the manner as given. Regards Combigas. Engineer extrodinaire.

totaly agree with last answer your boiler needs a qualified engineer to attend