Ariston 30 MFFI Hot Water Problem

[mark-bristol]

Can anyone help?

I have an Ariston 30 MFFI Combi Boiler. When demanding hot water, it is always scalding, irrespective of the temperature setting. I guess the only time the flame is cut off, is when the overtemperature circuit triggered.

The thermistors and the power PCB has recently been changed, so suspect the control PCB, anybody have other ideas?

 

[ChrisR]

What's the hw flow rate? Does it cycle on and off? Any competent bloke would measure the temp drop across the secondary heat exchanger - did they tell you what it was? Could be there's so much scale in there on the tap side that the flow rate is tiny so the water gets v hot as it goes through slowly. Sensors may be insulated by layers of scale, so react late if at all.