The high flow has its own storage of water underneath the burner chamber, unlike a standard wall hung combi. There is a tank overheat sensor in there so unless the boiler is tripping out it won't be that. It's more likely to be your plate to plate secondary heat exchanger which, with older systems, gets coated with 'magnetite', the metallic black gunge you get from dirty systems. This coats the plates & makes them thicker & therefore generates more heat. The same
matter can also coat the domestic hot water sensor on the outlet side therefore giving a false reading. Both can easily be checkedneither job for the novice, get an engineer in! NTC sensor up £20, plate heat exchanger could be up to £130+vat plus labour of course.
Hope this helps.
An Engineer, August 2016
