i had a similar issue on a ferroli last year, and when the boiler was installed the plumber installed flexi pipes in place of copper, i'm guessing due to laziness, any way, replaced them with copper pipe and found they were blocked, never had any issues since.
i did have another issue with the heat exchanger making strange banging noises but that magically fixed itself over the summer before i sold the house, i guess that it was particles from running various sytem cleaners through it and while the boiler was off over the summer the particles have sunk to a lower point in the system and have since been caught by the filter.
anyway the reason your boiler keeps shutting down is because the water going back into it is to hot because it is not cooling down due to it not radiating the heat through the radiators. basically it is bypassing your entire system and going straight back to the boiler.
was the boiler a back boiler or open vent system before the combi was installed?
Have you inspected the system for any potential valves that should have been removed and are now seized?
the fact your flow and return are hot suggests its not a problem with anything on the boiler
Thanks for the reply, not sure though did find old large copper pipes capped off and cut in the combi boiler cupboard and in access hatch in fire place, Adey filter working great so quiet and very little magnatite found on magnet. Central heating working throughout house though some radiators hotter than others great improvements from start of these posts..clearly need to do better in balancing the rads lol
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