These contracts are used and run to stop people fixing expensive problems. All issues with Kestons are expensive. Im surprised he didn't order a main heat exchanger and get the boiler BER'd. You will not win a fight about the system being dirty. EVER. :-)
There is a venturi for the DHW diaphragm in the right hand brass block. Its the nut below the 2 small copper pipes. The nut is 24mm i think and needs ring spanner to remove. If the venturi fails the microswitch will not make and the diverter will stay in ch mode. This explains all your issues...
I have put this here for Gas engineers. I know joe public could watch this stuff but its not aimed or intended for them.
This is one of my really unusual fixes.
I do understand that a lot of people wont like this.
Sorry..
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