I have a Baxi Solo2 80pf. It fires up ok (set on No2, quite low) but after 15 min or so the Rads get really hot and the boiler makes a boiling /Kettling noise and overheats. I switch it to Zero and wait for it to cool down, the pump is running during this time and only switches off when I put the boiler back on. I've had a heating engineer out who's 'trying things' by process pof elimination with a call out charge each time! Is it really that difficult. Any clues on how to proceed would be gratefully appreciated.