Boiler Overheating maybe caused by faulty shower valve

We have a Worcester combi-boiler 5-6 years old in good condition located at the first floor. The boiler works fine with heating for hours and doesn't create any problems. It has been checked by a specialist.
The overheating of the boiler happens when we use any hot water taps (shower, bath tap, kitchen sink and small wc toilet downstairs, APART the bathroom sink tap upstairs. The expert for the boiler said that is the shower cartridge/valve (non return valve?) causing the problem; basically the cold water "pushes" the hot water back to boiler causing to overheat.

Shouldn't be only the shower to overheat the boiler? Supposing the problem is in the shower cartridge valve, rather than all the taps in the house apart the bathroom sink?
Please help me
Thank you
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Sounds like the "expert" is the problem. Get a real Plumber that does heat and air. Maybe you do need a new shower valve mixing cartridge, but neither it nor any other fixture would ever cause a problem with the boiler's operation. - If he's was even on the right track, then your Expansion Tank/Vessel should've been immediately and easily (literally nothing simpler) re-pressurized or replaced if it was failed. - Otherwise, it's a Worcester/Bosch German problem, German is another word for Garbage and thus a problem with the boiler. Typically, it's either air within the system or the need to descale and flush the Heat Exchanger or to replace it entirely.
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