Combi boiler issues

[Macsandy]

Hi, just had two repairs on our combi boiler to fix a leak. The last engineer was here for over 5 hours and took the whole boiler apart. The leak appears to be fixed. But every time we flush a loo or run a Cold water tap the boiler comes on?
I have another bg engineer coming tomorrow to try to fix. I have read that it is air trapped somewhere but could it be something that was done to the boiler, seems very coincidental that this has just started now?

Thanks
.

[gas4you]

Sounds like a dead leg full of air.

Run all taps and washing machine/ dishwasher outlets then see if this cures it