Got an open central heating system with a Baxi in the garage. middle of the night I hear banging in the rads and pipes. now boiler wont start up. I push reset button, lights for a few seconds then stops. Chap put a new pump on system, still no joy. Could there be some blockage in the system that makes the boiler overheat, three way valve or air lock. I suggest it must be close to the boiler. the pipes are micro bore ground floor 15mm on 1st floor, don't know if that's relevant. Cheers!
Sounds like it. might be worth checking your F&E tank to ensure ball valve isn't stuck. favourite will be the tee where the feed meets the return. You could also have an airlock.
If this happened since pump exchange & water level is ok more than likely air lock or pump speed needs adjustment.
microbore can clog up a lot easier than 15mm , might need a good flushing out, has it ever been drained and flushed out ??
Did he drain the system to change the pump? He shouldn't have had to as the pump should have iso valved on both sides. But if he did, did he tie the ballcock up to stop the tank refilling, and if he did, did he untie it again?