Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.
Yes, previous boiler was a combi. New boiler is also a combi.
The installer added an inhibitor but didn't powerflush or add any cleaner. I remember asking him about cleaning the system and he said he'd do "everything required".
When I cleaned the system (a few months after install) there was quite a bit of black water coming out.
There is a filter on the return side (assuming the return side is correctly plumbed!).
The pipework on the bottom two floors of the property are fairly old as far as I know. From the second floor to third floor it's JG Speedfit.
If I understand I need to:
- see if it's hydrogen in the system
- if it is hydrogen, something is corroding in the system
- if it is corrosion, then I need to powerflush the system and add inhibitor