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so just like you'd do to bleed them?Maybe crack open each/every rad vent with boiler/circ pump on and check that water is issuing.
No problem and thanks. It was actually the issue with the upstairs rad' being cold and requiring bleeding that brought me here, but I thought it was worth mentioning the banging sound. It's difficult to know if it's requiring bleeding less frequently or whether there's less air emitting each time as it's in the spare bedroom. Will eventually be a home office/spare bedroom but other than rewiring when we moved in and just done the bathroom and created an ensuite, we haven't touched upstairs yet. I'll make a point of putting the heating on once a week and checking it.I suspect the TRVs are a red herring.
IME, it's normal for a re-piped system to evolve gas for several (typically six) months after the work has been completed. This is due to a 'passivating layer' forming on the inside of the fresh pipework. Inhibitors slow this process but don't stop it. If the amount of gas you are having to vent is gradually reducing each time then this is probably what's happening.
The banging noise that brought you here is probably unrelated to the above process. My guess is that pipe-clips that were nice and tight at the time of installation have now had time to relax. What you are hearing is slip-stick friction occuring when the pipework contracts as it cools. Somewhere a pipe will be passing over/through a somewhat tight gap that needs opening up a bit and a piece of hemp lagging used to let the pipe slide rather than stick.
Edit: I haven't read all 30 posts on this topic very carefully so my apologies if I'm repeating stuff that others have already said.
I know you're clutching at straws and I could be wrong myself but I'm not convinced that the above you mentioned would occur. If it did we would see a lit more air ingress than we do. The only other thing I thought of and again I could be wrong is air being drawn in at the highest point of the system where the static head is at a minimal?
I also wondered if it might be the highest point in the system and if that might explain anything.