Airlock open vented system?

[lndwl3]

Hello new to the forum Hope someone can help please.

New gloworm flexicom 18hx fitted last year,not much problem with the boiler until changed out pump and replaced rad upstairs i thought i take the opportunity to drain down and flush the system out at the same time.Bad mistake i think, because the boiler fires up and cuts out on fault code F22 (Low water pressure,not enought water in the system the user manual states a couple of other possible causes ie flow and return thermistors but this maybe because its nots seeing any temperature difference.
I have tried various remedies that have been posted on site for airlocks but cant shift it
Any advise would be welcome,Thanks

 

[gasmarc]

i suppose you could run the mains through the boiler or any drain point

 

[lndwl3]

Thanks will try that.