The "Fd" is a water flow issue so are all the valves open and the boiler properly vented the fact that it goes to "C" makes me think there is possibly an air lock in the system. Was the system properly flushed when the new boiler was fitted? have seen an old system recently where a new boiler and radiators were fitted but part of the pipework was that badly sludged up that it had blocked the rad valves after the system had been used for 12 - 18 months