themo compact 637e?

hi i have recently been called to look at a thermocompact boiler and i has been fed from an f&e tank it has no pressure on the gauge the boiler is mounted upstairs next to the cylinder it is coming up with an f28 fault the gas valve is opening ignition is taking place but the burner is not firing. does it need to be made into a sealed sytem and be filled with a filling loop,please advise Regards,Terry.