Baxi solo was normally put on in a traditional system, in which case the tap would normally be permently on to fill the header tank in he loft.
If the system was created as a pressured system for some reason such as radators in the roof space or insuficent head then the tap should be turned on till the pressure is 1 to 1.5 Bar. It should then be turned off and the filling loop removed to comply with water regulations. (Removing it also means if the tap leaks the pressure won't go up)
Peter, November 2009
