Hi . the heating system water is sealed and will not fill itself back up . when you drain the system froM the drain valve ,open the top radiator bleed valve to help drain the system
When filling back up , shut all radiator bleeds and then fill the system up using the filling loop.
Fill up until pressure gauge gets to 1.2 bar .
If you can't find the filling loop ythe boiler may have an internal one
Hi if happy with your answers , would you mind rating ,please. Thanks