combi boiler?

(more info in your question would have helped, make, model etc)
most combi boilers you refill the system via the use of the filler loop hose,(metal braided hose) usually filled up to the 1,to 1.5 bar pressure mark, with the system cold. Bleed any air out of the whole system, re top up the pressure if need be. The filler hose is then removed/detached to comply with the water bylaws.

go to your boiler makers web site, see if they do any videos on how to do it.

wirecutterman
December 2009