Look underneath your boiler it should have a little key called a filling valve. Get someone to check the pressure gauge as you are filling it up as the pressure should be between 1 and 1.5. If you don't have anyone to check the pressure gauge for you, fill slowly and turn off and check yourself and fill again.
Turn it slowly and fill the unit with water until the required amount has been added, remember to turn it off when you have finished. check the pressure gauge is between 1 and 1.5, Good luck
Claudette
November 2008