Hello.
Bear in mind i am no heating expert, However when i needed to increase the pressure on my boiler i was informed the following.
Turn the power off to the boiler and under the boiler there should be a flexible hose with a plastic tap on it . Turn the tap to allow water back into the system and turn it off when the pressure reaches number 1 0n the dial.
I was told if the pressure keeps dropping then there must be a leak on the system..normally from a radiator valve.
Hope it helps
Paul, September 2007