more info required. type of boiler i.e. combi, system,conventional.
If its a combi there will be a filling loop under the bioler
(one or two valves connected be a flexable tube) open the valve(s) until the pressure indicated on the bioler gauge reaches 1bar then turn off. you my need to vent some rads and fill again depending on the possition of the bioler in the house.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.