what boiler? if worcester bosch - ive cut and pasted a previous answer i gave - the location of these are on the underside of the boiler but you have to slide/unclip the underside plastic cover on the bottom of boiler
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I have the greenstar junior 24 -if it is similar the steps are as follows:
Make sure you push the key fully up and turn slightly until you feel that you have engaged correctly - note you can only turn the key a maximum of say a quarter of a turn - what you then need to do is to turn the square white plastic 'thingy' that is next to the key - hey presto - you'll hear water then charging the system!(Unless plumber installed a shut off valve within the supply line outside of boiler).
keep an eye on the level as you fill the system - when upto level turn the square thingy to stop charging the system - turn the key that quarter of a turn and pull out - put things back as they should be and pat yerself on the back for a job well done!
Ed.
