If the cH system has pressure gauge then it will probably have a filling loop too.
find the filling loop. it is a connection from the cold water mains pipe to the central heating pipes. It is usually a silver, flexible pipe with an on off valve at one end, it is usually under the boiler.
have a look at the boiler manual if you have it. If not then if I were you I would try and download it or write to the manufacturer for a new one.
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