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Hi the filling loop is usually just under the boiler and you can recognize as it has a silver braided flexible pipe attached to two valves.
Some however have an integral filling loop but we will need to know the make and model of the boiler before we can identify what you have.
If you find the loop with the flexible hose then open the valves either side of this and fill your system to about 1.2.
Failing this please tell us make and model also if you can please send pictures of the pipework below the boiler. and of the boiler.