Step by step
Read the gauge
Check whether the cold pressure is below the range shown in your boiler manual.
Find the filling controls
Identify the correct valves or integral filling key from the manual. Do not guess which pipe to open.
Fill slowly
Open the specified controls carefully and watch the gauge rise to the manual's recommended cold range.
Close everything
Shut the filling controls fully. If the loop is detachable, follow the manual's instructions for disconnecting it.
Check again
Restart only as the manual directs and monitor whether pressure holds.
Know when to stop
When to stop and call a professional
Stop if you cannot identify the controls, the gauge climbs too far, water leaks, or pressure falls again. A repeated pressure loss needs a qualified heating engineer.
General information for a UK home; always follow your product instructions and current local rules. Images are illustrative, not technical diagrams.