Start with the pattern
Record how often the pressure falls and whether the system is hot or cool when you read it. Keep any notes from a recent boiler service. A visible radiator joint or accessible fitting may provide a clue, but concealed pipework is only one possible cause.
Choose the right investigation
A boiler component fault and a leaking heating pipe require different work. A suitably qualified heating engineer can assess boiler-related causes. If earlier checks point to a concealed circuit leak, pressure testing and specialist tracing may help narrow the source.
Prepare useful information
Describe the system: radiators, wet underfloor heating or both. Explain whether there are separate zones and where the manifold is. Do not remove the boiler casing or attempt internal repairs. Follow the manufacturer’s instructions and get professional advice rather than repeatedly topping up without a diagnosis.