Boiler frequently loses pressure and needs to be topped up every few days, no visible leaks and no error code.

Safety first: Do not repeatedly overfill the system above the recommended pressure. This can cause further damage.

User-level checks:

• Check around radiators, valves, and visible pipework for damp patches or staining.
• Observe the safety valve discharge pipe outside for signs of dripping when the boiler is running.

Technical checks (qualified personnel):

• Test the expansion vessel pre-charge and replace or recharge if faulty.
• Inspect the safety valve for signs of passing; replace if it does not seal properly.
• Use leak detection methods (e.g. pressure test, dye) to locate hidden leaks in the system.
• Check the plate heat exchanger for internal leaks between the heating and DHW circuits.

Once the underlying cause is repaired, refill, vent, and monitor the system pressure over several days.