Bleed a radiator or use one of the drain covks and run a hose outside.
Check the filling loop is properly shut off
If it's a combi boiler you may have a heat exchanger with a leak between the mains pressure incoming water and the primary circuit.
Think about getting a maintenance contract with callout and spares coverage. I think Vailla t do quite a good one.
Vhu6, October 2022
