Boiler shows F.20 and shuts down, casing may feel hot

Safety: Isolate power and allow the boiler to cool fully before touching pipework. Do not bypass safety devices.

• Check system pressure: Ensure pressure is 1.0–1.5 bar when cold. Top up via the filling loop if low and bleed radiators.
• Verify all valves open: Confirm all isolation valves under the boiler and on the heating circuit are fully open. Check that thermostatic radiator valves are not all closed.
• Check pump operation: When calling for heat, listen for the pump running and feel for vibration. If the pump is silent or very hot but not circulating, it may be seized or airlocked. Bleed the pump via the bleed screw (with a cloth ready) and try again.
• Clean system filter: If a Vaillant or third-party magnetic filter is installed, isolate it and clean out sludge that may be restricting flow.
• Descale if necessary: In hard water areas, the primary heat exchanger can scale up and cause local overheating. This requires chemical descaling or replacement by a professional.
• Reset: Once the cause is addressed and the boiler has cooled, reset the boiler and monitor operation.