Boiler shows F20 and shuts down, casing may feel very hot

Safety: Switch off power and gas immediately if the boiler is overheating. Do not attempt to run the boiler until the cause is identified.

Checks:

• Allow the boiler to cool fully. Check system pressure and top up if low. Bleed air from radiators and the boiler’s automatic air vent.
• Confirm that the circulation pump operates correctly. Replace if seized or not circulating water.
• Inspect the primary heat exchanger for signs of scale or sludge blockage (uneven heating, kettling noises). A professional power‑flush or heat exchanger replacement may be required.
• Test the flow and return NTC sensors for correct resistance. Replace any sensor that is out of specification.

After repair: Reset the boiler and run at a moderate temperature setting, monitoring for normal operation without F20. Persistent overheating requires professional investigation.