Safety: Turn off power and allow the boiler to cool fully before touching pipework. Do not repeatedly reset an overheating boiler.

Check system pressure: Ensure the pressure gauge reads around 1.0–1.5 bar when cold. If low, top up via the filling loop following the manual.
Check radiator valves: Make sure at least some radiators are open; a fully closed system can cause rapid overheating.
Listen for pump operation: On a call for heat, you should hear the circulation pump running. If silent or very noisy, it may be seized or airlocked.
Bleed air: Bleed radiators and any automatic air vents to remove trapped air that can stop circulation.
Engineer tasks:
- Check pump speed and head, free the pump or replace if failed.
- Inspect the primary heat exchanger for blockage or scale.
- Test and replace the overheat thermostat if faulty.

Persistent F20 faults require professional attention to avoid damage to the heat exchanger.

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

Possible Causes