F.20 on a Glow-worm boiler means: safety switch-off — overheating temperature reached. The water inside the boiler has got too hot, so a safety device has shut it down.
This code applies to Glow-worm Energy, Ultimate, Betacom. Codes can mean different things on different models, so check the display against your boiler's user manual.
What you can safely check
- Look at the pressure gauge on the front of the boiler. With the heating off and the system cold, it should normally read 1 to 1.5 bar. Low pressure is a common cause of overheating.
- Make sure the valves on your radiators are open, and bleed any radiators that are cold at the top.
- Let the boiler cool down, then press reset once, following your user manual.
When to call an engineer
If it keeps overheating, the usual causes are a failing pump, a blockage (sludge) in the system or heat exchanger, or a faulty sensor. Call a Gas Safe engineer.
Never remove the boiler casing yourself — everything behind it must be handled by a Gas Safe registered engineer. If you smell gas, open windows, turn the gas off at the meter, and call the National Gas Emergency number on 0800 111 999.
Repair it, or replace it?
If your boiler is over 10 years old, compare a repair against a new combi before you pay for parts.
Applies to: Glow-worm Energy, Ultimate, Betacom
Meaning checked against: Glow-worm — fault finding codes (Energy range)
Last reviewed November 2026. Always follow your boiler's own user manual.