F.20 High

Glow-worm F.20 Fault Code

Also written as F20 — same fault.

The Glow-worm F.20 fault code means: your boiler has automatically shut down because it detected that the internal temperature has become too high, preventing it from overheating.. It appears on 24 Glow-worm models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Safety switch-off: Overheating temperature reached
Typical repair cost £100-280 (including labour)

DIY fix possible

Check that your radiator valves are fully open to allow water to flow.Ensure there is no air trapped in the system by bleeding your radiators.Press the reset button (indicated by a flame with a cross through it) to restart the boiler.

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Glow-worm models that show the F.20 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm F.20 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026