BoilerFaultCodesGlow-worm F.20 Fault Code
Also written as F20 — same fault.
The Glow-worm F.20 fault code means: your boiler has switched itself off because it has become too hot internally and needs to cool down for safety.. It appears on 6 Glow-worm models in our database.
Technical description: Safety switch-off – overheating temperature reached
What causes the Glow-worm F.20 fault?
This fault occurs when the water inside the boiler reaches an illegally high temperature, triggering a safety sensor to shut the system down. The most common reason is that the water isn't moving through the unit quickly enough, often due to a pump failure or a blockage caused by sludge buildup in the pipework. It can also happen if there is trapped air in the system or if the internal divertor valve has become stuck.
This fault is more frequent when the heating is first turned on after summer, as pump components can seize up during months of inactivity.
DIY fix possible
Check that your radiator valves are open and not turned to the off positionEnsure your boiler pressure is between 1.0 and 1.5 bar on the displayPress the reset button (a flame symbol with a cross through it) once the boiler has cooled down
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Gas Safe Register →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 27 April 2026