BoilerFaultCodesGlow-worm F.77 Fault Code
Also written as F77 — same fault.
The Glow-worm F.77 fault code means: your boiler has detected a blockage in the drainage pipe or an issue with the flue, preventing exhaust gases or water from leaving the system safely.. It appears on 6 Glow-worm models in our database.
Technical description: Fault: Condensate or smoke
What causes the Glow-worm F.77 fault?
This fault occurs when the boiler's internal sensors detect a backup of liquid or exhaust gases that cannot escape the system safely. The most common reason is a blockage in the condensate trap or a failure in the external drainage pipe, which causes water to fill up inside the heat exchanger. In some cases, it can also be triggered if the flue becomes obstructed or if the internal seals have perished, allowing smoke to recirculate.
This fault is significantly more common during freezing winter temperatures, as the external condensate pipe can freeze, causing liquid to back up into the boiler.
DIY fix possible
Check the white plastic pipe leading outside (condensate pipe) to see if it is frozen during cold weather.If frozen, pour warm (not boiling) water over the external pipe or apply a heat pad to thaw it.Reset the boiler once the pipe is clear to see if the error clears.
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Gas Safe Register →Glow-worm models that show the F.77 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.77 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