BoilerFaultCodesGlow-worm F.78 Fault Code
Also written as F78 — same fault.
The Glow-worm F.78 fault code means: your boiler has detected a fault with a safety sensor designed to prevent the main heating unit from overheating, causing the system to shut down.. It appears on 6 Glow-worm models in our database.
Technical description: The safety cut-out in the primary heat exchanger is defective
What causes the Glow-worm F.78 fault?
This fault occurs when the boiler's internal safety switch detects that the main heat exchanger is getting too hot, or if the sensor itself has failed and is sending incorrect data. The most common reasons include a lack of water flow caused by a trapped air bubble, a failing pump, or a build-up of sludge that prevents heat from being transferred away from the unit quickly enough.
This fault is more common in autumn when heating systems are first turned on after a long summer break, as internal pumps can seize up or air can become trapped in the pipes.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the F.78 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.78 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