BoilerFaultCodesGlow-worm F8 Fault Code
The Glow-worm F8 fault code means: your boiler is struggling to circulate water quickly enough through your pipes, which is causing it to stop to prevent overheating.. It appears on 5 Glow-worm models in our database.
Technical description: Microbore system or pump speed setting fault
What causes the Glow-worm F8 fault?
This fault occurs when the boiler detects that water isn't moving through the system fast enough to carry heat away from the heat exchanger. The most common reasons are a failing internal pump that has lost its pushing power or a blockage in the narrow pipework that is restricting flow. When the water sits still for too long, it overheats rapidly, forcing the boiler to shut down for safety.
This issue is most frequently noticed when the heating is first turned on in autumn, as pump components can seize up or sludge can settle during the summer months of inactivity.
DIY fix possible
Ensure all radiator valves (TRVs) are fully open to allow water to flow.Check that the boiler pressure is between 1 and 1.5 bar on the gauge.Gently bleed your radiators to remove any trapped air pockets.
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 F8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F8 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