BoilerFaultCodesGlow-worm F.23 Fault Code
Also written as F23 — same fault.
The Glow-worm F.23 fault code means: your boiler has shut down because the water is heating up too quickly or unevenly, suggesting a circulation problem.. It appears on 6 Glow-worm models in our database.
Technical description: Safety switch-off – temperature difference too great (NTC1/NTC2)
What causes the Glow-worm F.23 fault?
This fault occurs when the boiler's internal sensors detect that water is heating up too fast or that there is a massive temperature gap between the water leaving and returning to the unit. The most common reason is a pump failure or a blockage that prevents water from moving through the system quickly enough to carry the heat away. It can also be caused by air trapped in the heat exchanger or a buildup of limescale restricting the flow.
This issue often surfaces during the first cold spell of autumn when the heating system is turned on after being dormant all summer, which can cause internal pump parts to seize.
DIY fix possible
Ensure all radiator valves (TRVs) are turned to the highest settingCheck that the boiler pressure is between 1.0 and 1.5 bar on the displayGently bleed air from your radiators using a radiator key
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.23 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.23 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