BoilerFaultCodesGlow-worm F26 Fault Code
The Glow-worm F26 fault code means: your boiler has shut down because the water is heating up too quickly or hasn't been able to circulate correctly through the system.. It appears on 4 Glow-worm models in our database.
Technical description: Maximum delta temperature
What causes the Glow-worm F26 fault?
This fault occurs when the internal sensors detect that water inside the boiler is getting too hot too quickly, or there is too large a temperature difference between the water leaving and returning to the unit. The most common reason is a lack of water flow, often caused by a failing pump or a blockage in the pipework that prevents heat from being carried away to your radiators. It can also be triggered if the sensors themselves become faulty and provide inaccurate temperature readings to the control board.
This fault is frequently seen at the start of autumn when heating systems are turned back on after a long summer break, which can cause internal pump components to seize.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barSwitch the boiler off and back on again to reset the system
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Glow-worm models that show the F26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F26 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 7 July 2026