BoilerFaultCodesGlow-worm 25 Fault Code
The Glow-worm 25 fault code means: your boiler is heating up much faster than it should and has shut down to prevent itself from overheating.. It appears on 1 Glow-worm model in our database.
Technical description: The rate of temperature rise measured by the heating flow thermistor is too high.
What causes the Glow-worm 25 fault?
This fault occurs when the water inside the boiler gets hot too quickly because it isn't moving away from the heat source fast enough. The most common reasons are a pump that has failed or become stuck, or a blockage within the main heat exchanger that is restricting water flow.
This issue is more frequent in autumn or winter when the heating is first turned on after a long summer break, as pumps can often seize up from lack of use.
DIY fix possible
Check if your radiator valves are openEnsure your boiler pressure is between 1.0 and 1.5 barGently bleed your radiators to remove any trapped air
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Gas Safe Register →Glow-worm models that show the 25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 25 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