BoilerFaultCodesGlow-worm 24 Fault Code
The Glow-worm 24 fault code means: your boiler's temperature sensors have been swapped during installation or maintenance, causing the system to get confused about which pipe is heating up.. It appears on 1 Glow-worm model in our database.
Technical description: The heating flow thermistor and heating return thermistor are connected the wrong way round.
What causes the Glow-worm 24 fault?
This fault occurs when the water inside the boiler heats up much faster than it can be moved away, leading to a dangerous heat buildup. The most common reasons are a failed pump that isn't pushing water round or a blockage in the heat exchanger, such as a buildup of sludge or limescale. In some cases, it may simply be that the internal sensors have become faulty and are providing incorrect temperature readings to the control board.
This fault is more frequent in autumn when heating systems are first turned on after summer, as pumps can seize during long periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Glow-worm models that show the 24 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 24 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