BoilerFaultCodesGlow-worm 21 Fault Code
The Glow-worm 21 fault code means: your boiler has lost water pressure and does not have enough water inside it to operate safely.. It appears on 1 Glow-worm model in our database.
Technical description: LACK OF WATER IN PRIMARY CIRCUIT: PERFORM WATER TOPPING - UP
What causes the Glow-worm 21 fault?
This fault occurs when the water pressure inside your heating system drops below the minimum level required for the boiler to fire up. The most common reasons for this are a leak somewhere in your radiators or pipework, or a failure in the expansion vessel which helps manage pressure changes as the water heats up.
This issue often appears in autumn when the heating is turned on for the first time after summer, as the change in temperature can highlight minor leaks or a loss of pressure in the expansion vessel.
DIY fix possible
Locate the filling loop (the flexible silver hose) underneath or near your boilerSlowly open the one or two taps on the hose until you hear water flowingWatch the pressure gauge and close the taps once it reaches between 1.0 and 1.5 bar
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Gas Safe Register →Glow-worm models that show the 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 21 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