BoilerFaultCodesGlow-worm S.28 Fault Code
Also written as S28 — same fault.
The Glow-worm S.28 fault code means: your boiler is taking a short, programmed break to prevent it from turning on and off too frequently, which helps protect the internal components from wear and tear.. It appears on 9 Glow-worm models in our database.
Technical description: Anticycling period
What causes the Glow-worm S.28 fault?
This occurs when your boiler reaches its target temperature too quickly and shuts down to prevent 'short-cycling,' which wastes energy and puts stress on the components. It usually happens because the heat produced by the boiler cannot be moved away fast enough, often due to thermostatic radiator valves closing as rooms get warm. The boiler is simply waiting for the system water to cool down slightly before it is safe and efficient to fire up again.
This is more common during the 'shoulder' seasons of spring and autumn, when the weather is mild and your radiators don't need the boiler's full heat output to keep the house warm.
DIY fix possible
Wait 5 to 15 minutes for the countdown timer to finishCheck that your room thermostat is turned up higher than the current room temperatureEnsure your radiator valves are open to allow heat to dissipate
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Gas Safe Register →Glow-worm models that show the S.28 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.28 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