S.21 Low

Heatline S.21 Fault Code

Also written as S21 — same fault.

The Heatline S.21 fault code means: your boiler is currently checking that its internal air extraction system is working correctly before it attempts to ignite the flame.. It appears on 8 Heatline models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Pre-run fan
Typical repair cost £120-250 (including labour)

DIY fix possible

No action required; this is a normal operational status messageMonitor the display to ensure it progresses to ignition (S.4)If the code stays on screen indefinitely, try a resetIf the boiler fails to fire after this stage, contact a Gas Safe engineer

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Heatline models that show the S.21 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Heatline S.21 fault code information on this page is sourced and cross-checked against the official Heatline service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026