S.20 Low

Heatline S.20 Fault Code

Also written as S20 — same fault.

The Heatline S.20 fault code means: your boiler is currently preparing to fire by checking the water circulation, which is a normal part of the start-up process.. 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 pump

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer

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Heatline models that show the S.20 fault code

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

Source

Heatline S.20 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 27 April 2026