S.20 Low

Vaillant S.20 Fault Code

Also written as S20 — same fault.

The Vaillant S.20 fault code means: your boiler has received a request to provide hot water but is currently unable to complete the cycle, often because it is already cooling down or busy with another task.. It appears on 42 Vaillant 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: Hot water requirement error

What causes the Vaillant S.20 fault?

This status occurs when the boiler receives a signal to heat your water but cannot start the burner because it has recently finished a heating cycle and needs to cool down internally. It is often caused by a communication lag between your external timer and the boiler, or a mechanical delay in the internal valve that directs water to your taps. In many cases, it is simply a 'waiting' state rather than a permanent mechanical failure.

Typical repair cost £60-120 (including labour)

DIY fix possible

Check if any hot water taps have been left slightly openTurn the boiler power off and back on again to reset the internal clockEnsure your external timer or thermostat is not stuck in an 'on' position

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Vaillant models that show the S.20 fault code

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

Source

Vaillant S.20 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026