S.8 Low

Vaillant S.8 Fault Code

Also written as S8 — same fault.

The Vaillant S.8 fault code means: your boiler is taking a short rest to prevent it from switching on and off too frequently, which helps save energy and protect its internal components.. It appears on 26 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: Anti cycling mode (after heating operation)

What causes the Vaillant S.8 fault?

This status occurs when the boiler has reached its target temperature and shuts down briefly to prevent 'short-cycling,' which is when the unit turns on and off too quickly. It physically happens because the heat produced by the burner cannot be moved away into your radiators fast enough, often because the house is already warm or the radiator valves have closed. This pause protects the internal components from unnecessary wear and tear while ensuring the system runs efficiently.

This is more common during the autumn and spring when the weather is milder and your home requires less heat to stay comfortable.

DIY fix possible

Check that your room thermostat is turned up high enough to call for heat.Ensure your radiator valves are open to allow heat to dissipate.Wait 15-20 minutes for the countdown to end, as the boiler will restart automatically.

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Vaillant models that show the S.8 fault code

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

Source

Vaillant S.8 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