S.10 Low

Vaillant S.10 Fault Code

Also written as S10 — same fault.

The Vaillant S.10 fault code means: your boiler is currently working to provide hot water to your taps or shower, which is a normal status message rather than an actual fault code.. It appears on 40 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 demand (hot water operation)

What causes the Vaillant S.10 fault?

This status appears when the boiler detects a flow of water through the internal heat exchanger, signaling a request for hot water. It confirms that the sensors have successfully identified that a tap or shower is turned on and the burner is firing to heat that water. Since this is an operational message rather than a breakdown, it simply means the system is doing exactly what it was designed to do.

DIY fix possible

Check if any hot water taps are currently turned onEnsure there are no leaking or dripping hot water taps in the propertyIf the code remains when all taps are off, check your pre-heat settings or potential internal leaks

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Vaillant models that show the S.10 fault code

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

Source

Vaillant S.10 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