S.39 Medium

Vaillant S.39 Fault Code

Also written as S39 — same fault.

The Vaillant S.39 fault code means: your boiler has paused because it detected that the water temperature is too high for your underfloor heating system, triggered by a safety safety switch to protect your floors.. It appears on 39 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Contact thermostat has been activated (under floor systems)

What causes the Vaillant S.39 fault?

This fault occurs when an external safety sensor detects that the water flowing into your underfloor heating pipes has exceeded a safe temperature. It is usually triggered by a failure in the mixing valve or the underfloor pump, which prevents the hot boiler water from being cooled down properly before entering the floor. This safety measure is essential to prevent high temperatures from damaging your flooring or the pipes underneath.

This issue is most common during the first cold spells of autumn/winter when underfloor heating systems are turned on for the first time after being idle all summer.

Typical repair cost £0-120 (including labour)

DIY fix possible

Wait for the system to cool down naturally for 30-60 minutes.Check that the underfloor heating pump is running and hasn't seized.Ensure the mixing valve for the underfloor heating is not set too high.

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Vaillant models that show the S.39 fault code

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

Source

Vaillant S.39 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