BoilerFaultCodesVaillant S.20 Fault Code
Also written as S20 — same fault.
The Vaillant S.20 fault code means: your boiler is simply letting you know that the 'warm start' feature is active, which keeps a small amount of water pre-heated so you get hot water at your taps more quickly.. It appears on 49 Vaillant models in our database.
Technical description: Warmstart demand
What causes the Vaillant S.20 fault?
This fault occurs when the boiler attempts to ignite to provide hot water but fails to detect a consistent flame or correct operation. The most common reason is a communication error between the sensors and the burner, often triggered by a faulty diverter valve sticking or a sensor incorrectly reporting water flow. It essentially means the boiler’s safety controls have stepped in to stop the burner because it cannot guarantee correct operation in hot water mode.
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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Vaillant models that show the S.20 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £0-120
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX
- ecoMAX 613
- ecoMAX 618
- ecoMAX 622
- ecoMAX 635
- ecoMAX 824E
- ecoMAX 828E
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC
- ecoTEC 65kW
- ecoTEC exclusive
- ecoTEC plus
- ecoTEC Plus 610
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 826
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus
- ecoTEC Pro 24
- ecoTEC Pro 28
- ecoTEC Pro 30
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
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.
- Vaillant ecoFIT Pure 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026