BoilerFaultCodesVaillant S.21 Fault Code
Also written as S21 — same fault.
The Vaillant S.21 fault code means: your boiler is currently preparing to supply hot water and is in a normal sequence of starting up its internal fan.. It appears on 31 Vaillant models in our database.
Technical description: DHW mode: Fan start-up. The fan start-up for domestic hot water mode is activated.
What causes the Vaillant S.21 fault?
This status code appears when you turn on a hot tap and the boiler begins its startup sequence to heat your water. It occurs as the internal fan begins to spin up to the correct speed to safely vent exhaust gases, which is a vital part of the ignition process. This is not a fault, but rather a brief progress update showing that the boiler is responding correctly to your request for hot water.
DIY fix possible
This fault may be fixable without an engineer. Check your manual first.
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Vaillant models that show the S.21 fault code
Click a model for the full fault code list and model-specific repair notes.
- 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 824/2 E
- ecoMAX 828/2 E
- ecoMAX 835 E
- Ecomax Condensing 600/800 series
- ecoTEC exclusive
- ecoTEC plus
- ecoTEC plus VU GB 656/4-5 A
- ecoTEC pro
- ecoTEC sustain
- Home Combi 25
- Home Combi 30
- Home Combi 35
- 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.21 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 425 manual (PDF)
- Vaillant ecoFIT Pure 435 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026