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 46 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.
- Atmomax Repair: £180-350
- 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 610
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 826
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus Repair: £180-450
- 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.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 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026