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 it is currently busy heatng up your hot water tank or keeping its internal water reserve warm.. It appears on 30 Vaillant models in our database.
Technical description: Hot holding request / Cylinder charging request
What causes the Vaillant S.20 fault?
This occurs when the boiler prioritizes your hot water over your central heating to ensure you have a full tank or a primed internal heat exchanger. It is triggered when the physical sensors detect the water temperature has dropped below a set point, prompting the boiler to fire up specifically for hot water. This isn't a failure, but rather a notification that the system is temporarily busy focusing its energy on your taps and showers.
This status is more noticeable in winter when the incoming mains water is colder, requiring the boiler to work longer to reach the necessary temperature for your cylinder.
DIY fix possible
Check your hot water timer or thermostat settings to see if it is currently scheduled to be onWait a few minutes to see if the status changes back to heating or standby naturallyTurn off 'Comfort Mode' using the radiator dial if you want to stop the boiler from pre-heating its internal tank
Find a Gas Safe engineer
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A cover plan would handle faults like S.20: no call-out fees, no surprise bills.
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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.
- 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 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.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 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