BoilerFaultCodesVaillant F.20 Fault Code
Also written as F20 — same fault.
The Vaillant F.20 fault code means: your boiler has shut down because it has detected an unsafe increase in internal temperature, likely caused by a sensor fault or an issue with water flow.. It appears on 39 Vaillant models in our database.
Technical description: Safetyswitch-off:Temperaturelimiter | Incorrectearthconnectionbetweencableharnessandproduct,flowor returnNTCdefective(looseconnection),straysparkviaignitioncable, ignitionplugorignitionelectrode
What causes the Vaillant F.20 fault?
This fault occurs because the boiler's internal sensors have detected that the water temperature has risen too quickly or exceeded a safe limit. It is usually caused by a lack of water flow, often due to a pump failure or a blockage, which prevents the heat from being carried away from the heat exchanger. In some cases, the sensor itself may have become detached or faulty, sending an incorrect reading to the control board.
This fault is more common in autumn when heating systems are first turned on after summer, as circulation pumps can seize due to months of inactivity.
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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A cover plan would handle faults like F.20: no call-out fees, no surprise bills.
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Vaillant models that show the F.20 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT pure 612 Repair: £150-350
- ecoFIT pure 615 Repair: £150-350
- ecoFIT pure 618 Repair: £150-350
- ecoFIT pure 630 Repair: £150-350
- ecoFIT Pure 825 Repair: £120-250
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835 Repair: £150-300
- ecoFIT sustain 615 Repair: £150-300
- ecoFIT sustain 618 Repair: £150-300
- ecoFIT sustain 630 Repair: £150-300
- ecoFIT sustain 825 Repair: £150-300
- ecoFIT sustain 830 Repair: £150-300
- ecoFIT sustain 835 Repair: £150-300
- ecoMAX Repair: £120-£350
- ecoMAX 613 Repair: £100-£350
- ecoMAX 618 Repair: £100-£350
- ecoMAX 622 Repair: £100-£350
- ecoMAX 635 Repair: £100-£350
- ecoMAX 824/2 E Repair: £100-£350
- ecoMAX 828/2 E Repair: £100-£350
- ecoMAX 835 E Repair: £100-£350
- Ecomax Condensing 600/800 series Repair: £150-300
- ecoMAX Pro 28E Repair: £150-350
- ecoTEC exclusive Repair: £120-£280
- ecoTEC plus Repair: £100-280
- ecoTEC plus VU GB 656/4-5 A Repair: £150-350
- ecoTEC pro Repair: £100-£350
- ecoTEC sustain Repair: £100-280
- Home Combi 25 Repair: £150-350
- Home Combi 30 Repair: £150-350
- Home Combi 35 Repair: £150-350
- Home Regular 12
- Turbomax 824/828 Repair: £120-250
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E Repair: £100-280
- Turbomax Plus 828 Repair: £120-350
- Turbomax Plus 828 E Repair: £100-280
- Turbomax Plus VUW 242E Repair: £100-280
- Turbomax Plus VUW 242EH Repair: £100-280
Source
Vaillant F.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 630 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026