BoilerFaultCodesVaillant F.77 Fault Code
Also written as F77 — same fault.
The Vaillant F.77 fault code means: your boiler has detected an issue with the drainage system that removes leftover liquid from the exhaust pipe or a fault with the safety flap inside the flue.. It appears on 28 Vaillant models in our database.
Technical description: Flue non return flap condensate pump error
What causes the Vaillant F.77 fault?
This fault occurs when the boiler's internal safety sensors detect that water is backing up in the flue system or that the mechanical flap designed to stop exhaust fumes from returning is stuck. The most common reason is a failed condensate pump that can no longer push waste liquid away, or debris interfering with the flue flap's movement. Because this affects how exhaust gases and liquids leave the unit, the boiler shuts down as a safety precaution.
This fault is more frequent during particularly cold winters, as frozen external drainage pipes can cause a backup of water that eventually triggers the condensate pump alarm.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the F.77 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £120-280
- ecoFIT Pure 412 Repair: £150-300
- ecoFIT Pure 418 Repair: £150-350
- ecoFIT Pure 425 Repair: £150-350
- ecoFIT Pure 435 Repair: £150-350
- ecoFIT Pure 825 Repair: £150-300
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £150-350
- ecoTEC Repair: £120-280
- ecoTEC 65kW Repair: £150-350
- ecoTEC exclusive Repair: £120-280
- ecoTEC Plus 610 Repair: £150-300
- ecoTEC Plus 615 Repair: £120-350
- ecoTEC Plus 620 Repair: £120-350
- ecoTEC Plus 625 Repair: £120-350
- ecoTEC Plus 630 Repair: £120-350
- ecoTEC Plus 635 Repair: £120-350
- ecoTEC Plus 826 Repair: £120-250
- ecoTEC Plus 840 Repair: £120-350
- ecoTEC Plus 938 Repair: £120-350
- ecoTEC Pro Repair: £120-280
- ecoTEC Pro / ecoTEC Plus Repair: £120-280
- ecoTEC Pro 24 Repair: £120-280
- ecoTEC Pro 28 Repair: £120-280
- ecoTEC Pro 30 Repair: £150-300
- ecoTEC sustain Repair: £120-350
- Home Combi Repair: £150-300
- Home System Repair: £120-350
Source
Vaillant F.77 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 ecoTEC manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026