Verified boiler fault code

Vaillant F.77 Fault Code (ecoTEC plus/pro (manual models))

Condensate pump or accessory feedback blocks heating.

Safety disclaimer

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Fault code summary

Manufacturer
Vaillant
Fault code
F.77
Fault type
Condensate, Safety lockout
Severity
high
Reset
Not specified
Applicable models
ecoTEC plus 612, ecoTEC plus 615, ecoTEC plus 618, ecoTEC plus 624, ecoTEC plus 630, ecoTEC plus 637, ecoTEC plus 824, ecoTEC plus 831, ecoTEC plus 837, ecoTEC pro 28

Quick answer

Condensate pump or accessory feedback blocks heating.

What does this fault code mean?

Condensate pump or accessory feedback blocks heating.

Common causes

  • Condensate pump defective.
  • Flue flap feedback triggered.

Symptoms

  • Heating is blocked.

What can the homeowner safely check?

This information was not provided in the verified manufacturer source.

Boiler reset information

Reset information was not provided in the verified manufacturer source. Do not repeatedly reset an unresolved fault.

When should I call an engineer?

Contact a suitably qualified heating engineer when the fault persists, the boiler locks out, or the manufacturer instructions do not resolve it. For gas appliance work in the UK, use a Gas Safe registered engineer.

Engineer diagnosis

  • Check condensate pump operation and accessory/flue-flap feedback.

Applicable boiler models

Related fault codes

Manufacturer documentation

Instructions for installation and servicing ecoTEC

F.77 | condensate pump or feedback of accessorie blocks heating | condensate pump defective or flume flap feedback triggered

Last verified

2026-09-12

Frequently asked questions

What does Vaillant F.77 mean?

Condensate pump or accessory feedback blocks heating.