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Vaillant VU 446/5-5 / VU 606/5-5 fault F.77

Accessory fault (flue non-return flap, condensate pump, etc.)

Gas boiler

What it means

Fault indicating a problem with an accessory such as the flue non-return flap or condensate pump, due to missing or incorrect feedback.

Likely causes

  1. No feedback from the flue non-return flap or feedback is incorrect
  2. Flue non-return flap defective
  3. No feedback from the condensate pump or feedback is incorrect
  4. Defective pump

Where to start

  1. Check that the flue non-return flap works without any problems.
  2. Check that the condensate pump is working correctly.
  3. Pull out the plug (5) for the pump

The full answer, free

  • 4 more checks in order
  • How to put it right, step by step
  • The manual page for this fault (page 63), right under the answer
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Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.

From the Vaillant manual for the VU 446/5-5 / VU 606/5-5. Check the model on the data plate before you order parts.

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