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Viessmann Vitodens 050-W fault F8

Fuel valve closes too late

Gas boiler

What it means

Burner enters a fault state because the fuel valve closes too late relative to control signal.

Likely causes

  1. Fuel valve closes too late
  2. Leak in the balanced flue system
  3. Faulty gas valve

Where to start

  1. Check gas train
  2. Check both control paths
  3. Press Reset

The full answer, free

  • 5 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 57), 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 Viessmann manual for the Vitodens 050-W. Check the model on the data plate before you order parts.

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