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Viessmann Vitodens 100-W fault 77

HBMU heat management unit data memory fault

Gas boiler

What it means

Burner enters a fault state due to a data memory issue within the HBMU heat management unit.

Likely causes

  1. HBMU heat management unit data memory fault
  2. Burner locked out due to a fault condition

Where to start

  1. Reset the appliance
  2. If fault re-occurs, replace the HBMU heat management unit
  3. Tap up/down arrows to display the fault number

The full answer, free

  • 3 more checks in order
  • How to put it right, step by step
  • The manual page for this fault (page 72), 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 100-W. Check the model on the data plate before you order parts.

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