Low
A.12 on Viessmann Vitodens 100-W WB1A

Your boiler has lost its internal time settings because the small backup battery that remembers them has run out of power.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

Real time clock battery flat. Measure: Replace the battery (type CR2032) in the HMU heat management unit.

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Switch off the power supply to the boiler at the fused spurOpen the front casing to access the Heat Management UnitLocate the CR2032 coin cell battery on the circuit board and swap it for a new oneClose the casing, restore power, and reset the time and date on the display

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