F.12 Low

Viessmann F.12 Fault Code

Also written as F12 — same fault.

The Viessmann F.12 fault code means: your boiler is reporting that it is not receiving heat from your solar water heating panels, meaning you will rely entirely on the gas boiler for your hot water.. It appears on 4 Viessmann models in our database.

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.

Technical description: No solar DHW heating
Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the F.12 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann F.12 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026