Thermostat shows B4 and indicates that the heating curve or weather compensation parameters are invalid.

Safety first: No electrical work is required.

• Review heating curve settings: In the installer or expert menu, check the heating curve slope and parallel shift values. Compare them with the recommended values in the Viessmann documentation for your system type (radiators vs underfloor).
• Restore defaults: Use the option to restore factory default heating curve parameters, then adjust gradually if needed.
• Check system type selection: Ensure the thermostat is set to the correct emitter type (radiators, underfloor, mixed) so that valid curve ranges are used.
• Save and reboot: After correcting settings, save and restart the thermostat. If B4 persists, a firmware issue may be present and Viessmann support should be contacted.