Smart thermostat shows F.12 and refuses to control heating because the system configuration is incomplete.

Safety: No internal electrical work required, but ensure boiler is safe and serviced.

• 1. Run commissioning wizard: On the thermostat, start the installation or commissioning wizard. Follow each step to select boiler model, heating circuits, hot water cylinder, and zones.
• 2. Assign devices to zones: Ensure each Vaillant room thermostat, sensor, and actuator is correctly assigned to a zone in the controller menu.
• 3. Remove unsupported modules: If you have added third‑party controls or unsupported Vaillant modules, temporarily disconnect them and re‑run configuration.
• 4. Factory reset if needed: If configuration is inconsistent, perform a factory reset and re‑commission the system from scratch using the Vaillant installation guide.
• 5. Technician assistance: Complex multi‑zone or cascade systems may require a Vaillant‑trained engineer to configure correctly.