A3
Viessmann
Heating Circuit Assignment or Addressing Error
Thermostat shows A3 and cannot control the heating circuit; boiler does not respond to setpoint changes.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Incorrect heating circuit configuration in thermostat settings, Duplicate KM‑BUS address with another room unit, Incompatible boiler configuration for assigned circuit, Firmware mismatch between thermostat and boiler control
How to Fix / Troubleshooting
Safety first: No internal live wiring work is required for this step, but do not open the boiler cabinet unless qualified.
- Check heating circuit configuration: In the thermostat menu, navigate to installation or expert settings and verify that the correct heating circuit (e.g. HC1, HC2, mixed or direct) is assigned to the room unit.
- Verify unique address: If multiple Viessmann room units are installed, ensure each has a unique KM‑BUS address as per the installation manual. Adjust addresses if duplicates are found.
- Confirm boiler supports circuit: On the boiler control panel, verify that the corresponding heating circuit is enabled and configured (e.g. mixing valve present, pump installed).
- Update firmware: If the thermostat is app‑connected, check the Viessmann app for available firmware updates and apply them. Also check for boiler firmware updates via a Viessmann technician.
- Reboot system: Power cycle both boiler and thermostat. If A3 persists, professional re‑commissioning of the system may be required.
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Repair Difficulty
Medium
3/5
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