C8
Viessmann
Auxiliary Heat Source Control Fault
Thermostat shows C8 and cannot control the auxiliary heater or backup boiler in a bivalent system.
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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 bivalent configuration in controller, Faulty relay output to auxiliary heater, Wiring error to backup boiler enable input, Auxiliary heater lockout or fault
How to Fix / Troubleshooting
Safety first: Turn off power to all heat sources before working on control wiring. Only qualified personnel should modify bivalent system wiring.
- Check configuration: In the system controller or thermostat installer menu, verify that the auxiliary heat source is enabled and correctly configured (bivalent temperature, priority, etc.).
- Inspect control wiring: Ensure the control signal from the Viessmann controller to the auxiliary heater or backup boiler is wired to the correct terminals.
- Check auxiliary device status: Confirm that the backup boiler or electric heater is not in lockout and is ready to operate.
- Test relay output (technician): A professional can test the controller’s relay output to ensure it is switching correctly.
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Repair Difficulty
Professional Required
5/5
Required Part
Auxiliary heat source control relay
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