C9
Viessmann
Hydraulic Configuration Error
Thermostat displays C9 and indicates that the system hydraulic configuration is inconsistent; some functions are disabled.
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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 selection of system layout (e.g. with/without buffer tank), Mismatched number of heating circuits configured vs installed, Incompatible combination of components in settings, Corrupted configuration data
How to Fix / Troubleshooting
Safety first: No direct electrical work is required, but do not change hydraulic components without a heating engineer.
- Review system type: In the installer menu, verify that the selected hydraulic scheme matches the actual installation (e.g. one direct circuit, one mixed circuit, buffer tank, solar integration).
- Match circuits to hardware: Ensure that only the heating circuits and components that physically exist are enabled in the configuration.
- Restore default scheme: If unsure, restore the default hydraulic scheme recommended for your boiler/heat pump and reassign circuits.
- Recommission system: A Viessmann‑trained installer may need to fully recommission the system to clear C9.
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Repair Difficulty
Hard
4/5
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