A4
Viessmann
DHW Control Communication or Configuration Fault
Thermostat displays A4 and cannot control domestic hot water (DHW) temperature; DHW schedule is ignored.
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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
DHW sensor not configured or missing in boiler, Incorrect DHW mode selected in thermostat, Faulty DHW temperature sensor on boiler cylinder, KM‑BUS configuration not allowing DHW control from thermostat
How to Fix / Troubleshooting
Safety first: Turn off power to the boiler before accessing any internal components. DHW circuits can be hot; avoid contact with hot pipes.
- Check DHW mode in thermostat: In the thermostat settings, verify that DHW control is enabled and that the correct DHW type (tank, combi, etc.) is selected.
- Verify DHW sensor presence: At the boiler or cylinder, confirm that a Viessmann DHW temperature sensor is installed and wired to the correct terminals on the boiler control board.
- Inspect DHW sensor wiring: Look for loose connections or damaged cable between the cylinder sensor and the boiler control.
- Check boiler DHW configuration: On the boiler’s service menu (installer level), confirm that DHW control via KM‑BUS/room unit is enabled.
- Replace DHW sensor if faulty: If the boiler shows unrealistic DHW temperatures or sensor error, replace the Viessmann DHW temperature sensor with the specified part.
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Repair Difficulty
Hard
4/5
Required Part
Viessmann DHW temperature sensor
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