Viessmann
DHW temperature control issue
Domestic hot water temperature fluctuates or is too hot/cold on a Viessmann combi or indirect 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
Scaled plate heat exchanger, Faulty DHW NTC sensor, Incorrect flow rate through boiler, Mixing valve malfunction on cylinder
How to Fix / Troubleshooting
Safety: Risk of scalding. Test hot water carefully and adjust controls to safe temperatures.
- Check DHW setpoint: On the Viessmann control, verify the DHW temperature setting is appropriate (typically 50–60°C).
- Stabilize flow: Avoid opening multiple hot taps simultaneously. Maintain a steady flow at one outlet and observe temperature stability.
- Inspect mixing valve: On cylinder systems, check the thermostatic mixing valve for correct setting and free movement. Replace if seized.
- Professional descaling: A technician can descale or replace the plate heat exchanger and verify DHW sensor operation.
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Repair Difficulty
Medium
3/5
Required Part
Plate Heat Exchanger
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