Viessmann
DHW Temperature Fluctuation
Hot water temperature fluctuates between hot and cold during a shower
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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
Dirty or sticking DHW flow sensor, Scale in plate heat exchanger causing rapid overheating and cycling, Inadequate minimum flow rate, Thermostatic mixing valve malfunction on outlet
How to Fix / Troubleshooting
Safety: Turn off power and close water valves before removing sensors or valves.
- Stabilize flow: Avoid mixing hot and cold at the tap during testing. Open only the hot tap fully and observe temperature stability.
- Clean DHW flow sensor: Remove and clean the flow sensor turbine and inlet filter to ensure accurate flow measurement.
- Descale plate heat exchanger: Flush the plate exchanger with descaling solution to remove limescale that can cause rapid temperature swings.
- Check external mixing valve: If a thermostatic mixing valve is installed on the hot outlet, verify its operation and replace if it sticks or hunts.
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Repair Difficulty
Medium
3/5
Required Part
Thermostatic Mixing Valve
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