Vaillant
Inconsistent DHW Temperature Control
Domestic hot water temperature fluctuates or runs hot then cold on Vaillant combi boiler
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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
Partially blocked plate heat exchanger, Faulty DHW NTC sensor, Modulating gas valve not responding correctly, Flow regulator or turbine sensor issues
How to Fix / Troubleshooting
Safety: Risk of scalding. Reduce hot water temperature setting and test carefully.
User checks:
- Ensure cold water inlet stopcock is fully open.
- Avoid running multiple hot taps simultaneously to see if stability improves.
Professional checks:
- Inspect and clean or replace the plate heat exchanger if restricted by scale or sludge.
- Test the DHW NTC sensor and replace if readings are incorrect.
- Check the flow sensor/turbine for debris and correct operation.
- Verify gas valve modulation and combustion settings.
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Repair Difficulty
Hard
4/5
Required Part
Plate Heat Exchanger
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