Vaillant
DHW Temperature Instability
Water heater temperature fluctuates between very 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
Faulty DHW NTC sensor, Flow sensor signal instability, Scale build-up causing intermittent restriction, Incorrect minimum flow setting or low inlet pressure
How to Fix / Troubleshooting
Safety: Beware of scalding when testing hot water. Isolate power and gas before internal checks.
- Check inlet pressure: Measure cold water inlet pressure; ensure it meets Vaillant’s minimum requirement. Install a pressure regulator if pressure is highly variable.
- Clean filters and aerators: Remove and clean tap/shower aerators and the appliance inlet filter to ensure stable flow.
- Test DHW NTC and flow sensor: Replace any sensor with erratic readings.
- Descale DHW circuit: In hard water areas, descale the plate heat exchanger and internal DHW passages.
- Verify appliance settings: Ensure DHW temperature setpoint is appropriate and that any eco/comfort modes are configured correctly.
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Repair Difficulty
Medium
3/5
Required Part
DHW Temperature NTC Sensor
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