Vaillant
Intermittent DHW Temperature / Flow Modulation Fault
Water heater delivers hot water initially, then temperature quickly drops to lukewarm or cold
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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 increasing output, Flow sensor misreading flow rate
How to Fix / Troubleshooting
Safety: Turn off power and gas before internal work.
- Check flow rate: Ensure the flow rate is within the appliance’s specified range. Excessive flow can cause temperature drop; partially close the tap to see if temperature stabilises.
- Descale plate heat exchanger: Remove and chemically descale the DHW plate heat exchanger to restore heat transfer.
- Test DHW NTC: Compare sensor readings to actual outlet temperature. Replace the DHW NTC if readings are inaccurate or unstable.
- Check gas modulation (engineer): Verify that the gas valve increases output as demand rises. Replace or adjust as per Vaillant procedures if modulation is faulty.
- Inspect flow sensor: Clean or replace if it is under-reporting flow, causing incorrect modulation.
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Repair Difficulty
Hard
4/5
Required Part
DHW Plate Heat Exchanger
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