Vaillant
No DHW Production Without Fault Code
Water heater produces no hot water at any tap, but display shows no error code
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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 flow sensor not detecting demand, Stuck diverter valve in heating position (on combi-type units), Blocked plate heat exchanger, DHW temperature sensor failure
How to Fix / Troubleshooting
Safety: Isolate power and gas before removing covers. Water may be hot; take care when opening hydraulic components.
- Check demand recognition: Open a hot tap fully and observe the display. If the burner does not attempt to start and flow rate is not indicated, suspect the DHW flow sensor.
- Inspect and clean flow sensor: Remove the flow sensor/turbine and clean any debris or scale. Ensure it spins freely. Replace if worn or damaged.
- Check diverter valve (if fitted): On combination units, observe whether the diverter motor moves when hot water is demanded. If stuck in heating position, replace or service the diverter valve assembly.
- Inspect plate heat exchanger: If the burner runs but water remains cold or only lukewarm, remove and descale the DHW plate heat exchanger.
- Test DHW NTC: Measure the DHW temperature sensor resistance and compare to spec. Replace if faulty.
- Reassemble and test: Restore power and gas, then test hot water at several outlets.
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Repair Difficulty
Hard
4/5
Required Part
DHW Flow Sensor
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