Viessmann
DHW Priority / Diverter Valve Fault
No domestic hot water but heating works normally (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
Stuck diverter valve, failed DHW flow sensor, scaled plate heat exchanger, faulty microswitch on diverter
How to Fix / Troubleshooting
Safety: Hot surfaces and water can cause burns; allow components to cool.
- Check tap flow: Ensure adequate water flow at hot taps; very low flow may not trigger DHW demand.
- Observe boiler: When a hot tap is opened, the boiler should indicate DHW demand. If not, the flow sensor or microswitch may be faulty.
- Diverter valve movement: A technician can check if the Viessmann diverter valve motor moves between heating and DHW positions.
- Descale plate heat exchanger: If the boiler fires but water remains lukewarm, the plate heat exchanger may be scaled and require descaling or replacement.
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Repair Difficulty
Hard
4/5
Required Part
Diverter Valve Assembly
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