Viessmann
No Hot Water Without Fault Code
Water heater does not produce any hot water, display is on and no error code is shown
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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
DHW flow sensor not detecting low flow, Partially blocked inlet filter, DHW setpoint temperature too low, 3-way diverter valve stuck in heating position (on combi models)
How to Fix / Troubleshooting
Safety: Turn off power and close water isolation valves before removing filters or valves.
- Check hot water demand: Ensure a tap is fully opened. Some Viessmann flow sensors require a minimum flow rate to activate.
- Verify setpoint: On the control panel, confirm that the DHW temperature setpoint is not set too low or in vacation/eco mode.
- Clean inlet filter: Remove and clean the cold water inlet filter at the hydraulic block to restore flow.
- Check diverter valve (if applicable): On combi units, observe whether the diverter motor moves when hot water is requested. If the valve is stuck, it may need cleaning or replacement.
- Test again: Restore power and water, then open a hot tap and see if the burner starts and hot water is produced.
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Repair Difficulty
Medium
3/5
Required Part
DHW Flow Sensor
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