F17
Viessmann
DHW flow detection fault
Flow switch or flow sensor error on combi boiler, no hot water despite demand
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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 DHW flow switch, Blocked flow turbine sensor, Limescale buildup in DHW inlet, Faulty flow sensor wiring
How to Fix / Troubleshooting
Safety: Turn off power and close the cold water inlet before opening any DHW components.
- Check inlet filter: On many Viessmann combi units, there is a strainer on the cold water inlet. Close the inlet valve, remove the filter, and clean out debris.
- Inspect flow sensor: Locate the DHW flow sensor or turbine assembly. Check for visible scale or debris. Clean or replace as needed.
- Verify wiring: Ensure the sensor connector is firmly attached to the control board.
- Restore supply: Reassemble, open the cold inlet, check for leaks, then restore power and test hot water.
- Persistent F17: If the error remains, the flow sensor may need replacement by a technician.
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Repair Difficulty
Medium
3/5
Required Part
DHW Flow Sensor / Flow Switch
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