F11
Viessmann
Flow Sensor / DHW Sensor Fault
Boiler intermittently shows F11, especially during hot water 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
Defective domestic hot water (DHW) NTC sensor, scaled plate heat exchanger causing abnormal temperatures, wiring fault to DHW sensor
How to Fix / Troubleshooting
Safety: Hot water components can be very hot. Allow to cool before handling.
- Check DHW temperature setting: Ensure the setpoint is within normal range (e.g., 50–60°C). Extremely high settings can stress sensors.
- Inspect DHW NTC: Locate the Viessmann DHW sensor on the plate heat exchanger or cylinder coil. Check for secure connection and no visible damage.
- Descale plate heat exchanger (technician): If hot water fluctuates and F11 appears, the plate heat exchanger may be scaled and require chemical descaling or replacement.
- Replace DHW sensor: If wiring is sound, replacing the DHW NTC is often straightforward and resolves intermittent F11.
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Repair Difficulty
Medium
3/5
Required Part
DHW Temperature Sensor (NTC)
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