H17
Bosch DHW Flow Detection Fault

Boiler indicates a DHW flow sensor or flow switch fault; hot water may not start when a tap is opened.

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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 or scaled DHW flow sensor/turbine, Blocked inlet filter on cold water, Faulty DHW flow switch or Hall sensor, Wiring fault between flow sensor and PCB

How to Fix / Troubleshooting

Safety: Turn off power and isolate cold water supply to the boiler before removing any water components.

Steps:

  • Check inlet filter: Remove and clean the cold water inlet filter/strainer if accessible.
  • Inspect DHW flow sensor: On combi models, locate the DHW flow sensor or turbine assembly. Remove and check for debris or scale; clean or replace as needed.
  • Check wiring: Ensure the sensor connector is firmly seated on the PCB.

After reassembly, restore water, check for leaks, then restore power and test hot water demand. If still faulty, the sensor or PCB may require professional replacement.

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Repair Difficulty

Medium 3/5

Required Part

DHW Flow Sensor / Turbine
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