Navien NCB-150E Service Manual page 126

Fault
Boiler not working Defective flow sensor
Other trouble
Emergency measures for a defective flow sensor
Replacement of the part is usually required for the boiler to operate when an error occurs with the flow sensor. If it is impossible to replace the
part immediately, use the following emergency measures.
If the flow sensor impeller is stuck due to foreign substances:
Remove the flow sensor and blow air through the part from the inlet side. If the impeller does not rotate, disassemble the impeller, and
remove the foreign substance before reassembling it.
If the flow displayed on the front panel is 0.0, and the impeller rotates when you blow into it, the flow sensor will need to be replaced.
1. Drain water from the unit, then remove the flow sensor (refer to page 137).
2. Blow air through the flow sensor. Check that the impeller rotates freely.
Possible Causes
1. Check the current flow displayed on the front panel.
If the flow sensor is defective, the displayed flow value is 0.0 while water
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flows from the hot water tap.
Remove the flow sensor and blow into it. The rotation detector on the flow
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sensor is defective if the impeller rotates and display remains 0.0.
* Component Test mode may also be used to check the operation of the flow
sensor (refer to page 18).
2. Check connector assembly and contact of the flow sensor.
3. Check the input voltage of the flow sensor.
Operating voltage (Black + Red): Defective PCB if DC 12 V is not supplied.
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4. Replace the flow sensor if no flow is detected while the operating voltage is
normal.
Defective PCB If the issue continues despite the checking of items above the PCB is defective.
Check method
Troubleshooting 125