Navien NCB-24LSWE, Service Manual

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62 NCB Service Manual Version 1.0
4.9.3 Thermistor
Item Description
Function
Measure hot and cold water temperature at the boiler’s space heating outlet and inlet
connections.
Fault
Inaccurate water temperatures from inside the boiler.
Symptoms
1. If a thermistor fails, an error code is displayed before the boiler operates.
2. If resistance values are outside the allowable range, there will be hot water temperature
fluctuations.
Error codes
E047, E205, E218, E407, E421
Diagnostics
1. Visual inspection: Check wire connections are secure and inspect for wire damage.
2. Test the resistance of the sensor. Before testing, shut down the boiler and allow it to cool.
Testing/inspection
information
Resistance range: Refer to the table below
Test for an open hot water temperature sensor (Test result: MΩ open)
Temp (
℃
)
Thermistor (kΩ)
[Space heating line]
Thermistor (kΩ)
[DHW line]
Exhaust Limit Temperature
Sensor (kΩ)
0-5 23.4-19.1 27.5-22.2 162.3-125.9
6-10 18.3-15.5 21.2-18.0 119.8-98.5
11-15 14.8-12.6 17.3-14.7 93.8-77.6
16-20 12.1-10.7 14.2-12.1 74.0-61.6
21-25 10.3-8.5 11.6-10.0 58.8-49.2
26-30 8.2-7.0 9.6-8.3 47.1-39.5
35-40 6.8-4.9 8.0-6.9 37.9-32.0