Thermistor - Navien NCB-700-2S+/42K Service Manual

4.7.3 Thermistor

Item
Function
Fault
Symptoms
Error codes
Diagnostics
Testing/inspection
information
Temp (°C)
0-5
6-10
11-15
16-20
21-25
26-30
35-40
Measure hot and cold water temperature at the boiler's space heating outlet and inlet connections.
Inaccurate water temperatures from inside the boiler.
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.
E047, E205, E218, E407, E421
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.
Resistance range: Refer to the table below
Test for an open hot water temperature sensor (Test result: MΩ open)
Thermistor (kΩ)
[Space heating line]
23.4-19.1
18.3-15.5
14.8-12.6
12.1-10.7
10.3-8.5
8.2-7.0
6.8-4.9
Description
Wire colour: BLUE-BLUE
Inlet, Outlet
Exhaust limit temperature sensor
Wire colour: BLUE-BLUE
Thermistor (kΩ) Exhaust Limit Temperature
[DHW line]
27.5-22.2
21.2-18.0
17.3-14.7
14.2-12.1
11.6-10.0
9.6-8.3
8.0-6.9
Sensor (kΩ)
162.3-125.9
119.8-98.5
93.8-77.6
74.0-61.6
58.8-49.2
47.1-39.5
37.9-32.0
System Details 45