Navien NCB500-2S/37K Service Manual page 73

Testing method
Fault
Defective safety device
Temperature sensor error
Other potential issues
72 Troubleshooting
Possible Causes
Check if the overheat controller's contacts are faulty.
Defective overheat
●
controller
Check if the overheat control wiring is disconnected (normal resistance: 0.3Ω)
1. If the hot water temperature is measured at levels lower than it actually is,
DHW output temperature
sensor error
2. Measure the temperature sensor resistance, and determine if the sensor
1. If the Max switch is ON, change the switch to the normal operating
Capacity setting
2. PCB DIP switch capacity setting errors can result in sudden increases to
The surface temperature rises due to heavy scale deposits in the primary heat
Primary heat exchanger
exchanger.
overheat
●
PCB fault If the error condition continues after checking these items, replace the PCB.
Testing method
Use a multimeter to test the resistance. Normal resistance is 0.3Ω and a fault
condition would be infinity (∞).
Overheat controller
test to confirm if the temperature sensor is faulty.
Check the output temperature displayed on the front panel.
●
is faulty.
position.
DHW temperature.
Flush the primary heat exchanger.