Navien NCB-24LSWE, Service Manual

Page: 97 / 144

NCB Service Manual 95 Version 1.0
Testing method
Fault Possible Cause Testing method
Defective safety
device
Defective
overheat
controller
Check if the overheat controller’s contacts are faulty.

Use a multimeter to test the resistance. Normal resistance is 0.3Ω and a fault
condition would be infinity (∞).
Overheat controller
Check if the overheat control wiring is disconnected ( normal resistance: 0.3 Ω )
Temperature
sensor error
DHW output
temperature
sensor error
1. If the hot water temperature is measured at levels lower than it actually is, test to
confirm if the temperature sensor is faulty.

Check the output temperature displayed on the front panel.
2. Measure the temperature sensor resistance, and determine if the sensor is faulty.
Other potential
issues
Capacity setting
1. If the Max switch is ON, change the switch to the normal operating position.
2. PCB DIP switch capacity setting errors can result in sudden increases to DHW
temperature.
Primary heat
exchanger
overheat
The surface temperature rises due to heavy scale deposits in the primary heat
exchanger.

Flush the primary heat exchanger.
PCB fault
If the error condition continues after checking these items, replace the PCB.