Navien NHB-055 Service Manual page 99

Check method
Fault
Heat exchanger
overheated
Defective part
Other trouble
Version 1.0
Possible Causes
1. The error occurs due to high exhaust gas temperature caused by a damaged or
Damaged or clogged
heat exchanger 2. Flush the heat exchanger to remove scale deposits.
3. Replace the heat exchanger if it is damaged or cannot be unclogged.
Defective contact point of the exhaust gas overheat controller 230°F (110°C) Max
Check connection of the overheat controller.
Defective temperature
●
sensor
If the resistance is abnormal, replace the temperature sensor (refer to page 67).
●
Check the output temperature displayed on the PCB.
●
Check if the heating water temperature sensor is open (Error type : MΩ Open)
Defective PCB If the issue continues despite checking the items above, replace the PCB.
clogged heat exchanger.
Check method
NHB Service Manual 99