Navien NHB-055, Service Manual

Page: 99 / 147

NHB Service Manual 99 Version 1.0
Check method
Fault Possible Causes Check method
Heat exchanger
overheated
Damaged or clogged
heat exchanger
1. The error occurs due to high exhaust gas temperature caused by a 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 part
Defective temperature
sensor
Defective contact point of the exhaust gas overheat controller 230°F (110°C) Max
●
Check connection of the overheat controller.
●
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)
Other trouble Defective PCB If the issue continues despite checking the items above, replace the PCB.