Navien NHB-055 Service Manual page 97

Check method
Fault
Defective safety device
Temperature sensor error
Other potential issues
Version 1.0
Possible Causes
Check if the contact point of the overheat controller is defective.
Defective overheat
controller
●
Check if the overheat controller wire is disconnected ( Normal resistance : 0.3Ω )
1. If the hot water temperature is sensed lower than it actually is due to a
Defective hot water
output temperature
sensor
2. Measure the temperature sensor resistance, and determine if the sensor is
1. If the Max switch #1 of 1-1 is on,
Capacity setting
2. PCB DIP S / W capacity setting error can suddenly increase the hot water
The surface temperature rises due to heavy scale deposits in the primary heat
Primary heat exchanger
exchanger.
overheated
●
Defective PCB If the issue continues despite checking the items above, the PCB is defective.
Use a multi-meter to see if the resistance is normal (0.3Ω) or abnormal (∞).
Overheat controller
defective sensor, check if the deviation of temperature is large due to a
defective temperature sensor.
Check the output temperature displayed on the front panel.
●
defective.
Set the switch to the normal operation position.
temperature.(refer to page 18)
Flush the primary heat exchanger.
Check method
NHB Service Manual 97