Navien NCB300-28K, Service Manual

Page: 69 / 108

68 Troubleshooting
Testing method
Fault Possible Causes 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: 0.3Ω)
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.