Navien NHB-055, Service Manual

Page: 114 / 147

114 NHB Service Manual Version 1.0
5.2.17 291Error
Error conditions and Check Items
Error Description
E291
Supply / Return inversion limit
If the return temperature is higher than “supply tempe 10°F (5.5°C)”, the system stops
operation and displays error code (E291) on the front panel.
Check items
1. Check whether the space heating pipe connection is inversed.
2. Check the supply temperature sensor connection.
3. Check the return temperature sensor connection.
Is the space heating pipe
connection normal?
Check the supply and return
temperature sensor
Replace space heating pipe
connection
No
Yes
Check method
Fault Possible Causes Check method
Space heating pipe
connection
Defective connection Check whether the space heating pipe connection is inversed.
Temperature sensor
1. Check the supply temperature sensor connection.
2. Check the return temperature sensor connection.