Navien NFC Series User's Information Manual page 15

4. Remove the screw that secures the air intake filter (the filter is
located at the top left of the boiler).
5. Pull the filter out of the boiler.
14 Maintaining the Boiler
6. Remove the filter from the plastic assembly. Dust and rinse it
with clean running water.
7. Allow the filter to dry completely.
8. Place the filter assembly back into the boiler and secure it with
the screw.
9. Replace the front cover.
10. Reconnect the power supply to the boiler.
11. Press the Power button on the front panel to turn on the boiler.