What does 'nF' flashing on the display mean?

The 'nF' flashing on the display is a reminder to replace the air filter, which appears after 2,880 hours of use. After 15 seconds, the display will revert to its previous state. To reset the reminder, press the LED button on the remote control 4 times or press the MANUAL CONTROL button 3 times. If not reset, the 'nF' indicator will flash again when the unit is restarted. (Page 37)