Wall-mounted split
Rinse the filter every couple of weeks under heavy use, and keep leaves and clutter clear of the outdoor body.
Ceiling cassette
Wipe the visible grille regularly and book a coil clean before smells or weak airflow show, as access only gets harder later.
Centralized and ducted
Read uneven room temperatures as an early sign, and have the ducting checked for damp ahead of the wet season.
Portable and window
Empty the tray often and keep the filter clear so the small build is not overworked in the heat.