The units are designed for ease of maintenance. They usually require a filter change or cleaning at least once a year. Where dust is a problem, such maintenance should be more frequent. At this time, the condenser coil should be brushed with a soft brush and flushed with water. The filters should be vacuumed and then washed to remove dust. The outside of the case should be wiped clean with a soapy cloth. Needless to say, the cleaner the filter, the more efficient the unit.