Filters are in almost every air conditioner manufactured, and they are generally located upstream of the evaporator coil. The filters can be located in the air return grille or special duct slots. The primary reasons for filters are to clean the air being cooled and to protect the cooling equipment.
Without a filter, dust, lint, pollen and other airborne pollutants can clog the coils and cause the equipment to fail as well as circulate breathing irritants in the home. Filters must be cleaned or changed regularly. Clogged filters can put excess load on the system and drastically reduce energy efficiency. Filters vary in their construction and quality, so they are rated on a minimum efficiency reporting value scale from 1 to 20, with the higher number being the better filter.