There are six types of air conditioning (AC) units: central AC, ductless AC, window units, portable units, hybrid systems, and geothermal systems. PriceMyAC.com specializes central AC systems, ductless systems, and hybrid systems. In the Phoenix area, a centralized AC system is the most common. A centralized system combines the evaporator, condenser, and compressor in a single unit that is placed on a roof or a concrete slab near the house/building foundation.