First, air returns back to the packaged rooftop unit from the space the unit is ventilating. Some units also mix outdoor air with the returning air to produce fresher air for the space. The air then moves through a rack of filters. Then it continues to the cooling coil which chills air by running it over refrigerant. The refrigerant is cooled with a standard refrigerant cycle. The air then flows through a blower, leaving it conditioned and ready for the space. There is usually a clear, insulated divide between the evaporator coils that cool the return air, and the condenser coils and compressor that eject heat to the atmosphere. A basic thermostat typically controls packaged rooftop units. However, depending on the application they can be a part of a more complex control system. There are also some specialized applications for packaged rooftop units. For example, some units condition 100 percent outdoor air. So all the air running through the unit comes from outside, and the unit uses no return air. These units are known as 100 percent dedicated outdoor air units, or makeup air units. They require increased insulation, additional parts, and potentially more energy to run. Their obvious advantage is that they circulate fresher air than normal packaged rooftop units.