An air handling unit ( AHU ) conditions and circulates air for the purpose of maintaining suitable indoor air quality for people and equipment. “Two-stage” refers to simple binary (ON/OFF) heating or cooling in the form of resistive electric elements or direct expansion (DX) cooling with a closed-system refrigerant. An AHU controller regulates all the different aspects of the AHU system, with the goal of reducing energy expenses while maintaining a safe and comfortable living or working environment. The main functions of an AHU include: heating/cooling the air to a desired temperature, mixing and filtering air to ensure proper air quality, providing enough air pressure for adequate circulation, dehumidification, and sometimes humidification (not to be discussed here).

.