An air handling unit is a huge indoor unit of commercial air conditioning equipment. This massive “indoor unit” is installed on a rooftop or along an external building wall. It produces cool/warm air and sends it to the rooms for cooling/heating. Unlike ordinary commercial air conditioners, air handling units are usually made to order for individual buildings. Furthermore, air handling units can be customized by internally mounting a filter, humidifier and other devices for the control of temperature, humidity, airflow and air cleanliness.