More than half of all commercial floor space is cooled by self-contained, packaged air-conditioning (AC) units. Most of those are rooftop units (RTUs). Also called unitary air conditioners, these mass-produced machines include cooling equipment, air-handling fans, and sometimes gas or electric heating elements. Condensing units are also very common and the only difference is the cooling coil or evaporator is not contained within the unit. Condensing units often reside on the rooftop and the cooling coil or evaporator resides in some sort of air handling unit within the facility.