The airhandler is the basic unit of operation of space conditioning. It is used to keep occupied spaces comfortable (Figure 11.1) or unoccupied spaces at desired levels of temperature and humidity. In addition to supplying or removing heat and/or humidity from the conditioned space, the airhandler also provides ventilation and fresh air makeup. Depending on the type of space involved, from 75,000 to 300,000 BTU/year (19,000 to 76,000 cal/year) are required to condition one square foot (0.092 m2) of office space. Depending on the energy sources used, this corresponds to a yearly operating cost of a few dollars per square foot of floor space.