The class and the size of the building has a major effect on system design. Low-rise buildings often use multiple rooftop cooling-only units, cooling and heating units, or heat pumps serving the zones. They may also be designed as a unitary system with packaged terminal units or heat pumps, or a closed water loop heat pump system (also called a California heat pump design). Some buildings are also designed with separate variable air volume water-cooled units on each floor or zone, and a central condenser water loop and a cooling tower.

High rise and owner occupied buildings often use a central chilled and hot water distribution system to multiple room fan-coil units, or constant volume or VAV air handling units (one for each of the various zones). Other popular alternatives include the closed water loop heat pump system, as well as the geothermal heat pump system.

The special needs of data processing and other computer areas are often handled by a separate refrigeration and air side system.

In mild climates, a lower cost perimeter heating system with cooling only units is another popular alternative.

Separating the perimeter heating system from the cooling system has the advantage of selecting the cooling system air distribution devices for a specific duty, rather than a compromise between cooling and heating. Care should be taken to make sure these systems are energy-balanced -- that is, no s