Full air conditioning with windows fixed closed and tempered air from ceiling diffusers is now not necessarily the only design choice for new commercial, deep-plan buildings in Europe, although it is still required for high heat gain buildings. It is also still very common in warmer and more humid climates in America, the Near East and the Far East. In Europe the alternative design choices are mixed-mode air conditioning or natural ventilation. Mixed-mode air conditioning is where window opening, thermal fabric storage and night ventilation are involved, in addition to air conditioning, to save energy and to reduce the use of refrigerants in the cooling system. There are several mixed-mode systems and several variations, but in a number of them the cooled air is delivered from floor or low-level diffusers and extracted at higher level due to displacement ventilation. The cool air warms up around heat sources, e.g. occupants and computers, then it rises. Chilled ceilings, with cool water flowing through pipes bonded to ceiling panels, provide radiant cooling and convective cooling. Chilled beams are water-to-air heat exchangers placed near or in the ceiling; they can be used near to, or above, windows for coping with solar gains in summer. These low-energy air-conditioning systems are discussed in Chapter 12; this chapter considers full air-conditioning systems and the basic control of traditional, conventional air-conditioning plant.