In recent years the term air conditioning has come into common use in every city and town. Today it is no longer true, for air conditioning engineers, contractors, and technicians are modifying indoor weather in every part of the country. Our homes and business houses are warmed in winter, cooled in summer, and made comfortable the year around by the proper application of air conditioning principles and equipment. Development of the Industry. The development of the air conditioning industry is an interesting story, illustrating well the necessity for close working relationships among scientific, engineering, production, and sales personnel. The basic science of air conditioning has been well known for many years. The term HVAC has been applied to six important functions that are controlled in order to meet comfortable environmental requirements heating, cooling, humidification, dehumidification, air movement, and air cleaning. Although there has been consid¬erable effort to gather these terms together under the term air conditioning. That HVAC is the common expression in use today, while air conditioning generally applied only to the cooling function. The reason for the existence of any HVAC system is to provide control of a specific environment. The environment may be for humans at home, work or shopping center, be an industrial place which an industrial process takes place, enabling machinery to work smoothly or permitting plants to grow, a hospital, or a school, or a bank or any places . There is no specific way that we can put those in separate groups but the expres¬sion applications is in wide use in the following three different categories area: 1. Commercial applications refer to an HVAC system that controls the environment of stores, offices, banks, restau¬rants, and generally those places where people come and go, 2. Residential applications refer to the environmental control of homes, apartments, condominiums, and similar places where people live, sleep, and cook. 3. Industrial applications refer to controlling the environment for industrial processes, storage, computer operations, as¬sembly areas, and the like. The control that we would like to have over the HVAC system is depended to the function of the building.it is useful to think about the function of the building. The HVAC system of an uncertain building must be flexible because we do not know its final purpose or not, what would be the future use of the building. It must be economical because unpredictable investments are risky and the less invested the better. It must be include the great energy saving equipment to reduce the operating cost. But Buildings with a specific purpose of use, The HVAC system can be inflexible, since we know its usage exactly, yet it should also be flexible to accommodate change and the expansion of its intended activity. It should be simple enough to facilitate maintenance but sufficiently special to meet exacting requirements. It should minimize energy consumption, yet be inexpensive enough to meet the build¬ing construction budget.