Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. HVAC (pronounced either H-V-A-C or aetch-vak) is an acronym that stands for the closely related functions of Heating, Ventilating, and Air Conditioning- the technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the field's abbreviation as HVAC & R or HVACR, or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is particularly important in the design of medium to large industrial and office buildings such as skyscrapers and in marine environments such as aquariums, where safe and healthy building conditions are regulated with temperature and humidity, as well as fresh air from outdoors.