Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air. Ventilating or ventilation (the 'V' in HVAC) is the process of exchanging or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, carbon dioxide, and other gases. HVAC is an important part of residential structures such as single family homes, apartment buildings, hotels, and senior living facilities medium to large industrial and office buildings such as skyscrapers and hospitals vehicles such as cars, trains, airplanes, ships and submarines and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. ' Refrigeration' is sometimes added to the field's abbreviation as HVAC&R or HVACR, or 'ventilation' is dropped, as in HACR (as in the designation of HACR-rated circuit breakers). HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Its goal is to provide thermal comfort and acceptable indoor air quality. Heating, ventilation, and air conditioning ( HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. The safety interlock switch is at the bottom left. The fan enclosure is directly behind the board, and the filters can be seen at the top. The wires connecting to the blue terminal block on the upper-right of the board lead to the thermostat. The control circuit in a household HVAC installation. In the middle is a damper to open and close the vent to allow more or less air to enter the space. These are installed throughout a building to move air in or out of rooms. Rooftop HVAC unit with view of fresh-air intake vent Ventilation duct with outlet diffuser vent. HVAC may also refer to high-voltage alternating current.

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