heating, ventilation, and air conditioning

Diagram showing main components of an HVAC system Rooftop HVAC unit with view of fresh-air intake vent Ventilation duct with outlet diffuser vent. These are installed throughout a building to move air in or out of rooms. In the middle is a damper to open and close the vent to allow more or less air to enter the space. The control circuit in a household HVAC installation. The wires connecting to the blue terminal block on the upper-right of the board lead to the thermostat. The fan enclosure is directly behind the board, and the filters can be seen at the top. The safety interlock switch is at the bottom left. In the lower middle is the capacitor. Heating, ventilation, and air conditioning (HVAC /ˈeɪtʃˌvæk/) systems regulate temperature, humidity, and indoor air quality in residential, commercial, and industrial buildings, and in enclosed vehicles. They are designed to provide thermal comfort and to control airborne contaminants through heating, cooling, ventilation, filtration, and humidity control. HVAC system design is a sub-discipline of mechanical engineering based on the principles of thermodynamics, fluid mechanics, and heat transfer. Design considerations include energy efficiency, indoor air quality, maintenance, and environmental impact, particularly in green building projects. In building design, mechanical, electrical, and plumbing engineers may integrate HVAC systems with other building systems and use energy modeling to evaluate performance and operating costs. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR, while ventilation is sometimes omitted, as in HACR.

Ventilation (the "V" in HVAC) is the process of exchanging or replacing air in any space to provide higher air quality. This involves temperature regulation, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, carbon dioxide, and other gases. Ventilation removes unpleasant smells and excessive moisture, introduces outside air, and keeps interior air circulating. Building ventilation methods are generally classified as mechanical (forced) ventilation or natural ventilation.