<P> Heating, ventilation, and air conditioning (HVAC) is the technology of indoor and vehicular environmental comfort . Its goal is to provide thermal comfort and acceptable indoor air quality . HVAC system design is a 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 (as in the designation of HACR - rated circuit breakers). </P> <P> 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, on 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 . </P>

What does the r in hvacr systems refer to