HVAC

HVAC (heating, ventilation, and air conditioning) is the group of related technologies that are combined as a unit or system to provide temperature regulation and air-quality control for buildings and vehicles. The technologies are often studied and taught as a group or in conjunction with refrigeration, which is closely related in design if differing in application. HVAC engineers in the United States, for instance, are usually members of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). HVAC engineering is a special application of mechanical engineering that is usually taught as a two-year associate’s degree program.

HVAC and its maintenance is fundamental to modern building design, as it is necessary to keep temperatures safe and comfortable for living and working and to preserve indoor air quality in order to replenish oxygen and remove other gases, built-up heat, dust, and airborne bacteria and molds.

Background

The HVAC systems of modern buildings and other enclosed spaces often integrate some of their functions. For example, private homes often use the same duct and ventilation system for ordinary ventilation as they do for heating and, if installed, for central air conditioning. This integration developed organically, as HVAC innovations developed side by side with one another and with modern architecture, in large part in response to the spread of factories and office buildings—both of which posed significant air-quality and temperature-regulation challenges—during and after the Industrial Revolution.

The subsequent rapid urbanization of the United States and other countries led to a construction boom that in turn drove demand for qualified HVAC engineers with expert knowledge in the increasingly improved science of heating and ventilation. ASHRAE was founded in 1894 in New York City, originally as the American Society of Heating and Ventilating Engineers; large-scale electrical air conditioning was not introduced until 1902 and was not incorporated into ASHRAE’s name for some decades. ASHRAE holds annual meetings and publishes the four-volume ASHRAE Handbook (the principal resource for HVAC engineers) as well as a series of standards and guidelines for HVAC systems. While these standards and guidelines have no legal standing themselves, they are referenced by so many building codes and government agencies that they have the de facto force of law and are almost universally consulted by architects and engineers. ASHRAE, which has supported energy efficiency and sustainability initiatives, also publishes books for engineers and several journals.

The Uniform Mechanical Code (UMC), developed by the American National Standards Institute (ANSI) and its consensus procedure, is the model code for the United States that covers the installation, inspection, and maintenance of HVAC systems. It is updated every three years, based on input from the public as well as expert testimony from the professional community, including mechanical engineers, plumbers, the building industry, public utilities, government agencies and regulatory authorities, the insurance industry, manufacturers, and research, standards, and testing laboratories.

While there is some overlap between the UMC and ASHRAE standards, the UMC focuses principally on installation and subsequent maintenance, while ASHRAE standards include vastly more detail on overall design practices that come into play long before the installation of the system.

Overview

Ventilation is one of the fundamental HVAC technologies. It involves changing or circulating air in an enclosed space, whether that means circulating the air within a building or exchanging indoor air with outdoor air. The purpose of ventilation is to avoid stagnation and improve air quality by replenishing oxygen (consumed by breathing) and removing unwanted elements. It also assists in temperature regulation. Ventilation can be accomplished through either forced air (mechanical ventilation) or natural ventilation.

Natural ventilation is the older method, relying on high windows, trickle vents, and other openings that allow warm air to rise and flow outside, which in turn draws cool outside air in through lower building openings. This process is more challenging in hotter weather and humid climates. While natural ventilation was first developed before electrical methods of forced-air ventilation were possible, it remains in use in many locations where doors open and close frequently or remain open most of the time, such as bars, nightclubs, and restaurant kitchens. The more crowded a space is expected to be, the greater the air-change rate (the rate at which the volume of air in the space is exchanged for new air by ventilation) needs to be. Crowded bars and clubs need an air change every other minute.

Forced air uses an air handler containing fans, filters, heating and/or cooling units, a humidifier and/or dehumidifier, and a mixing chamber. Air that circulates through the ventilation system enters the handler, where it is filtered, exchanged, heated or cooled, and humidified or dehumidified before being mixed back into the system. Rather than relying on temperature differences to create airflow as with natural ventilation, a fan forces air through the ventilation system at a controllable rate.

Heating can be generated with various fuels in order to heat the air that is conducted through the ventilation system, a common practice in residential HVAC systems. Alternately, because electrical resistance generates heat as a waste product, a space can be heated by conducting electricity through high-resistance wire in a portable heater or along baseboards. Given fuel prices, this is usually not an energy-efficient way to provide heating and is typically found only in warmer climates where heating is used for only a small portion of the year.

In commercial spaces, a heat pump is often used for heating. This technology extracts heat from groundwater, soil, or the air, depending on what is appropriate for the location, and transfers that heat to the system.

Air conditioning is the newest area of HVAC technology, having been introduced at the turn of the twentieth century. Building air-conditioning systems use refrigerants to remove heat from indoor air and an evaporator to dehumidify it. Air conditioning is more maintenance intensive than even heating systems are. Evaporator coils can become caked in ice if the air-conditioner unit runs for too long, especially in humid conditions, and systems are equipped with filters that require regular replacement or washing because of the accumulation of dust, dirt, and other particulate matter as air passes through. Therefore, in addition to the complicated electrical wiring involved with the unit, HVAC mechanics play an important role in the installation and maintenance of the system, ensuring that it functions properly and does not instead engender poor air quality.

Bibliography

Althouse, Andrew D., et al. Modern Refrigeration and Air Conditioning. 22nd ed., Goodheart-Willcox, 2025.

Auvil, Ronnie J. HVAC Control Systems. 4th ed., American Technical Publishers, 2017.

Bell, Arthur A., Jr., and W. Larsen Angel. HVAC Equations, Data, and Rules of Thumb. 3rd ed., McGraw Hill, 2015.

Kleinert, Eric. HVAC and Refrigeration Preventive Maintenance. McGraw Hill, 2014.

Nall, Daniel H. "Integrating HVAC & Life Safety." ASHRAE Journal, vol. 55, no. 12, 2013, pp. 48–51.

Sugarman, Samuel C. HVAC Fundamentals. 3rd ed., Fairmont Press, 2016.

Sugarman, Samuel C. Testing and Balancing HVAC Air and Water Systems. 5th ed., River Publishers, 2014.

Tomczyk, John. Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems. 2nd ed., ESCO Institute, 1995.