Glossary of HVAC Terms: What to Know

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You probably know what AC stands for, but that’s just one in a long list of acronyms, abbreviations, and terms that make up the spine of the HVAC lexicon. Indeed, the HVAC industry operates with its own language, one based on a shared understanding of the fundamentals of heating, cooling, and ventilation.

Even current HVAC professionals occasionally need a reference guide, as this is a living language. New environmental standards and new technologies come with new terms and new abbreviations. Certain HVAC specialties have their own dialects, as do different geographic regions and competing brands. But one person’s shorthand can be another person’s incomprehensible alphabet soup.

No HVAC glossary is ever complete, but below we’ve collected a glossary of terms that includes both the most common and the most tricky shorthand references in the industry. Whether you’re an HVAC student, a new HVAC professional, or even just a curious customer, read on to turn that alphabet soup into something more digestible.


AFUE: Acronym for Annual Fuel Utilization Efficiency, a ratio which calculates the percent of heat produced for every dollar of fuel consumed by a furnace or boiler. The higher the rating, the lower the cost. Any furnace with an AFUE of 90 or higher is considered high efficiency, while the federal minimum is 80.

AHRI: Acronym for Air Conditioning, Heating, and Refrigeration Institute, a trade association for HVAC specialists that also develops standards for measuring and certifying HVAC product performance.

AHU: Acronym for Air Handling Unit, the part of an HVAC system designed to regulate, circulate, and condition airflow; they’re typically connected to a ductwork ventilation system.

ASHRAE: Acronym for American Society of Heating, Refrigerating, and Air-Conditioning Engineers, a global society that sets standards for the built environment.


BEMS: Acronym for Building Energy Management System, a system that allows broad control over a large commercial building’s HVAC and energy tools — programming temperature, airflow, lighting, etc.

BTU: Acronym for British Thermal Unit, a measurement of heat energy. One BTU translates to the total heat required to raise a pound of water by one degree Fahrenheit.


CAV: Acronym for Constant Air Volume, a system that provides constant airflow. Most residential forced-air systems are small, simple CAV systems with constant supply flow, but the temperature varies.

CFC: Abbreviation for chlorofluorocarbons, ozone-depleting gasses once commonly used in refrigerants but now phased out in favor of hydrofluorocarbons (HFCs).

CFM: Acronym for cubic feet per minute, a measurement of airflow.

CO: Abbreviation for carbon monoxide, a dangerous gas whose lack of odor, color, and taste can make it difficult for homeowners to detect.


DOE: Acronym for the Department of Energy, a federal agency responsible for monitoring the consumption of energy sources and setting standards for HVAC products made in the US.


ENERGY STAR: A program of the Environmental Protection Agency, ENERGY STAR is designed to help customers save money by choosing energy-efficient products; products with an ENERGY STAR label meet US government guidelines for energy efficiency.


FAI: Acronym for Fresh Air Intake, an opening through which outside air is drawn into a building, either to replace air that has been exhausted through the ventilation system or to provide fresh air for fuel combustion.

FCU: Acronym for Fan Coil Unit, a device that heats or cools by moving air over a coil; its typical components are a heating or cooling coil, a fan, a valve or valves, and a local controller.

Forced Air System: An HVAC system that pushes temperature-controlled air through ductwork.


HEPA: Acronym for High-Efficiency Particulate Arrestance, an efficiency standard for air filters; in the US, HEPA filters must remove 99.97 percent of particles with a diameter 0.3 micrometers or greater.

HFCs: Abbreviation for hydrofluorocarbons, a class of manmade gasses commonly used in refrigeration and air conditioning; according to the EPA, they have a high global warming potential, and efforts are underway to reduce their usage.

HSPF: Acronym for Heating Seasonal Performance Factor, a measurement of the efficacy of heat pumps; the higher the rating, the more energy efficient it is.


IAQ: Acronym for Indoor Air Quality, a measurement of an indoor space’s air quality level; IAQ is typically denominated qualitatively rather than quantitatively.


MAU: Acronym for Makeup Air Unit, a type of air handler that conditions 100 percent outside air.

MERV: Acronym for Minimum Efficiency Reporting Value (MERV), a rating used to rank the efficiency of air filters; higher MERV ratings indicate smaller holes in the filter itself, and, therefore a more efficient filter.

Micron: A unit of measurement equal to one-millionth of a meter, or 1/25,000th of an inch; microns are a common measurement for tiny airborne particles.


NATE: Acronym for North American Technician Excellence, a form of certification available to HVAC technicians who have sufficient knowledge and experience


OAT: Acronym for Outside Air Temperature, a measurement that is used to determine whether outside air can be used for heating or cooling.


PAG: Abbreviation for polyalkylene glycol, a synthetic oil used in air conditioning systems to lubricate the compressor.

Package Unit: In HVAC, a package unit refers to having a heating and cooling system in a single outdoor unit.

Psychrometrics: The study of how atmospheric conditions, particularly how air-water vapor mixtures behave, which has important implications in energy transfer and HVAC system design.


R-22: The abbreviation for freon, an old standard for residential air conditioners that is now being phased out by the US; after 2020, only recycled or reclaimed R-22 is available.

R-410A: A chlorine-free refrigerant that meets the EPA’s newest and strictest environmental standards.


SEER: Acronym for Seasonal Energy Efficiency Ratio, a rating that takes a unit’s cooling output and divides it by the total energy input; the higher the SEER, the more energy efficient a unit is.

Split System: In HVAC, a split system typically includes an indoor unit, an outdoor unit, a thermostat, an air filter, and ductwork; split systems may be contrasted with package units.


TXV: Acronym for Thermostatic Expansion Valve, a device that regulates the flow of refrigerant through a system to control evaporator temperature.


UFAD: Acronym for Under Floor Air Distribution, a strategy for ventilation and conditioning of buildings through a supply plenum under a raised floor; such strategies are typically used in office buildings.


VAV: Acronym for Variable Air Volume, a descriptor for an HVAC system that varies its airflow rate in order to meet different temperatures; they are generally considered more energy efficient than constant air volume systems.

Matt Zbrog

Matt Zbrog

Matt Zbrog is a writer and researcher from Southern California. He's been living abroad since 2016. Long spells in Eastern Europe, Southeast Asia, and Latin America have made the global mindset a core tenet of his perspective. From conceptual art in Los Angeles, to NGO work on the front lines of Eastern Ukraine, to counterculture protests in the Southern Caucasus, Matt's writing subjects are all over the map, and so is he.

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