HVAC terms, explained for beginners

Plain-English definitions from official sources · Last reviewed October 12, 2026

A

ACR (Air conditioning and refrigeration)

ACR is short for air conditioning and refrigeration. You'll see it most in Texas, where the Texas Department of Licensing and Regulation runs its HVAC licenses as the Air Conditioning and Refrigeration Contractors program and refers to "ACR companies." Texas law defines an air conditioning and refrigeration contractor as "a person who engages in air conditioning and refrigeration contracting." See Texas HVAC licensing. [3][4]

ACCA (Air Conditioning Contractors of America)

A nationwide nonprofit association for professionals who install and maintain HVACR (heating, ventilation, air conditioning and refrigeration) systems. It is also one of the organizations EPA (U.S. Environmental Protection Agency) has approved to give the EPA 608 test. [40][41]

AFUE (Annual fuel utilization efficiency)

The federal efficiency rating for furnaces and boilers, shown as a percent. For example, ENERGY STAR certified boilers need an AFUE of 87% or more for oil and 90% or more for gas. [24][28][29]

B

Boiler

A heating unit that heats water, or makes steam, for heating a building, instead of heating air. It can burn gas or oil or use electricity. Home boilers in federal rules supply low-pressure steam or hot water; heating with water in pipes is called hydronic heating. Efficiency is rated as AFUE. [24][28]

BTU (British thermal unit)

One BTU is the amount of heat needed to raise the temperature of one pound of water by 1 degree Fahrenheit. It's a small amount, so heating and cooling equipment is rated in BTUs per hour (see BTUH). It's also written Btu. [23]

BTUH (BTU per hour)

BTUs per hour, also written Btu/h. It measures how much heat a furnace or boiler puts out, or an air conditioner removes, each hour, which is how equipment size is stated. Federal energy rules use it too: their consumer-product rules cover central air conditioners rated below 65,000 Btu per hour. One ton of cooling equals 12,000 BTUH (British thermal units per hour). [24][25]

C

CFC (Chlorofluorocarbon)

CFCs are older refrigerants, such as CFC-12, that damage the ozone layer. EPA calls them Class I ozone-depleting substances, and the United States phased them out in the 1990s. [13][20]

Clean Air Act

The federal law that "defines EPA's responsibilities for protecting and improving the nation's air quality and the stratospheric ozone layer." HVAC technicians meet it through Section 608 (refrigerant handling and technician certification) and Section 609 (car air conditioning). [6][10][11]

D

Ductwork

Also called: duct, air ducts

The system of ducts that carries air from heating and cooling equipment to the rooms (supply), back to the equipment (return), and outside (exhaust). Michigan's licensing law defines it that way and includes flues, vents and chimneys. Ducts are often made of sheet metal, and installing them is a big part of the sheet metal worker trade. Michigan licenses ductwork as its own work classification. [1][14][31]

E

EPA (Environmental Protection Agency)

The EPA is the federal agency whose mission is "to protect human health and the environment." For HVAC workers it matters because it writes and enforces the refrigerant rules under the Clean Air Act, including the requirement that nearly all HVAC technicians be certified to handle refrigerant. The EPA doesn't give the test itself; approved organizations do. See EPA 608 certification. [2][5][6]

EPA 608 (EPA Section 608 certification)

EPA 608 (the U.S. Environmental Protection Agency's refrigerant rule) is the everyday name for the certification required by Section 608 of the Clean Air Act. Anyone who maintains, services, repairs or disposes of equipment that could release refrigerant into the air must be certified, by passing an EPA-approved test from an EPA-approved certifying organization. There are four types: Type I, Type II, Type III and Universal. The certification doesn't expire, and with limited exceptions only certified technicians can buy refrigerant. Registered apprentices may work without it only while closely and continually supervised by a certified technician, and only for two years from when they first registered. Full details: EPA 608 certification. [6][7][8][9]

F

Freon

A trademark name, often used loosely for refrigerant in general. EPA explains it has been used for several different refrigerants, including CFCs such as CFC-12 and HCFCs such as HCFC-22, often called R-22. [13]

Furnace

A heating unit that warms air and sends it through ducts to the rooms of a building. Federal rules describe gas or oil furnaces that "supply heat through a system of ducts with air as the heating medium," and electric furnaces that heat the air with electric elements and move it with a fan or blower. Efficiency is rated as AFUE. [24]

H

Heat pump

An electric system that both heats and cools a building. Instead of burning fuel, it moves heat: it pulls heat from outside into the home in winter and moves heat out of the home in summer. ENERGY STAR suggests thinking of a ducted heat pump as a central air conditioner that also works in reverse. Because it holds refrigerant, working on one takes EPA 608 certification. [6][30]

HVAC (Heating, ventilation and air conditioning)

HVAC stands for heating, ventilation and air conditioning. It means the equipment that controls the temperature, humidity and air quality inside homes, schools, hospitals and other buildings, and the trade that installs, maintains and repairs it. The U.S. Bureau of Labor Statistics calls these workers heating, air conditioning and refrigeration mechanics and installers, and notes they are "often called HVAC technicians." See how to become an HVAC technician. [1][2]

HVACR (Heating, ventilation, air conditioning and refrigeration)

HVACR is HVAC with an R for refrigeration added. Besides keeping people comfortable, refrigeration keeps things cold, for example the coolers and freezers in grocery stores, hotels and restaurants. The Bureau of Labor Statistics uses HVACR for the whole field. [2]

HVAC-R (HVACR, written with a hyphen)

Another way to write HVACR (heating, ventilation, air conditioning and refrigeration). It means the same thing. [2]

HVAC/R (HVACR, written with a slash)

Another way to write HVACR (heating, ventilation, air conditioning and refrigeration). It means the same thing. [2]

HVAC&R (HVACR, written with an ampersand)

Another way to write HVACR (heating, ventilation, air conditioning and refrigeration). It means the same thing. [2]

HCFC (Hydrochlorofluorocarbon)

HCFCs were developed as transitional replacements for CFCs. They still harm the ozone layer, less than CFCs, and EPA calls them Class II ozone-depleting substances. The best known is HCFC-22, or R-22. The United States must complete its HCFC phaseout in 2030. [21]

HFC (Hydrofluorocarbon)

HFCs replaced many ozone-depleting refrigerants. They don't harm the ozone layer, but most are potent greenhouse gases. EPA's refrigerant rules cover them as substitutes, so you still need EPA 608 certification to handle them. The American Innovation and Manufacturing (AIM) Act lets EPA phase down their production and use and push the move to newer refrigerants. For example, most new home air conditioners installed since 2025 can't use R-410A. [8][13][17][18]

Hydronic heating

Also called: hydronic, hydronics

Heating or cooling a building by circulating water, other fluids or steam through pipes, rather than blowing air through ducts. Michigan's licensing law calls it "the controlled forced circulation of fluids or vapors in pipes." A boiler is a common heat source. Some states license it separately: Ohio, for example, has a hydronics contractor license alongside its HVAC license. [31][32]

I

SMART (International Association of Sheet Metal, Air, Rail and Transportation Workers)

SMART is a union whose members include sheet metal workers and service technicians, as well as rail and bus workers. It says it has 230,000 members. [38]

IFGC (International Fuel Gas Code)

A model code from the International Code Council that applies to the installation of fuel-gas piping systems and fuel gas appliances. HVAC workers meet it because gas furnaces, boilers and their venting fall under it rather than the IMC. [15][16]

IMC (International Mechanical Code)

A model code published by the International Code Council (ICC). It regulates the design, installation, maintenance, alteration and inspection of mechanical systems that are permanently installed to control conditions inside buildings. Gas piping and gas appliances are covered by a companion code, the IFGC. [15]

M

Mechanical contractor

Also called: mechanical license, mechanical contractor license

In building codes and licensing, "mechanical" usually means heating, cooling and ventilation systems and related piping, rather than machines. Some states license HVAC work as mechanical work. In Michigan, for example, licensed mechanical contractors "can perform installations, alterations, servicing, and secure permits," and license by work classification, such as HVAC equipment, ductwork, refrigeration, and hydronic heating and cooling and process piping. Check your state's page under HVAC licensing. [14][15]

Montreal Protocol

An international treaty that is gradually ending the production and use of ozone-depleting substances. The United States signed it in 1987, and it has been signed by 197 countries. Under it and the Clean Air Act, the United States phased out CFCs in the 1990s and is phasing out HCFCs. [12][13]

N

NATE (North American Technician Excellence)

A nonprofit certification organization, founded in 1997, for heating, ventilation, air conditioning and refrigeration technicians. NATE certification is an industry credential, not a government license: the Bureau of Labor Statistics notes that many such certifications are optional, though some employers prefer or require them. [2][42]

O

Ozone depletion

Also called: ozone-depleting, ozone layer

The ozone layer, 15 to 40 kilometers up, absorbs much of the sun's harmful ultraviolet (UVB) light. Chemicals such as CFCs and HCFCs, once common refrigerants, rise to it and break it down, which raises the risk of skin cancer and cataracts. This is why the Montreal Protocol and the Clean Air Act phase them out and why releasing refrigerant is restricted. [13][20][22]

P

PTAC (Packaged terminal air conditioner)

A heating and cooling unit built to mount through an outside wall, in a wall sleeve. Federal energy rules define it that way; a PTAC heat pump heats mainly by running its cooling system in reverse. EPA counts packaged terminal heat pumps among small appliances. [8][33]

Pipefitter

Also called: steamfitter, pipe fitter, steam fitter, pipefitters and steamfitters

Pipefitters and steamfitters, often just called fitters, install and maintain pipes that may carry chemicals, acids and gases, mostly in commercial and industrial buildings, power plants, and the heating and cooling systems of large buildings. Steamfitters specialize in liquids or gases at high pressure. Most learn through a 4- or 5-year apprenticeship. In the union sector, fitters belong to the United Association (UA), which also represents HVACR service technicians. [36][37]

Process piping

Piping that isn't part of a building's air conditioning or refrigeration system: for example, pipes that move chemicals and other fluids, gases or vapors in industrial settings. Michigan's licensing law defines it this way, and pipefitters often do this work. [35][36]

R

R-22

Also called: HCFC-22

R-22 (HCFC-22) is an HCFC refrigerant used in older home air conditioners. Because it damages the ozone layer, no new HCFC-22 has been made or imported into the United States since January 1, 2020. Existing R-22 systems can still be serviced, using refrigerant that has been recovered and reclaimed or made before 2020. [13]

R-410A

A refrigerant made from a blend of two HFCs. It became the most common replacement for R-22 in home air conditioners; brand names include Puron. It doesn't deplete the ozone layer, but EPA notes most HFCs are potent greenhouse gases, so it must be handled under the same refrigerant rules. Under a newer federal law, the American Innovation and Manufacturing Act, new home and light commercial air conditioning and heat pump systems installed from January 1, 2025, must use a refrigerant whose global warming potential (EPA's measure of how strongly a gas warms the climate) is within a limit of 700, which rules out R-410A. Systems whose main parts were all made or imported before that date can still be installed, and existing R-410A systems can still be repaired with R-410A parts. [13][17][18][19]

Reclaimed refrigerant

Also called: reclaiming, reclamation

To reclaim refrigerant means to reprocess recovered refrigerant until it meets a set purity standard listed in EPA's rules and test it to prove it. Reclaimed R-22, along with R-22 made before 2020, keeps older home air conditioners running now that no new R-22 is made. [8][13]

Refrigerant

The fluid inside air conditioners, heat pumps and refrigerators that carries heat out of the space being cooled. EPA's rules define it as a substance used "for heat transfer purposes" that "provides a cooling effect." Many refrigerants harm the ozone layer or the climate if released, so EPA requires EPA 608 certification to handle them, and the Bureau of Labor Statistics notes that refrigerants are hazardous and call for safety gear. [2][6][8]

Refrigerant recovery

Also called: recover refrigerant, recovering refrigerant, recovery and recycling

To recover refrigerant means to remove it from equipment "in any condition" and store it in an outside container, without necessarily testing or cleaning it. Technicians do this before opening up a system so refrigerant doesn't escape. The Bureau of Labor Statistics notes that HVAC technicians must follow rules on the recovery and recycling of refrigerants. [2][8]

Refrigerant recycling

Also called: recycled refrigerant, recycle refrigerant

To recycle refrigerant means to remove it from equipment and clean it for reuse in equipment of the same owner, without meeting the full purity standard for reclaimed refrigerant. It is usually cleaned by separating out oil and passing it through filter-driers. [8]

S

SEER (Seasonal energy efficiency ratio)

A rating of how efficiently an air conditioner or heat pump cools over a whole cooling season: the heat it removes, in BTUs, divided by the electricity it uses, in watt-hours. A higher number means it uses less electricity for the same cooling. Since January 1, 2023, the Department of Energy uses a new test, and ratings are given as SEER2. [26][27]

SEER2 (Seasonal energy efficiency ratio 2)

The version of SEER measured under the Department of Energy's updated test procedure, in effect since January 1, 2023. Like SEER, it divides the heat removed over a cooling season by the electricity used. The new name marks the new test. Heat pump heating ratings changed the same way. [26][27]

Section 608

Also called: Section 608 certification

Short for Section 608 of the Clean Air Act, the law behind the EPA's refrigerant rules. "Section 608 certification" is the same thing as EPA 608 certification: the credential you need to work on equipment that holds refrigerant. Car air conditioning falls under a different rule, Section 609. [6][10]

EPA Section 608 (Section 608 of the Clean Air Act)

Section 608 is the part of the Clean Air Act under which the EPA regulates refrigerants in stationary equipment such as home and commercial air conditioners and refrigeration. Its rules (40 CFR Part 82, Subpart F) require technician certification, set the four certification types and limit who can buy refrigerant. "EPA Section 608," "Section 608" and "EPA 608" all point to the same requirement. See EPA 608 certification. [6][9]

Section 609

Also called: Section 609 certification

The EPA certification for anyone who repairs or services a motor vehicle's air conditioning for pay. It comes from an EPA-approved training and certification program. Section 609 technicians can't buy refrigerant meant for stationary equipment such as home and building systems; that takes Section 608 certification. [9][10]

SMACNA (Sheet Metal and Air Conditioning Contractors' National Association)

An international trade association representing about 3,500 signatory contracting firms in the sheet metal and air conditioning industry, with more than 100 chapters. It works on industry education, labor relations and standards development. [39]

Sheet metal worker

Sheet metal workers make or install products from thin metal sheets. In construction, many install HVAC ducts; others install metal roofs, siding and gutters, repair ductwork, or do testing and balancing. Most construction sheet metal workers learn through a 4- or 5-year apprenticeship. SMART is the union whose members include sheet metal workers. [1][38]

Small appliance

An EPA term for equipment that is fully built, charged and sealed at the factory with 5 pounds or less of refrigerant. Examples include refrigerators and freezers, window and portable air conditioners, packaged terminal air heat pumps, dehumidifiers, ice makers, vending machines and water coolers. Servicing them takes Type I (or Universal) certification. In EPA's rules, "appliance" means any device that uses refrigerant, including any air conditioner, refrigerator, chiller or freezer. [7][8]

T

TAB (Testing, adjusting and balancing)

Specialized work that checks a finished HVAC system and adjusts it so it heats and cools rooms properly, for example by adjusting ducts to get the right airflow. The Bureau of Labor Statistics lists testing and balancing as a specialty of both HVAC technicians and sheet metal workers. The Testing, Adjusting and Balancing Bureau (TABB) is one certification program for it. [1][2][34]

Ton of cooling

Also called: tons of cooling, tons cooling, ton of refrigeration

Air conditioners and heat pumps are sized in tons. One ton of refrigeration equals 12,000 BTU per hour of cooling, so a 5-ton system removes 60,000 BTU of heat an hour. It isn't the unit's weight. [25]

Type I

The EPA 608 certification type for people who maintain, service or repair small appliances, such as refrigerators, window and portable air conditioners, dehumidifiers and vending machines. [7][8]

Type II

The EPA 608 certification type for people who maintain, service, repair or dispose of medium-, high- or very high-pressure appliances, other than small appliances and car air conditioning. "Pressure" here refers to the refrigerant the equipment uses: EPA's examples of high-pressure appliances include those using R-22 and R-410A, common home air conditioning refrigerants. [7][8]

Type III

The EPA 608 certification type for people who maintain, service, repair or dispose of low-pressure appliances. EPA defines these by the refrigerant they use; its examples include equipment using R-11, R-123, R-113 and R-245fa. [7][8]

U

Universal certification

Also called: 608 Universal, EPA Universal

The EPA 608 certification that covers all three types (I, II and III), so you can work on any stationary refrigerant equipment. EPA notes that a core test taken open book can't be used toward Universal; the core must be taken as a proctored exam. [6][7]