Common HVAC Terms

Ever wonder what a condenser coil or compressor does? Or what “plenum space” means? Here’s a glossary of terms that will help you increase your knowledge of your HVAC system so you’re better equipped to ask questions and even diagnose problems yourself.


Advanced Reciprocating Compressor

Type of compressor that uses a more efficient process for compressing refrigerant for better cooling efficiency.

Annualized Fuel Utilization Efficiency (AFUE)

Annual Fuel Utilization Efficiency. Indicated as a percentage, your furnace’s AFUE tells you how much energy is being converted to heat. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other 10% escapes as exhaust with the combustion gases. The higher the AFUE, the more efficient the furnace.

AHRI (Air Conditioning, Heating and Refrigeration Institute)

AHRI is a trade association that develops standards for measuring and certifying product performance. AHRI standards and guidelines are used throughout the world. Equipment with the AHRI Certified® mark has AHRI’s assurance that it will perform accurately and consistently.

Air changes per hour

The hourly ventilation rate divided by the volume of a space. For perfectly mixed air or laminar flow spaces, this is equal to the number of times per hour that the volume of the space is exchanged by mechanical and natural ventilation. Also called air change rate or air exchange rate. Abbreviated ACH or ac/hr.

Air conditioner

An appliance, system, or mechanism designed to dehumidify and extract heat from an area.

Air Handler

The portion of your air conditioner or heating system that forces air through your home’s ductwork. Abbreviated AH or AHU.

Air Purifier

A device that purifies 100% of the air flowing through your HVAC system before it even circulates, removing particulates, bacteria and viruses from the air.


The distribution or movement of air.


Annual Operating Hours is the total ration of full- and part-load operating hours in a geographical area.


Beckett Burner

A burner assembly within your oil furnace that provides combustion for heating oil.


A British Thermal Unit is a unit of heat energy. One BTU is the amount of heat required to raise one pound of water by one degree Fahrenheit. The higher the BTU rating, the greater the heating capacity of the system.



The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

Carbon Monoxide (CO)

Carbon monoxide (sometimes referred to by its chemical abbreviation, CO) is an odorless, colorless, tasteless and poisonous gas produced when fuels such as natural gas burn with insufficient air. When carbon monoxide leaks into your home’s air, it can lead to serious health problems for your family. Regular maintenance of your home’s heating and ventilation system will help ensure it’s not a source of a carbon monoxide leak.

Carbon Monoxide Alarm

A device that reads and detects levels of carbon monoxide in your home. When unsafe levels of CO are present, a loud, high-pitched alarm will sound to alert you.

Central Air Conditioning

Central air conditioning (or central AC) is a system in which air is cooled at a central location and distributed to and from rooms by one or more fans and ductwork.

Centrifugal fan

A mechanical device for moving air or other gases.

Cubic Feet Per Minute (CFM)

Cubic Feet per Minute is a measurement that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air being forced through the ductwork by the system.


A device that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan—coil units, or other systems, cooling and usually dehumidifying the air in the building. There are two types of chillers: air-cooled and water-cooled. Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air. Water-cooled chillers are usually inside a building, and heat is carried by recirculating water to a heat sink such as an outdoor cooling tower.


There are typically two HVAC coils in a system—the condenser coil in the outside air conditioning condenser unit and the evaporator coil in the indoor unit. The coils increase or decrease the temperature via heat transfer.

Combined Annual Efficiency (CAE)

A measure of the amount of heat produced for every dollar of fuel consumed for both home and water heating.


Part of the outdoor air conditioner or heat pump unit that controls the pressure applied to the refrigerant, necessary for taking in heat to warm your home or removing heat to keep your home cool.

Condenser Coil

Part of the outdoor portion of a split-system air conditioner or heat pump. By converting refrigerant that is in a gas form back to a liquid, the coil sends heat carried by the refrigerant to the outside. Also referred to as an outdoor coil.

Constant air volume

A system designed to provide a constant air flow. This term is applied to HVAC systems that have variable supply-air temperature but constant air flow rates. Most residential forced-air systems are small CAV systems with on/off control. Abbreviated CAV.

Continuous Fan Switch

A feature on an oil furnace that allows the furnace’s fan to blow continuously in order to improve system efficiency and maintain even temperatures.



A movable plate located in the ductwork that regulates airflow. Dampers are used to direct air to the areas that need it most. Typically used in a zoning application.

Decibel (dB)

A decibel (dB) is a unit used to measure the relative intensity of sound. For example, sound levels in a library are normally about 40 dB, normal conversations range from about 50 to 60 dB, and an operating motorcycle or garbage truck can be as high as 100 dB.


A device that extracts and removes humidity from the air. It works by cooling air until water turns to liquid from vapor form after which the liquid is removed.

Department Of Energy (DOE)

A federal agency responsible for monitoring the consumption of energy sources and setting industry efficiency standards for all heating and cooling units manufactured in the U.S.


A device placed over ductwork that separates air with vanes going in differing directions. It evenly distributes air flow in the desired directions.

Dry bulb thermometer

A device that measures air temperature independently of humidity. It is freely exposed to the air it is measuring and is protected from the radiation and moisture.

Dry-Charged Unit

An air conditioner or heat pump that is shipped dry and charged with refrigerant at the place of installation. Dry-charged units are appropriate for homeowners who need a replacement unit compatible with R-22 refrigerant.

Dual Fuel

A comfort system that pairs an electric heat pump with a gas furnace and alternates between the two fuel sources to maximize comfort and efficiency.


A square channel or round tube that distributes air from the air handler (e.g.,rooftop unit, furnace, fan coil or VAV box) to the room.


Energy Efficiency Ratio (EER)

The ratio of cooling capacity to the power input (in watts). The higher the EER rating, the more efficient the air conditioner.

Electronic Air Cleaner (EAC)

An electronic device that filters out large particles and contaminants in indoor air. It then electronically pulls out tiny particles that have been magnetized, such as viruses and bacteria, drawing them to a collector plate.

Energy Star®

ENERGY STAR® is a program of the U.S. Environmental Protection Agency (EPA) designed to help consumers and businesses save money by choosing energy-efficient products and technologies. Heating, ventilation and air-conditioning (HVAC) equipment with the ENERGY STAR® label meets or exceeds federal guidelines for energy-efficient performance.

Environmental Protection Agency (EPA)

The U.S. Environmental Protection Agency (EPA) is a federal agency that develops and enforces federal environmental regulations and oversees the nationwide ENERGY STAR® program.

Evaporator Coil

Part of a split-system air conditioner or heat pump that is located indoors. The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas, which absorbs heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil). Also referred to as an indoor coil.



A component of an HVAC system that adds heat to air or an intermediate fluid by burning fuel (natural gas, oil, propane, butane, or other flammable substances) in a heat exchanger.


Gas furnace heat exchanger

A device that transfers heat from inside the furnace into the air outside the furnace. The duct system then transfers this exchanged air to different rooms in the building or space.

Gas Furnaces

A gas furnace is the part of an HVAC system that converts natural gas or propane into high-temperate heat for the home.

Geothermal Heat Pump

Carrier geothermal heat pumps tap into the earth’s surface to use the energy and consistent heat found in the ground, instead of using outside air like traditional heat pumps.


A facing across a duct opening, often rectangular in shape, containing multiple parallel slots through which air may be delivered or withdrawn from a ventilated space. The grille directs the air flow in a particular direction and prevents the passage of large items.


Heat Exchanger

The part of a furnace that transfers heat to nearby air. That air is then distributed through the ductwork throughout your home.

Heat gain, heat load, heat loss

Terms for the amount of cooling (heat gain) or heating (heat loss) needed to maintain desired temperatures and humidities in controlled air. Regardless of how well-insulated and sealed a home is, homes gain heat from sunlight, conduction through the walls, and internal heat sources such as people and electrical equipment. Homes lose heat through conduction during cold weather. Engineers use heat load calculations to determine the HVAC needs of the space being cooled or heated.

Heat Pump

An HVAC unit that heats or cools by moving heat. During the winter, a heat pump draws heat from outdoor air and circulates it through a home’s air ducts. In the summer, it reverses the process and removes heat from the house and releases it outdoors.

Heat transfer

Heat transfer happens when heat moves from one area to another. It is an important and vital step in the process of cooling a space.

Heating coil

The part of the system that conducts heat. It allows electricity to act as fire.

Heating Seasonal Performance Factor (HSPF)

The heating efficiency rating for heat pumps. The higher the rating, the more efficient the heat pump.


An indoor air-quality device that introduces moisture to heated air as it passes from the furnace into the ductwork for distribution throughout the building.



A humidistat is a device that works with a home’s heating and cooling system to automatically adjust the amount of moisture in the air to maintain a specific humidity level throughout the home.


Abbreviation for Heating, Ventilation and Air Conditioning.

HVAC Zoning System

An HVAC zoning system (also referred to as “zoned HVAC”) uses dampers in the ductwork or multiple systems to regulate and redirect air to specific areas of the home. This allows for the creation of customized temperature zones throughout the home, resulting in increased comfort and efficiency.


Indoor Air Quality (IAQ)

Refers to the cleanliness of the air in a home. IAQ factors include particulate count (pollen, mold), humidity and volatile organic compounds (VOCs) found in a home’s air – all of which can aggravate allergy and asthma symptoms.


Load Estimate

A series of studies performed to determine the heating or cooling requirements of your home. An energy load analysis uses information such as the square footage of your home, window or door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner. When referring to heating, this is often known as a Heat Loss Analysis, since a home’s heating requirements are determined by the amount of heat lost through the roof, entryways and walls.


Minimum Efficiency Reporting Value (MERV)

The standard comparison of the efficiency of an air filter. Every air filter has holes that allow air and particles to pass through. The smaller the holes, the fewer particles that can pass through. Higher MERV ratings indicate smaller holes The MERV scale ranges from 1 (least efficient) to 16 (most efficient) and measures a filter’s ability to remove particles from 3 to 10 microns in size.


A unit of measure equal to one millionth of a meter, or 1/25,000 of an inch. Airborne particles such as dust, dander, mold and viruses are measured in microns. These tiny particles can circulate in your home’s air and have harmful effects on your family’s health and comfort.


NATE Certification

North American Technician Excellence (NATE) conducts rigorous, independent testing verifying the real-world knowledge and application of HVAC technicians and installers.


Oil Furnace

Part of an HVAC system that converts heating oil (similar to diesel) into high-temperate heat for the home.

Operating Cost

The day-to-day cost of running your home comfort equipment, based on energy use.

Outside air damper

An automatic louver or damper that controls the fresh air flow into an air handler and modulates to the most energy efficient setting.



Any substances measuring less than 100 microns in diameter. The EPA has found that small particles (less than 2.5 microns) are responsible for some of the most serious health effects of poor air quality.

Plenum space

An enclosed space inside a home or other structure that is used for airflow. Often refers to the space between a dropped ceiling and the structural ceiling, or a raised floor and the hard floor. A plenum is distinct from ductwork since it is a part of the structure itself.

Programmable Thermostat

A thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day.


R-22 Refrigerant

The old standard for residential air conditioners, now being phased out by the U.S. EPA.

R-410A Refrigerant

A chlorine-free refrigerant that meets the EPA’s newest, most stringent environmental guidelines.

Radiant floor

A type of radiant heating system where the home’s floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room is heated from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced-air heating systems.


A chemical that produces a cooling effect while expanding or vaporizing.

Refrigerant Lines

Two copper lines that connect the outdoor air conditioner or heat pump to the indoor evaporator coil.

Remote Room Sensor

A sensor that feeds information about comfort conditions (such as temperature and humidity) to the main thermostat or control, used when the location of the thermostat or control is not optimal for assessing such conditions (for example, when it’s located near an exterior door).


Seasonal Energy Efficiency Ratio (SEER)

An energy efficiency rating for air conditioners. The higher the SEER, the more efficient the unit.

Smart Recovery

Actively manages system ramp-up during “away” periods to meet homeowner comfort needs while saving money.

Smart Setback Programming

Uses information about indoor and outdoor conditions and the specifics of the system’s capabilities to determine the best temperature setback during “away” periods and when to ramp up to save energy while staying within homeowner min/max temperature preferences.

Split System

Refers to an air conditioner or heat pump that has components in two locations. Usually, one part of the system is located inside (evaporator coil) and the other is located outside your home (condenser coil).



Usually found on an inside wall, this device controls heating and cooling equipment, allowing simple adjustments of temperature and other comfort control factors, such as humidity and airflow.


A unit of measure for cooling capacity. One ton = 12,000 BTUs per hour.

Two-Stage Operation

When a unit provides two levels of heat output for greater temperature control.


Underwriters Laboratories (UL)

A nonprofit testing and research organization that sets standards for different product categories and tests products to make sure they meet UL certification standards.

UV Lamps

Ultraviolet lamps attack and kill mold and bacteria preventing them from circulating through the home.


Variable-Speed Motor

A motor that provides a quiet, consistent flow of air for enhanced comfort, efficiency and humidity control. According to the Office of Energy Efficiency & Renewable Energy, a variable-speed motor running continuously at half speed may use up to 75% less power that a single-stage motor uses to move the same amount of air.


A device that replaces the stale, recirculated air inside your home with fresh, filtered outdoor air. An energy recovery ventilator (ERV) is ideal for warm and humid climates, since it cools and dehumidifies the incoming outdoor air. A heat recovery ventilator (HRV) is more appropriate for colder climates.

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