HVAC Terms You Should Know | Heating & Cooling Glossary

Use this glossary of HVAC (heating, ventilation, and air conditioning) terms and definitions to make better purchasing decisions.

Understanding your heating, ventilation, and air conditioning (HVAC) system will help you better communicate your problems/needs with a tech professional and make better purchasing decisions. Not only that, you will learn the basic mechanical engineering behind heating and cooling principles that apply to your car, refrigerator, and more.

It will also help with basic maintenance that you can perform on your own, saving you both time and money. A big part of this equation is the extensive glossary of terms and acronyms associated with the typical heating and cooling system. Get to know your HVAC unit by learning the basic components; many of which are highlighted in this easy-to-understand glossary.

HVAC Terms & Definitions:


ACH (Air Changes Per Hour)

This refers to the number of times per hour a room is supplied (or removed) of air through mechanical and natural ventilation.


AC (Air Conditioner)

An appliance/mechanism (or system) designed to extract heat and dehumidify a room or building. A room air conditioner is installed in a window or a wall and delivers conditioned are without a ductwork system. Central air conditioning uses fans and ducts to deliver cool air from a central unit to the rooms in your home.

AFUE (Annual Fuel Efficiency Ratio)

Similar to SEER (Seasonal Energy Efficiency Ratio), AFUE measures the energy efficiency of your heating system (furnace or boiler). It tells you how much of your fuel is used to heat your home versus how much energy is lost through exhaust. For example, a furnace with a 80% AFUE rating means that 80% of the fuel is converted into heat while 20% is lost through exhaust. AFUE and SEER ratings go down over time. If your AFUE rating is below 80%, consider replacing your unit with a more energy-efficient one.

BTU (British Thermal Unit)

A measurement of energy (heat) used to indicate the rate of cooling, dehumidifying, or heating in an HVAC system. One BTU is the amount of energy needed to raise the temperature of one pound of pure water by one degree Fahrenheit.

FACT: One BTU is approximately: 1.05 to 1.060 kilojoules, 252 to 253 calories (or little calories), 0.25 kilocalories (or food calories), and 778 to 782 foot pound force (ft. lbf.)


This device removes heat from a liquid through a vapor-compression or absorption refrigeration cycle. The 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. There are two types of designs; air-cooled and water-cooled.


Equipment that implements heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it.

Condenser Coil

This component in the basic refrigeration cycle ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condenser coils are heat exchangers, and can transfer heat to air or to an intermediate fluid to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers). You central HVAC’s condenser coils are usually located in your outdoor air conditioning or heat pump unit. You should have a minimum 2-foot clearance around your outdoor unit for proper airflow.


Device that controls the operation of all (or part) of an HVAC system. It might turn a device on and off, or it may more subtly modulate the set point of components. Most controllers are automatic but have user input such as temperature set points, e.g. a thermostat.


Specialized pipes or channels for the airflow (including supply air, return air and exhaust air) within a home.

FACT: Ducts can be made from several different materials including galvanized steel (the most common), aluminum, fiberglass, polyurethane, and plastic (called flexible ducting).


Equipment that reduces the level of humidity from the air. It works by cooling air to the point where water turns to liquid from vapor form, which is then removed.


Placed over ductwork, it separates air with vanes going in differing directions. It evenly distributes airflow in the desired directions.

Evaporator Coil

Coil in the basic refrigeration cycle that absorbs or adds heat to the HVAC system. It can be used to absorb heat from air or from a liquid. The evaporator is the cold side of an air conditioner or heat pump, and is located inside your air handler or attached to the furnace.

Fresh Air Intake

The opening through which outside air is drawn into the building. It either replaces air in the building that has been exhausted by the ventilation system, or provides fresh air for combustion of fuel.


The facing across a duct opening, often rectangular in shape, containing multiple parallel openings through which air may be delivered to a ventilated space. The grille directs the airflow in a particular direction and prevents the passage of debris.

Heating Coil

Part of the HVAC system that conducts heat. It allows electricity to act as fire.

Heat Pump

A compressor that cycles both hot and cold air. It is designed to move thermal energy in the opposite direction of heat flow by absorbing heat from a cold space, which is released to a warmer space.

Heat Transfer

This occurs when heat moves from one area to another. It is a vital step in the process of cooling a space.


The acronym for Heating, Ventilation and Air Conditioning. Modern systems also include air cleaning and moisture control.

OAT (Outside Air Temperature)

A measure of the air temperature outside a building.

Packaged Unit or RTU (Rooftop Unit)

An air-handling unit, defined as either “recirculating” or “once-through” design, created for outdoor installation. They most often include, internally, their own heating and cooling devices.

Plenum Space

An enclosed space inside a building or other structure, used for airflow.


The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism).


Substance that produces a cooling effect; used in most air conditioning and cooling systems.

SEER (Seasonal Energy Efficiency Ratio)

The seasonal energy efficiency ratio measures the energy efficiency of your air conditioning system. It is equal to the Total Cooling Output Over the Cooling Season / Total Electrical Energy Input Over the Cooling Season. It measures the total cooling of your air conditioner or heat pump in BTUs compared with the energy output (in watt hours) used within the same period. The higher the SEER rating on your air conditioner, the more energy efficient it is, meaning lower energy costs to run it.

Split System (Zoned)

A type of central HVAC system that allows you to control the thermostat/temperature in different areas of the home. The unit does not typically use a duct system, but rather has small wall-mounts that deliver heating or cooling to different rooms in the home. It is composed of one or more indoor units, and outdoor unit, and one or more thermostats.


The condition where liquid refrigerant is colder than the minimum temperature required to keep it from boiling which would change it from a liquid to a gas phase. Subcooling is the difference between its saturation temperature and the actual liquid refrigerant temperature.


A temperature-control device that monitors and regulates a heating or cooling system. It can be used to set the desired temperature at which it keeps the environment either heated or cooled.

FACT: One of the earliest examples of thermostat technology is attributed to Dutch innovator Cornelis Drebbel who, around 1620, invented the first mercury thermostat. It was created to regulate the temperature of a chicken incubator.

VAV (Variable Air Volume)

An HVAC system that has a stable supply-air temperature, and varies the airflow rate to meet the temperature requirements. Compared to constant air volume systems, they conserve energy through lower fan speeds during times of lower temperature control demand. Most new commercial buildings have VAV systems.

For additional HVAC terms, visit these glossaries from around the web:

Now that you’re more familiar with the basic terms from the HVAC glossary, let’s start a conversation. Call us today to discuss your HVAC problems or concerns.

Spring is the best time to schedule your professional air conditioning tune-up. Call us today to make sure your cooling system performs at its peak.

Don’t hesitate to contact us for more information on air cleaners, air purifiers, and air filtration.

Dirty and leaking air ducts also contribute to poor indoor air quality. Find out how regular air duct cleaning could benefit your home.

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