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Glossary of HVAC Terms and Definitions

Whether you need to thaw out after being outside on a chilly winter's day or you can't wait to step into a cold, fresh, dry air-conditioned house on the hottest day of summer, we simply can't live without our heating and cooling. These complex, essential systems have their own set of terminology that is used by HVAC professionals but isn't always so well known outside of the HVAC industry. If you're wondering what an evaporator coil is or what exactly SEER means, check this glossary from the Dallas HVAC experts at Aire Serv of Dallas for those definitions and many more.


Accumulator: Also called a suction line accumulator, it is a device located on an air conditioner suction line between the evaporator outlet and compressor suction port to prevent liquid from entering the compressor.

ACH (Air Changes Per Hour): Also called air change rate, it is a measure of the volume of air added or removed from a space over an hour, divided by the total air volume of the space.

AHRI (Air Conditioning, Heating and Refrigeration Institute): An HVAC industry association that develops standards for certifying and measuring heating and cooling product performance.

AFUE (Annual Fuel Utilization Efficiency): A ratio that measures the heating efficiency of furnaces and boilers by comparing the heater's annual output of thermal energy to the amount of fuel it consumes over the course of the year. It is often expressed as a percentage. For example, a heater with an AFUE rating of 80% converts 80% of the fuel it uses into heat, while 20% is lost.

Air Conditioner: A cooling system that lowers the temperature in a space to the desired level and also controls humidity and ventilation for increased indoor comfort.

Air Handler: Also referred to as an air handling unit, it is the indoor component of a heating and cooling system that circulates air throughout a room or building.

Ambient Air: 1. Indoor or outdoor air that has not been treated by an HVAC system. 2. Fresh air that has not been contaminated by airborne pollutants.

AOH (Annual Operating Hours): An estimate of the number of hours an HVAC system such as a furnace or air conditioner may operate over the course of a year.


Bioaerosols: Microscopic airborne particles of biological origin from plants, animals, or microbes, such as pollen, viruses, fungal spores, bacterial cells, and more.

Boiler: A closed vessel heating system in which water is warmed and circulated throughout a series of pipes to provide radiant warmth to a space.

BTU (British Thermal Unit): A measurement of thermal energy needed to raise the temperature of one pound of water by one degree Fahrenheit. One BTU is equal to about 1,055 joules of energy. The heating capacity of furnaces, boilers, heat pumps and other systems that provide warmth is generally measured in BTUs.

Building Envelope: All the components of a building that enclose the interior space, including walls, floors, roof, windows, doors, and foundation.

Burner Flame: The part of a combustion furnace where gas mixes with air and is consumed by fire to provide thermal energy. The flame should be mostly blue in color; a yellow burner flame is an indication you need heater maintenance services.


Carbon Footprint: A representation of the total greenhouse gas emissions, expressed as a carbon dioxide equivalent, caused by an entity such as an individual, event, organization, service, place, or product.

Carbon Monoxide (CO): A colorless, odorless, tasteless gas produced by burning fuel. It can be very harmful or even fatal when it is inhaled in large quantities. A carbon monoxide leak from a deteriorated HVAC system that has not been maintained properly can pose a serious health risk in a closed space.

Centrifugal Fan: A mechanical device for moving air or other gasses in a specific direction by using the kinetic energy of impellers to move the stream and displace it radially.

CoP (Coefficient of Performance): A measurement of heating efficiency represented by a ratio of thermal energy output versus the amount of fuel or electric power consumed. It is the rated capacity of the system compared to its rated power input, often expressed as a single figure or percentage.

Condenser Coil: A component of an outdoor unit that takes in refrigerant from the compressor in the form of hot gas and cools it into a liquid phase, then moves it to the evaporator coil inside to provide cooling for an indoor space.

Conditioned Space: An enclosed space that has been warmed, cooled, and/or dehumidified by an HVAC system.

Constant Air Volume System: A type of system that provides constant indoor airflow when active, particularly systems with a variable supply temperature but a constant flow rate, like most residential forced-air systems.

Compressor: The part of the outdoor unit of an air conditioner or heat pump that maintains the flow of refrigerant through the unit and compresses the liquid refrigerant into a gaseous form. That hot gas is then moved to the condenser coil for cooling or the evaporator coil to provide warmth.

CFM (Cubic Feet Per Minute): A measurement of airflow volume representing the amount of air, in cubic feet that passes by a stationary point over the course of a minute.


Damper: A moveable device inside the ductwork that regulates airflow by closing off areas that are at the desired temperature and increasing airflow to spaces that need heating or cooling. They are usually used in zone control systems.

Decibel (dB): A unit that measures the intensity of sound as a degree of loudness. For example, normal conversations are usually around 60 decibels, while about 45 decibels are considered an acceptable level of indoor noise from an HVAC system.

Dehumidifier: A component of a cooling system that removes humidity by cooling air until it condenses from vapor into liquid, then removing the liquid to decrease indoor humidity.

Diffuser: A grate placed over a ductwork opening that can send air in different directions with vanes to evenly distribute airflow.

Drain Pan Heater: This separate heating element can be installed on an outdoor compressor in a cold climate. It warms the drain pan, so ice doesn't form in the pan or at the bottom of the condenser.

Dry Bulb Temperature: True thermodynamic temperature is measured by a dry bulb thermometer that is exposed to the air but sheltered from radiation and moisture. It is a measurement of heat intensity independent of humidity.

Dual Duct System: An HVAC system with a heated air duct and a cooled air duct. The desired room temperature is accomplished by mixing the correct amount of air from each duct.

Duct: A pipe or hose that houses airflow to move airflow from one space to another.


Electric Air Cleaner: An electronic device that removes large airborne particles and bioaerosols from indoor spaces.

Energy Star: A program operated jointly by the U.S. Department of Energy and the U.S. Environmental Protection Agency to promote energy efficiency and certify energy-efficient buildings, consumer products, and systems. The Energy Star logo can be found on HVAC components that meet certain standards of energy efficiency.

Evaporative Cooler: A type of cooling system that removes heat from a building through the process of evaporation. Also referred to as swamp coolers, this type of AC installation solution works best in arid climates with low humidity.

Evaporator Coil: A component of an indoor air handler that works in tandem with the condenser in the outdoor unit to complete the cycle of refrigerant. In cooling mode, the warm air in a room is absorbed by the refrigerant in the evaporator coil, then moves to the condenser via the suction line. In heating mode, the heat from the refrigerant in the evaporator coil is released into the room to warm it up.


Fan Coil Unit (FCU): A small terminal unit that is typically composed of a blower and cooling and/or heating coil, commonly used in apartments, condominiums, and hotels.

Fresh Air Intake (FAI): A portal that allows the HVAC system to draw fresh air from outside into a building to replace air from the ventilation system or facilitate fuel combustion.

Furnace: An enclosed appliance that facilitates the combustion of fuel such as gas, oil, or wood to create thermal energy to heat a space.


Heat Pump: A heating and/or cooling system that uses the refrigeration cycle to transfer thermal energy from a cold space to a warmer space. It moves thermal energy in the opposite direction of heat flow that occurs naturally, which is heat moving to a colder space. When a heat pump is in cooling mode, it removes heat from an indoor space to make it colder, and when it is in heating mode, it gathers thermal energy from a heat source such as the air, the ground, or a body of water.

Horizontal Flow: The product of an air handler that has been installed horizontally, so air moves across a space and exits out another duct. This is typically done in attic or crawl spaces that need climate control.

HSPF (Heating Seasonal Performance Factor): Also called the seasonal coefficient of performance (SCoP), it is a measure of the energy efficiency of air source heat pumps that represents the ratio of heat output in BTUs throughout the heating season versus the amount of electricity used annually in kilowatt-hours. Unlike CoP, which is more general, HSPF measures efficiency performance on an annual basis specifically.

HVAC (Heating, Ventilation, and Air Conditioning): An encompassing term for the technology that provides environmental comfort inside buildings and vehicles by changing the temperature of the airflow, regulating humidity and ventilation, and in some cases, filtering the air.


Indoor Air Flow: The movement of air inside a building, calculated in cubic feet per minute (CFM).


Kilowatt (kW): A measure of electric power equal to 1,000 Watts.

Kilowatt Hour (kWh): A measurement of the amount of electric power used over the course of an hour, equal to one kilowatt of power sustained for one hour, or 3,600 kilojoules.


Latent Heat: The energy released or absorbed by a substance when it is changing to a different phase, such as turning into liquid from vapor form, without changing its temperature.

Line-Set: A set of two copper tubes that connect a condenser coil and evaporator coil so the refrigerant can move between them in liquid or vapor form. The smaller tube is a liquid or discharge line that moves liquid refrigerant to the evaporator, and the larger tube is a suction line that moves refrigerant in its vapor form back to the condenser.

Load Calculation: The exact amount of BTUs required to heat or cool a space to the desired indoor temperature, based on factors such as the square footage, climate, ceiling height, level of insulation, sunlight exposure, and more.


Makeup Air Unit (MAU or MUA): The function of this unit is to make up air in an indoor space that is lost through the kitchen, bathroom, and dryer exhaust vents.

Mini-Split: A ductless air conditioning or heat pump system in which an outdoor compressor unit is connected to multiple air handlers inside, usually one in each high-use room that needs cooling or heating.


Outdoor Unit: Also referred to as the compressor or condenser unit, it is the part of an air conditioner or heat pump that is outside of the building and contains the compressor and condenser components of a refrigerant system.

Outside Air Temperature (OAT): A measure of air temperature outside of a building.


Package Unit: A unit that combines heating and cooling in one unit and uses a central ductwork system to blow cool or warm air into rooms.

Plenum Space: An enclosed space inside a building structure used for airflow, usually above the ceiling or below the floor. It is different from ductwork because it is part of the structure, not a separately installed pipe or hose.


Radiant Floor Heating: An electric or water-based system that is installed underneath flooring, such as tiles or engineered wood, to warm the room. Because thermal energy rises evenly from the floor, it provides a uniform temperature throughout the room.

Refrigerant: A fluid substance used in air conditioning and heat pump systems that is capable of repeatedly changing from a liquid to a vapor state and back again. While there are numerous types of refrigerant, they are categorized into three broad groups based on how they extract or absorb thermal energy. It is a highly regulated substance due to its flammability and toxicity, and some types are banned because of their negative effect on the ozone layer.


Seasonal Energy Efficiency Ratio (SEER): A measure of the efficiency of air conditioning systems represented by a ratio of the total annual cooling output of a system compared to the amount of electric power it uses in one year. Higher SEER ratings indicate better energy efficiency, which is why high energy costs caused by a unit with a low SEER rating are one of the signs you need a new AC unit that can perform with improved efficiency.

Single-Zone: A ductless system in which a single compressor is connected to a single air handler instead of one outdoor unit connected to multiple indoor air handlers. There is often more than one compressor and air handler unit, one for each space that needs to be cooled or, in the case of a heat pump, heated.

Subcooling: The difference between saturation temperature and the actual liquid refrigerant temperature when the liquid refrigerant is below the minimum temperature that would cause it to boil and shift from a liquid to a vapor phase.


Terminal Unit (TU): A small, ductless unit that controls the temperature in a single room and is typically composed of a heating coil, a cooling coil, an automatic damper, or some combination of those elements.

Thermal Zone: An individual indoor space or a group of neighboring spaces such as a cluster of rooms that are expected to have similar thermal loads.

Thermidistat: A device that regulates both temperature and humidity levels in a building or enclosed space by controlling the HVAC system and signaling when it should operate.

Thermostat: A temperature regulating device that senses the temperature in a space and controls the HVAC system to achieve the desired temperature that has been programmed. A faulty thermostat is one of the more common heating and cooling problems. Some thermostats can be programmed with a pre-set schedule, so they do not need to be manually adjusted like a conventional thermostat.

Thermostatic Expansion Valve (TXV or TEV): An equipment component that meters the flow of refrigerant into the evaporator while also measuring the refrigerant leaving the evaporator in vapor form. Despite the name, they do not control the temperature in the evaporator.


Upflow Furnace: A type of furnace that pulls air in at the bottom and releases it through the top of the unit.


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