HVAC Glossary


AC (Alternating Current):  A type of current where the polarity is perpetually reversing, causing the directional flow in a circuit to reverse at regular intervals.

ACCA:  ACCA is a non-profit association whose membership includes more than 60,000 professionals and 4,000 businesses in the indoor environment and energy services community. Their website can be found at www.acca.org.

Acoustical Relating to sound, the science of sound, or a sense of hearing.

AFUE:  Annual Fuel Utilization Efficiency is a measurement used to rate furnace efficiencies by dividing the ratio of heat output by heat input.

AGA:  American Gas Association, Inc.

AHRI: The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing manufacturers of HVACR and water heating. Their website can be found at www.ahrinet.org.

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 within the space is exchanged by mechanical and natural ventilation. Also called air change rate or air exchange rate.

Air Conditioner:  A device that changes humidity levels, temperature or quality of air. An appliance, system, or mechanism designed to dehumidify and extract heat from an area. Usually, this term is reserved for smaller self-contained units such as a residential system.

Airflow Volume:  Measured in cubic feet per minute (CFM), this is the amount of air circulated in a space.

Air Handler (AHU):  Indoor part of the air conditioning system including the circulating fan and evaporator (summer) / condenser (winter) coil. A central unit consisting of a blower, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.

ASHRAE:  American Society of Heating, Refrigerating and Air-Conditioning Engineers is a global society advancing human well-being through sustainable technology for the built environment. The Society and its members focus on building systems, energy efficiency, indoor air quality, refrigeration and sustainability within the industry. Their website can be found at www.ashrae.org.


BTU:  A British Thermal Unit is a measurement of the amount of heat required to raise or lower the temperature of one pound of water one degree Fahrenheit. Any of several units of energy (heat) in the HVAC industry, each slightly more than 1 kJ. One BTU is the energy required to raise one pound of water one degree Fahrenheit, but the many different types of BTU are based on different interpretations of this “definition”. In the United States, the power of HVAC systems (the rate of cooling and dehumidifying or heating) is sometimes expressed in BTU/hour instead of watts.

BTU/h:  British Thermal Units per hour

Burner:  The device that facilitates the combustion of air and gas.

Burner Orifice:  The opening in the burner through which the gas or fuel passes prior to combustion.


Capacity:  HVAC capacity is the output produced by the heating or cooling unit and is measured in BTUs per hour.

Celsius:  A temperature scale that registers the freezing point of water as 0° and the boiling point as 100° under normal atmospheric pressure.

Centrifugal Fan: A centrifugal fan is a mechanical device for moving air or other gasses.

CFM (Cubic Feet per Minute):  A measurement of airflow volume.

Charging a System:  Adding coolant, or refrigerant, to an HVAC system.

Chiller: 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. Chillers are of two types; air-cooled or 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 from these chillers is carried by recirculating water to a heat sink such as an outdoor cooling tower.

Coil: The coil, or evaporator coil, is connected to the airflow outlet of the furnace. The conditioned refrigerant is circulated through the coil to cool the structure in the summer and heat in the winter. As warm indoor air passes through the indoor or evaporator coil, temperature and humidity are removed creating cooler indoor air. Installing a correctly sized and rated evaporator coil is essential for getting the highest performance and comfort from your central air conditioning or heat pump system.

Compressor:  A pump that increases the pressure of the refrigerant gas.

Condensate:  As warm air is pulled or pushed across the cool evaporator coil the coil perspires, creating a liquid, or condensate which is mechanically drained away from the equipment.

Condenser: A component in the basic refrigeration cycle that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers and can transfer heat to air or to an intermediate fluid (such as water or an aqueous solution of ethylene glycol) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers).

Condenser Coil:  Generally the outdoor coil, it removes heat from the refrigerant in the summer months, allowing the refrigerant to be converted from vapor to liquid and complete the refrigeration process.

Condenser Fan:  A fan that accelerates the movement of air over the condenser coil, facilitating the removal of heat from the refrigerant.

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

Controller: A device that controls the operation of part or all of a system. It may simply 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. Controls may be analog or digital.

CSA:  Canadian Standards Association


DC: Direct Current. A type of electrical current that only flows in one direction.

Damper:  Found at the junction points of ductwork, these sheet metal plates can be opened or closed to control the flow of air into a zone.

Degree-Day:  Calculated by subtracting the average outdoor temperature for an area from 65º Fahrenheit. This measurement is used to estimate the amount of heating or cooling a home or building will need.

Dehumidifier:  A device that removes humidity, or moisture, from the air.

Delta T (ΔT): a reference to a temperature difference. It is used to describe the difference in temperature of a heating or cooling medium as it enters and as it leaves a system.

Diffuser:  A grille over an air supply duct with vanes that distribute the discharging air in a specific pattern or direction.

DOE:  Department of Energy. Their website can be found at www.energy.gov.

Downflow Furnace:  A furnace with an intake on the top and an air discharge at the bottom.

Drain Pan:  Also a condensate pan. As the refrigerant vapor is liquefied, the drain pan collects the condensate and funnels it to the drain line.

Dry Bulb Temperature:  The temperature as measured without the consideration of humidity.

Dry Bulb Thermometer: A dry bulb thermometer is 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.

Duct work:  A network of metal, fiberboard or flexible material flowing throughout a space which delivers air from an HVAC unit to the respective zones of a home or office.


Economizer: An HVAC component that uses outside air, under suitable climate conditions, to reduce required mechanical cooling. When the outside air’s enthalpy is less than the required supply air during a call for cooling, an economizer allows a building’s mechanical ventilation system to use up to the maximum amount of outside air.

EER:  The Energy Efficiency Ratio of a particular cooling device is the ratio of output cooling energy (in BTU) to input electrical energy at a given operating point.

Energy Star®:  ENERGY STAR is a U.S. Environmental Protection Agency (EPA) voluntary program that helps businesses and individuals save money and protect our climate through superior energy efficiency. Energy Star’s website is www.energystar.gov.

Enthalpy: For a given sample of air, a measure of the total heat content (the sum of the heat energy of the dry air and heat energy of the water vapor within it). It is typically used to determine the amount of fresh outside air that can be added to recirculated air for the lowest cooling cost.

EPA:  The United States Environmental Protection Agency is an agency of the US federal government which was created for the purpose of protecting human health and the environment by writing and enforcing regulations based on laws passed by Congress. Their website can be found at www.epa.gov.

Evaporator: A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators 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.

Evaporator Coil:  Also an indoor coil. A device that is designed to absorb heat in the air in order to change the liquid refrigerant that flows through it into a vapor initiating the cooling process.

Expansion Valve:  A valve that meters the levels of refrigerant through a temperature or pressure control.


Fahrenheit:  A temperature scale in which water freezes at 32 degrees and boils at 212 degrees at normal atmospheric pressure.

Fan:  A device consisting of a motor and a blower wheel that creates air flow.

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

Filter:  A central heating and cooling system may use multiple filters. The air filter is integral to the system intake ducting, prevents contaminants from entering the equipment and must be maintained or replaced at regular intervals. There is also a filter in the refrigeration system, also referred to as a drier, which acts like a strainer to remove dirt and undesired particles from the system.

Flow: A transfer of fluid volume per unit time.

Flue:  A vent that removes the byproducts of combustion from a furnace.

Fresh Air Fraction: Fresh air fraction quantifies the action of the economizer, and is defined by (Mass flow rate of fresh-air / Total air flow rate).

Fresh Air Intake: An opening through which outside air is drawn into the building. This may be to replace air in the building that has been exhausted by the ventilation system or to provide fresh air for combustion of fuel.

Furnace: The major gas fired component in for heating a home. A device that facilitates the combustion of fuel and air to create heat and then circulates it through the home by means of a fan.

Fuse: A delicate metal strip connecting two parts of an electrical circuit. This strip works as a safety, or circuit protector, and breaks, or melts, in the event of an excess electrical charge, breaking the electrical circuit.


GAMAGas Appliance Manufacturers Association

Gas Furnace Heat Exchanger: A gas furnace heat exchanger is responsible for the transfer of 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.

GrilleA 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: A device through which heat is transferred to a cold or warm area or surface.

Heat Gain:  The amount of heat added or created in a designated area.

Heating Coil: A coil that acts as a heat source for a heating system.

Heat Loss: The amount of heat lost or subtracted from a designated area.

Heat Pump: A device used for either the heating or cooling of a space by transferring heat between two reservoirs.

Heat Transfer: The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space.

HSPF: The Heating Seasonal Performance Factor (HSPF) is a term specifically used to define the measurement of the efficiency of a heat pump system.

Humidifier:  A device that adds humidity, or moisture, to the air.

Humidistat: The device that measures humidity and turns the humidifier on and off.

Humidity: Dampness in the air caused by water vapor.


Ignition:  Elevating the temperature of a substance to the point of causing a combustive reaction.

Interconnection Agreement: A connection or link between power systems that enables them to draw on each other’s reserve capacity in time of need.

Intermediate Fluid: A liquid or gas used to transfer heat between two heat exchangers. An intermediate fluid is used when the hot and cold fluids are too bulky (such as air) or difficult to handle (such as halocarbon refrigerant) to directly transfer the heat.


Kilowatt (kW):  1,000 watts.


Latent Heat:  Is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process that creates a change of state. An example is the latent heat of evaporation which creates a phase transition from liquid to a vapor at a specified temperature and pressure.

Louver:  1.  Components made of multiple smaller blades, sometimes adjustable, placed in ducts or duct entries to control the volume of air flow. When used inside of ducts, their function is similar to that of a damper, but they can be manufactured to fit larger openings than a single-piece damper. 2.  Blades in a rectangular frame placed in doors or walls to permit the movement of air.


Makeup Air Unit (MAU): An air handler that conditions 100% outside air. Typically used in industrial or commercial settings, or in “once-through” (blower sections that only blow air one-way into the building), “low flow” (air handling systems that blow air at a low flow rate), or “primary-secondary” (air handling systems that have an air handler or rooftop unit connected to an add-on makeup unit or hood) commercial HVAC systems.

Manufacturer Approved System:  If replacing a condensing unit, furnace or air handler, the system must be manufacturer approved and Air Conditioning, Heating and Refrigeration Institute (AHRI) matched. NOTE: Installation of unmatched systems is strongly discouraged.

Media:  The fine material of a filter that traps dirt, dust, mildew or bacteria.

Minimum Outside AirThe lowest amount of fresh air flow that can be allowed into a recirculating system. This limit is sent to ensure that the interior air remains safe and comfortable to breathe.


NATE:  North American Technician Excellence is the nation’s largest non-profit certification organization for heating, ventilation, air conditioning and refrigeration technicians. NATE is the only technician certification organization governed, owned, operated, developed and supported by the HVACR industry. NATE’s website can be found at www.natex.org.

NEC:  National Energy Council / National Electric Code

NEMA:  National Electrical Manufacturing Association


Orifice: An opening or hole.

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.

Outside Air Temperature: A measure of the air temperature outside a building. The temperature and humidity of air inside and outside the building are used in enthalpy calculations to determine when outside air can be used for free heating or cooling.


Package Unit:  A heating and cooling system contained in one outdoor unit. Very common in some regions, particularly in single-story commercial buildings. Also called a rooftop unit (RTU).

Particulates:  The fine liquid or solid particles contained in combustion gasses. The quantity and size of particulates emitted by cars, power and industrial plants, wood stoves, etc are regulated by the U.S. Environmental Protection Agency.

Plenum:  A pressurized housing containing a gas (typically air) at positive pressure (pressure higher than surroundings). One function of the plenum is to manage and equalize pressure for more even distribution.

Programmable Thermostat:  A type of thermostat that allows the user to program into the devices’ memory a pre-set schedule of times and temperatures enabling or disabling the HVAC equipment.

PSI:  A pound per square inch is a unit of pressure resulting from the force of one pound-force applied to an area of one square inch.

PSIA:  Pounds per square inch, absolute is used to clarify that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi, this will be added to any pressure reading made in air at sea level.

PSIG:  Pounds per square inch gauge designates that the pressure is relative to atmospheric pressure.

Psychrometric:  The analysis of atmospheric conditions, particular to moisture in the air. The study of the behavior of air-water vapor mixtures. Water vapor plays an important role in energy transfer and human comfort in HVAC design.

PVC:  Polyvinyl chloride; a type of plastic.


Radiant Ceiling PanelsUsually metal panels suspended under the ceiling, insulated from the building structure. The primary cooling/heating agent temperature is close to the room’s temperature.

Radiant floor:  A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated.

Radiation:  The transfer of heat through matter or space by means of electromagnetic waves.

Reciprocating Compressor:  A type of compressor used in cooling systems to compress refrigerant by using a piston action.

Refrigerant:  The compound (working fluid) used in air conditioners, heat pumps, and refrigerators to transfer heat into or out of an interior space. This fluid boils at a very low temperature enabling it to exude and absorb heat.

Refrigerant Charge:  The amount of refrigerant in a system.


Scroll Compressor:  Used in both lower and higher efficiency air conditioners, scroll compressors are popular because they feature fewer moving parts than reciprocating compressors. This translates to more efficient operation, higher tolerance to liquid refrigerant, less mechanical failure and smoother, quieter operation.

SEER:  The efficiency of air conditioners is often rated by the Seasonal Energy Efficiency Ratio which is defined by the Air Conditioning, Heating, and Refrigeration Institute as the cooling output during a typical cooling-season divided by the total electric energy input during the same period.

Sensible Heat:  Heat added or subtracted that causes a change in temperature.

Sensor:  A device that reacts to a change in conditions.

Single-Speed:  A single-speed motor runs at top speed until it satisfies your temperature setting and then shuts off. They’re generally louder at start-up, consume more energy than alternative motor types and can cause more stress on mechanical parts.

Smoke Damper:  A damper or adjustable louver designed to augment the ventilation of a space during a fire.

Split System:  An outdoor unit combined with an indoor unit (as opposed to a package unit), generally providing more efficiency and configuration options.

Super Heat: The number of degrees a vapor is above its boiling point at a specific pressure.

Subcooling: 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.

System:  General term used to refer to the set or a subset of components that perform a specific HVAC function within a building.


Terminal Unit:  A small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. Used to control the temperature of a single room.

Thermal Zone:  An individual space or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads. Building codes may require zoning to save energy in commercial buildings. Zones are defined in the building to reduce the number of HVAC subsystems, and thus initial cost. For example, for perimeter offices, rather than one zone for each office, all offices facing west can be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as garages, attics, and crawlspaces, and basements.

Thermostat:  A wall-mounted device that monitors and controls the output of an HVAC system.

Thermostatic Expansion Valve (TXV):  A device that creates a constant evaporator temperature by regulation of refrigerant flow through the system.

Ton:  One ton is 12,000 BTUs per hour.

Two-Speed:  The base required for a high-efficiency air conditioner, two-speed motors cycle on in low gear and attempts to satisfy the cooling load for the home, shifting to high gear if necessary. Once it reaches the desired temperature, it cycles back down to low before shutting off. With just two speeds, it reduces start-up noise, operates with greater energy efficiency and causes less stress on mechanical parts compared to single-speed motors.

Two-Stage (cooling and heating):  A two-stage air conditioner is designed to operate on high and low settings during different weather conditions and seasons. The high setting is used during extreme weather, and the low setting is used during moderate weather. This type of air conditioner produces a balanced temperature and is in use for a longer period of time.


Underfloor Air Distribution:  A method for providing ventilation and space conditioning by using the air plenum below a raised floor to distribute conditioned air through diffusers directly to the occupied zone.

Unitary Controller:  A unitary controller is a device that controls only one zone in a building.

Upflow Furnace:  A furnace that pulls in air from the bottom and releases it through the top.


Vacuum:  A space where the pressure is significantly below that of standard atmospheric pressure.

Variable Air Volume (VAV):  An HVAC system that has a stable supply-air temperature, and varies the air flow rate to meet the temperature requirements. Compared to constant air volume systems, these systems conserve energy through lower fan speeds during times of lower temperature control demand. Most new commercial buildings have VAV systems. VAVs may be bypass type or pressure dependent. Pressure dependent type VAVs save energy while both types help in maintaining temperature of the zone that it feeds.

Variable-Speed:  Ideal for high-efficiency air conditioners, variable-speed motor functions much like a two speed, only with several speeds of operation. When compared with single- or two-speed motors, it facilitates smoother cycling and more precise performance control, as well as the quietest operation, highest energy efficiency and least stress on mechanical parts.

Ventilation:  The process of moving air (changing) into and out of an interior space either by mechanically induced (forced) means.

Volt:  Is the derived unit for electrical potential and electromotive force.

Voltage:  The force pushing an electrical current along wires and cables.


Watt:  Is defined as a joule per second and can be used to express the rate of energy transformation with respect to time.

Wet Bulb Thermometer:  A thermometer that measures the relative humidity in the air.


Zoning:  A system that divides a home, office or space into different regions in order to better control the temperature and effectiveness of a heating and cooling system.