HVAC Dictionary

HVAC Heating Ventilation and Air Conditioning Terms Dictionary
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This heat exchanger depends on two coils, placed diagonally and then connected in the middle. The result is a coil that looks like the letter “A”.

Absolute Humidity

The humidity of the air measured by the number of grains of water vapor present in one cubic meter of air.

Absolute Pressure

The sum of gauge and atmospheric pressure (psia).

Absolute Temperature

The temperature measured on the Kelvin scale.

Absolute Zero

The lowest temperature theoretically attainable on the Kelvin scale (approximately 273.16° C).


A material that extracts one or more substances from a fluid (gas or liquid) medium on contact, and which changes physically and/or chemically in the process. The less volatile of the two working fluids in an absorption cooling device.


In physics, the taking up of light, heat, or other energy by molecules. The absorbed energy is converted into heat. Absorption in chemistry is the taking up of one substance by another. For example, a gas such as oxygen may be absorbed, or dissolved, in water. In the HVAC industry heat energy is absorbed from the medium being cooled and transferred in the refrigerant.

Absorption Chiller

A type of air cooling device that uses absorption cooling to cool interior spaces.

Absorption Cooling

A process in which cooling of an interior space is accomplished by the evaporation of a volatile fluid, which is then absorbed in a strong solution, then desorbed under pressure by a heat source, and then condensed again at a temperature high enough that the heat of condensation can be rejected to a exterior space.

Absorption Refrigeration

A system in which a secondary fluid absorbs the refrigerant, releasing heat, then releases the refrigerant and reabsorbs the heat. Ammonia or water is used as the vapor in commercial absorption cycle systems, and water or lithium bromide is the absorber.


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.


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


A shell device installed in the suction line of a HVAC system to prevent liquids from entering the compressor.

Activated Carbon

A processed carbon used in filter driers and commonly used in air filters to clean the air.

Active Cooling

HVAC term for compressor driven air conditioning.

Active Power

The power (in Watts) used by a device to produce useful work. Also called input power.


A mechanical device for moving or controlling a mechanism or system.


A change in gas condition where no heat is added or removed except in the form of work.

Adjustable Speed Drive

An electronic device that controls the rotational speed of motor-driven equipment such as fans, pumps, and compressors. Speed control is achieved by adjusting the frequency of the voltage applied to the motor.


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


American Gas Association, Inc. founded in 1918, is an American trade organization representing over 200 natural gas supply companies and others with an interest in the manufacturing of gas appliances as well as the production of gas.


The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) is the trade association representing North American manufacturers of air conditioning, heating, water heating, and commercial refrigeration equipment.

Air Balance

HVAC term for distributing air through a system to precisely match the required amount.

Air Cleaning

In HVAC an IAQ control strategy to remove various airborne particulates and/or gases from the air. The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation, and gas sorption.

Air Conditioner

A device used to control temperature and humidity of the air.

Air Conditioning

In HVAC the control of the quality, quantity, and temperature-humidity of the air in an interior space.

Air Diffuser

HVAC term for an air distribution outlet, typically located in the ceiling, which mixes conditioned air with room air.

Air Exchange Rate

In HVAC the rate at which outside air replaces indoor air in a space. Expressed in one of two ways: the number of changes of outside air per unit of time – air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time – cubic feet per minute (CFM).

Air Handler

HVAC term for a fan-blower, heat transfer coil, and housing parts of a system.

Air Handling Unit (AHU)

In HVAC refers to equipment that includes a blower or fan, heating and/or cooling coils, and related equipment such as controls, condensate drain pans, and air filters. Does not include ductwork, registers or grilles, or boilers and chillers.

Air Infiltration

The unwanted entrance of air due to leakage, temperature difference, or wind.

Air Passages

Openings through or within walls, through floors and ceilings, and around chimney flues and plumbing chases, that permit air to move out of the conditioned spaces of the building.

Air Standard

HVAC term for air having a temperature of 68 degrees F (20 degrees C) and a relative humidity of 36% at 14.7 psia.

Air to Air

Where both the condensing and evaporating mediums are air.

Air Vent

HVAC term for a valve, either manual or automatic, that is used to remove unwanted air from the highest point of a piping system.

Airflow Volume

Measured in cubic feet per minute (cfm), this is the amount of air circulated in a space.

AK Factor

The amount of free space that the air may travel through.

Alternating Current (AC)

Electrical current that reverses direction periodically.

Ambient Air

The air external to a building or device.

Ambient Temperature

The temperature surrounding an object.


A continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e analogous to another time varying signal. It differs from a digital signal in that small fluctuations in the signal are meaningful.


ASHRAE stands for American Society of Heating Refrigeration and Air Conditioning Engineers.  Founded in 1894, it 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.

Atmospheric Pressure

Is the pressure that the atmosphere exerts on us. At sea level this is 14.7 psig also written as 29.92 ” Hg.

Atmospheric Condenser

A condenser operated with water that is exposed to the atmosphere.

Automatic Expansion Valve

A pressure actuated metering device to regulate the flow of refrigerant into the evaporator according to the evaporator pressure.

Available Heat

The amount of heat energy that may be converted into useful energy from a fuel.


Refrigerants that are mixtures or blends of different refrigerants that are at the same ratio in vapor as well as in liquid state.

Azeotropes – (Near)

Azeotrope having a temperature glide sufficiently small that it is inconsequential.

Azeotropic mixture

A combination of different refrigerants to mask one with desirable refrigerant properties, an example of this is R-502 which is a mixture of 48.8% R-22 and 51.2% R-115.


BTU (British Thermal Unit)

The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit, equal to 252 calories.


British Thermal Units per hour.

Back EMF

The emf that opposes the normal flow of current in a circuit.

Back Pressure

This is the refrigerant pressure in the low side of the system also called low side pressure or suction pressure.


BACnet is an ASHRAE building automation and control networking protocol, was designed specifically to meet the communication needs of building automation and control systems for applications such as heating, ventilating, and air-conditioning control, lighting control, access control, and fire detection systems and their associated equipment. The BACnet protocol provides mechanisms by which computerized building automation devices can exchange information, regardless of the particular building service they perform. As a result, the BACnet protocol may be used by head-end workstation, general-purpose direct digital controllers, and application specific or unitary controllers with equal effect.

Balanced Pressure

When the both the inside and outside pressure of a container equal each other.


Used to measure atmospheric pressure.

Barometric Damper

Also called draft damper, is a device installed in a chimney to allow for the adjustment of dilution air.

BAS (Building Automation System)

An integration of digital, electronic, and/or pneumatic controls and devices to provide unattended and automatic operation of buildings systems. Systems may include HVAC, elevators, fire suppression, smoke control, security, lighting, and other subsystems.


A corrugated cylindrical container which moves with a pressure change.

BEMS (Building Energy Management System)

The software, hardware, and services associated specifically with the intelligent (i.e., information and communication technology, or ICT-based) monitoring, management, and control of energy, as well as the enhancement of a building’s efficiency of operations for commercial buildings.


A temperature regulating or indicating device which works on the principal that two dissimilar metals with unequal expansion rates, welded together and will bend as their temperature changes.

Blast Freezer

A low-temperature evaporator that uses a fan to force air rapidly over a evaporator surface.


A valve with a small opening which permits a minimum fluid flow when the valve is closed.


A mixture consisting of two or more single components.


In HVAC the device in an air conditioner that distributes the filtered air from the return duct over the coil/heat exchanger. This circulated air is cooled/heated and then sent through the supply duct, past dampers, and through supply diffusers to the living/working space.

BMS (Building Management System)

A system for centralizing and optimizing the monitoring, operating, and managing of a building. Services may include heating, cooling, ventilation, lighting, security, and energy management.


A vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to generate hot water or steam for applications ranging from building space heating to electric power production or industrial process heat.

Boiler Pressure

The pressure of the steam or water in a boiler as measured, usually expressed in pounds per square inch gauge (psig).

Boiler Rating

The heating capacity of a steam boiler expressed in BTU per hour (BTU/H), horsepower, or pounds of steam per hour.


The sheet metal chamber in a furnace where heat collects before being distributed.


A term used for the first stage compressor in a cascading system.

Bottled Gas

A generic term for liquefied and pressurized gas, ordinarily butane, propane, or a mixture of the two, contained in a cylinder for domestic use.


A salt water mixture commonly used as a secondary refrigerant.


The device (often within a furnace) that facilitates the combustion of air and gas.

Burner (sealed combustion)

A piece of a furnace that conducts heat using the air outside of the system.

Burner Orifice

The piece of the burner that inputs gas. The gas is then mixed with air and used as fuel.



Ability of a circuit system to store electricity. The capacitance of a capacitor is measured in farads and is determined by the formula C = q/V, where q is the charge (in coulombs) on one of the conductors and V is the potential difference (in volts) between the conductors. The capacitance depends only on the thickness, area, and composition of the capacitor’s dielectric.


The capability of a heating or cooling system to fill a required space.

Carbon Dioxide (CO2)

A colourless, odourless non-combustible gas with the formula CO2 that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.

Carbon Filter

Air filter housing activated carbon.

Carbon Monoxide (CO)

A colourless, odourless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass.

Cascade System

Arrangement in which two or more refrigerating systems are used in series.

Ceiling Plenum

Space below the flooring and above the suspended ceiling that accommodates the mechanical and electrical equipment and that is used as part of the air distribution system. The space is kept under negative pressure.


The metric scale of temperature. When measured on water, the freezing point is 0°C, and the boiling point is 100°C. Conversion from Celsius to Fahrenheit is as follows:  Multiply by 9, then divide by 5, then add 32.

Central Air Conditioning

See Air Conditioner.

Central Air Handling Unit (Central AHU)

This is the same as an Air Handling Unit, but serves more than one area.

Central Heating System

In HVAC a system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.


Cloro-Floro Carbon

CFM (Cubic feet per minute)

HVAC term for the amount of air, in cubic feet, that flows through a given space in one minute. 1 CFM equals approximately 2 litres per second (l/s).


The process of adding refrigerant to an air conditioning system. Refrigerant is stored in a sealed system and aids in the overall cooling factor. Refrigerant moves from the indoor evaporator to the outdoor condenser and then back to the indoor evaporator.


To add a charge of refrigerant to a system.

Charging Scale

A tool used to measure the weight of refrigerant recovered from and charged into a system.

Check Valve

A check valve is a mechanical device normally applied to a piping system which allows fluid to flow in only one direction.

Chilled Water Valve

A mechanical valve which provides a variable water flow to an air-conditioning chilled water coil.


A cooling system used to cool water or brine.

Closed Loop

Any piping system where the internal fluids are sealed from their surroundings. Commonly referred to as glycol loops.


As a component of a heating or cooling appliance, rows of tubing or pipe with fins attached through which a heat transfer fluid is circulated and to deliver heat or cooling energy to a building.

Combined Annual Efficiency (CAE)

Combined Annual Efficiency (CAE) is a measure of the amount of heat produced for every dollar of fuel used to heat your home’s air and water. It is used to compare efficiencies of integrated water and space heaters.

Combination Foundations

Buildings constructed with more than one foundation type; e.g., basement/crawlspace or basement/slab-on-grade.


The process of burning; the oxidation of a material by applying heat, which unites oxygen with a material or fuel.

Combustion Air

Air that provides the necessary oxygen for complete, clean combustion and maximum heating value.

Combustion Chamber

Any wholly or partially enclosed space in which combustion takes place.

Combustion Gases

The gaseous by-products of the combustion of a fuel.

Communication Module

A communication module is a component of a building control system that sends performance data to a secure website, where you can view system status, energy production and environmental benefits online, including carbon offsets, in real time.

Compression Gauge

An instrument used to measure positive pressure.


A refrigeration component that make a pressure difference in the system which causes refrigerant to flow.

Compressor Reciprocating

Compressor which uses a piss cylinder mechanism to provide pumping action.

Compressor, Rotary

Compressor which uses vanes, a mechanisms, or other rotating devices to provide pumping action.


A fluid formed when a gas is cooled .


Liquid or droplets which form when a gas or cooled below its dew point.


The changing of a gas or vapour to a liquid.


The heat rejection component of a system where the refrigerant is condensed from a vapour to liquid.

Condenser, Air Cooled

Heat exchanger that transfers heat to the surrounding air.

Condenser, Water Cooled

Heat exchanger that transfers heat to water.

Condenser Coil

The device in an air conditioner or heat pump through which the refrigerant is circulated and releases heat to the surroundings when a fan blows outside air over the coils. This will return the hot vapour that entered the coil into a hot liquid upon exiting the coil.

Condense Comb

Comb-like device used to straighten the metal fins on condensers or evaporators.

Condenser Fan

A fan used to move air through air-cooled condenser.

Condensing Furnace

A high efficiency furnace that also removes latent heat form the combustion products.

Condensing Pressure

See head pressure

Condensing temperature

The temperature at which a substance will condense.

Condensing unit

Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser, and returns it to the metering device.

Conditioned Air

Air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the “comfort zone.” (Sometimes referred to as “tempered” air.)

Conditioned Space

The interior space of a building that is heated or cooled.

Constant Air Volume Systems

Air handling system that provides a constant air flow while varying the temperature to meet heating and cooling needs.


A method of heat transfer that involves the movement of fluids, such as liquids or vapors, and the heat energy they contain.

Cooling Capacity

The quantity of heat that a cooling appliance is capable of removing from a room in one hour.

Cooling Degree Day

A value used to estimate interior air cooling requirements (load) calculated as the number of degrees per day (over a specified period) that the daily average temperature is above 65 degrees Fahrenheit (or some other, specified base temperature). The daily average temperature is the mean of the maximum and minimum temperatures recorded for a specific location for a 24 hour period.


Computer Room Air Conditioner – a type of HVAC system designed to regulate the temperature, humidity, and air distribution in computer rooms and data centers.

Crankcase Heater

A piece of equipment that maintains the vapor form of refrigerant in the crankcase part of the system. By heating the crankcase oil to a higher temperature than the coldest part of the system, refrigerant remains in its vapor form.



HVAC term for controls that vary airflow through an air outlet, inlet, or duct. A damper position may be immovable, manually adjustable or part of an automated control system.

Damper Motor

An actuator which will drive a mechanical damper variably or 2 position.

Deadband / Deadzone

A Deadband (sometimes called a neutral zone) is an area of a signal range or band where no action occurs (the system is dead). Deadband is used in voltage regulators and other controllers. The purpose is common, to prevent oscillation or repeated activation-deactivation cycles (called ‘hunting’ in proportional control systems).


The process which removes frost buildup from the outdoor coil.  This usually takes place when the weather is cold and the air conditioner is not in use.

Degree Day

A unit for measuring the extent that the outdoor daily average temperature (the mean of the maximum and minimum daily dry-bulb temperatures) falls below (in the case of heating, see Heating Degree Day), or falls above (in the case of cooling, see Cooling Degree Day) an assumed base temperature, normally taken as 65 degrees Fahrenheit, unless otherwise stated. One degree day is counted for each degree below (for heating) or above (in the case of cooling) the base, for each calendar day on which the temperature goes below or above the base.

Degree Hour

The product of 1 hour, and usually the number of degrees Fahrenheit the hourly mean temperature is above a base point (usually 65 degrees Fahrenheit); used in roughly estimating or measuring the cooling load in cases where processes heat, heat from building occupants, and humidity are relatively unimportant compared to the dry-bulb temperature.


A device for reducing the level of humidity in a room or home.

Diffusers and Grilles

Components of the ventilation system that distribute and return air to promote air circulation in the occupied space. As used in this document, supply air enters a space through a diffuser or vent and return air leaves a space through a grille.


Used to refer to more than one concept. It can refer to discrete-time signals that have a discrete number of levels, for example a sampled and quantified analog signal, or to the continuous-time waveform signals in a digital system, representing a bit-stream.

Direct Current (DC)

The electric current that travels in only one direction.  See also Alternating Current (AC).

Direct Digital Control (DDC)

Direct digital control is the automated control of a condition or process by a digital computer. DDC is often used to control HVAC (heating, ventilating, and air conditioning) devices such as valves via microprocessors using software to perform the control logic. Such systems receive analog and digital inputs from the sensors and devices installed in the HVAC system and, according to the control logic, provide analog or digital outputs to control the HVAC system devices.

Direct Expansion (DX)

Refers to the more common refrigerant evaporator coil. The evaporator is in direct contact with the air stream, so the cooling coil of the airside loop is also the evaporator of the refrigeration loop. The term “direct” refers to the position of the evaporator with respect to the airside loop. This coil usually has a direct expansion valve connected to the coil distributor.

Direct Water Heater

A type of water heater in which heated water is stored within the tank. Hot water is released from the top of the tank when a hot water faucet is turned. This water is replaced with cold water that flows into the tank and down to just above the bottom plate under which are the burners.


Department of Energy.

Downflow Furnace

This type of furnace processes air from top to bottom.


A column of burning combustion gases that are so hot and strong that the heat is lost up the chimney before it can be transferred to the house. A draft brings air to the fire to help keep it burning.

Draft Diverter

A door-like device located at the mouth of a fireplace chimney flue for controlling the direction and flow of the draft in the fireplace as well as the amount of oxygen that the fire receives.

Draft Hood

A device built into or installed above a combustion appliance to assure the escape of combustion by-products, to prevent back-drafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.

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

A measurement of heat intensity independently of humidity. A dry bulb thermometer takes this measurement.

Dry Bulb Thermometer

An device that measures air temperature independently of humidity.

Dual Duct System

An air conditioning system that has two ducts, one is heated and the other is cooled, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.

Dual Fuel

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

Duct Fan

HVAC term for an axial flow fan mounted in a section of duct to move conditioned air.

Duct(s) or Ductwork

The round or rectangular tube(s), generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes air from heating, ventilation and air-conditioning (HVAC) equipment in buildings.

DX (Direct Expansion)

This refers to the expansion of refrigerant. This process conducts heat.  “DX Cooling” refers to any process which uses refrigerant to cool.


EER (Energy Efficiency Ratio)

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

EMS (Energy Mangement System)

A set of automated controls and software that will monitor and optimize the use of energy in commercial buildings. An energy management system can reduce waste by adjusting heating and cooling usage. A typical system will collect data from energy meters, analyze it and identify opportunities for greater efficiency.


The amount of internal energy within an HVAC system combined with the product of its pressure and volume.

EPA (Environmental Protection Agency)

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 (or Indoor Coil)

Also known as the indoor coil, this piece of equipment removes heat and humidity from the air inside the building.

Environmental Agents

Conditions other than indoor air contaminants that cause stress, comfort, and/or health problems (e.g., humidity extremes, drafts, lack of air circulation, noise, and over-crowding).


The intentional or unintentional leakage of conditioned air out of a building.

Exhaust Ventilation

HVAC term for mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).

Expansion Valve

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



A temperature scale in which water freezes at 32 degrees and boils at 212 degrees at normal atmospheric pressure.  Conversion to the Celsius temperature scale, use this equation: Deduct 32, then multiply by 5, then divide by 9

Fan Coil Unit (FCU)

A fan coil unit (FCU) is a device that uses a coil and a fan to heat or cool a room without connecting to ductwork. Indoor air moves over the coil, which heats or cools the air before pushing it back out into the room. FCUs can be less expensive to install than ducted systems, and are available in ceiling, floor-mounted and freestanding configurations.

Filter (air)

A device that removes contaminants, by mechanical filtration, from the fresh air stream before the air enters the living space. Filters can be installed as part of a heating/cooling system through which air flows for the purpose of removing particulates before or after the air enters the mechanical components.


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

Forced Air System or Furnace

HVAC term for a type of heating system in which heated air is blown by a fan through air channels or ducts to rooms.

Formicary Corrosion

A type of corrosion caused by organic acids such as formic acids and acetic acids. It affects coils in a cooling system in nearly all branches of the industry. This corrosion attacks copper and copper alloy tube walls, and is associated with pinholes in the copper tube walls in cooling systems.  Formicary corrosion cannot be seen with the naked eye, which makes it difficult to detect the problem before the damage is already too severe and causes a coil leak.


When a refrigerant blend’s composition changes because one or more of its components are lost or boiled off faster than the others.


A registered trademark for a chlorofluorocarbon (CFC) gas that is highly stable and that has been historically used as a refrigerant.


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.


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 excess electrical charge, breaking the electrical circuit.



Gas Appliance Manufacturers Association.

Gas Furnace Heat Exchanger

This part of the system moves heat from inside the furnace into the air outside the furnace. The duct system transfers this air to rooms in the building.

Garage Heaters

Natural or propane gas units that provide warmth for car garages of almost any size during the winter months. Separated combustion models provide heating for hard-to-heat applications.

Grid-Tie Inverter Solar System

A grid-tie inverter solar system is the most common and least expensive of all residential solar systems. It allows homeowners to use their own solar-generated electricity to save energy and reduce costs. The utility company’s grid, or network of power stations, serves as the backup power source.


A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both.


Head Pressure

Also known as discharge pressure or high side pressure, head pressure is the pressure generated by the compressor in an HVAC unit.


A form of thermal energy resulting from combustion, chemical reaction, friction, or movement of electricity. As a thermodynamic condition, heat, at a constant pressure, is equal to internal or intrinsic energy plus pressure times volume.

Heat Exchanger

The part of the system allows heat to be transferred from the hot parts of the machine to the cold parts of the machine.

Heat Gain

This is a total amount of heat an area receives from all heat conducting machines including furnaces, appliances, lighting, respiration and solar energy.

Heat Loss

The heat that flows from the building interior, through the building envelope to the outside environment.

Heat Rate

The ratio of fuel energy input as heat per unit of net work output; a measure of a power plant thermal efficiency, generally expressed as Btu per net kilowatt-hour.

Heat Pump

A heat pump is 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 Register

The grilled opening into a room by which the amount of warm air from a furnace can be directed or controlled; may include a damper.

Heat Transfer

The process where heat moves from one area to another.

Heating Capacity (Also Specific Heat)

The quantity of heat necessary to raise the temperature of a specific mass of a substance by one degree.

Heating Coil

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

Heating Degree Day(s) (HDD)

The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder the daily average temperature(s).

Heating Load

The rate of heat flow required to maintain a specific indoor temperature; usually measured in Btu per hour.

Heating Seasonal Performance Factor (HSPF)

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


High Efficiency Particulate Arrestor (filters).


The measurement of electrical energy per second. Standard frequency is 6- Hertz.

Home Energy Rating Systems (HERS)

A nationally recognized energy rating program that gives builders, mortgage lenders, secondary lending markets, homeowners, sellers, and buyers a precise evaluation of energy losing deficiencies in homes. Builders can use this system to gauge the energy quality in their home and also to have a star rating on their home to compare to other similarly built homes.

Horizontal Flow

Describes an air handler or furnace that is positioned on its side and circulates air in one end and out the other. Ideal for attic or crawl space installations.

Hot Air Furnace

A heating unit where heat is distributed by means of convection or fans.


A device for increasing the humidity in a room or home.  Built-in humidifiers may use the fan in a furnace or air handler to blow humidified air throughout the ductwork, or they may work independently of heating and cooling systems to maintain humidity levels even when the furnace or air handler isn’t operating.


A humidistat (sometimes called a humidistat control) 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.


A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.


HVAC (pronounced either “H-V-A-C” or “H-vak”) is an initialism or acronym that stands for “heating, ventilating, and air conditioning”. HVAC is sometimes referred to as climate control and is particularly important in the design of medium to large industrial and office buildings such as skyscrapers and in marine environments such as aquariums, where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within.

HVAC Damper

An HVAC damper (also called a duct damper) is a movable plate, located in the ductwork, that regulates airflow and redirects it to specific areas of the home. Dampers are typically used in zoning or “zone control” systems.

HVAC Zoning System

An HVAC zoning system (also referred to as “zoned HVAC”) is a heating and cooling system that uses dampers in the ductwork to regulate and redirect air to specific areas of a building. This allows for the creation of customized temperature zones throughout the home for increased comfort and efficiency.



Indoor air pollution.


Indoor air quality.

Ice Bank

A thermal energy storage (TES) tank that stores cooling capacity for HVAC systems.


Beginning of the combustion process, ignition requires a gaseous mixture to take flame form.

Indoor Air

The air the people breathe inside a built environment.

Indoor Air Pollutant

Particles and dust, fibres, mists, bio-aerosols, and gases or vapours.


Air leakage inward through cracks and interstices and through ceilings, floors, and walls of a space or building.

Integrated Heating Systems

HVAC term for a type of heating appliance that performs more than one function, for example space and water heating.

Interconnection Agreement

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


Kilowatt (kW)

The electrical measurement of 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.


LonTalk is a protocol optimized for control created by Echelon Corporation for networking devices over media such as twisted pair, powerlines, fiber optics, and RF. It is popular for the automation of various functions in industrial control, home automation, transportation, and buildings systems such as lighting and HVAC.


Make-Up Air Unit (MUA)

A larger air handler that conditions 100% outside air, and no recirculated air.

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.

Mechanically Ventilated Crawlspace System

In HVAC a system designed to increase ventilation within a crawlspace, achieve higher air pressure in the crawlspace relative to air pressure in the soil beneath the crawlspace, or achieve lower air pressure in the crawlspace relative to air pressure in the living spaces, by use of a fan.


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

MERV rating (Minimum Efficiency Reporting Value)

Numbered from 1 to 16, this measurement gives the efficiency value 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, and therefore a more efficient filter.


Constructed of parallel flow aluminum tubes that are mechanically brazed to enhanced aluminum fins, resulting in better heat transfer and a smaller, lighter, corrosion resistant coil.


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.


Like central split systems, mini splits have two main components: an outdoor compressor/condenser, and an indoor air-handling unit. A conduit, which houses the power cable, refrigerant tubing, suction tubing, and a condensate drain, links the outdoor and indoor units.  It is a ductless system which supplies air though indoor unit.



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.

Natural Ventilation

In HVAC the movement of outdoor air into a space through intentionally provided openings, such as windows and doors, or through non-powered ventilators or by infiltration.


National Energy Council / National Electric Code.


National Electrical Manufacturing Association

Negative Pressure

Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.


A network is a collection of computers or devices connected to each other. The network allows these computers or devices to communicate with each other and share resources and information.



Original Equipment Manufacturer.

Off-Grid or Off-The-Grid

An off-grid or off-the-grid system is an electricity-generating system that operates independently from the utility grid, providing all of the electricity needed in the home, and is generally more expensive than grid-tied system ideal for buildings located in remote areas without utility service, or where installing power lines would be extremely costly.

Oil Furnace

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

OLE for Process Control (OPC)

A set of connectivity standards for industrial automation from the OPC Foundation. OPC added extensions to Microsoft’s COM and DCOM object technology in order to provide a set of common interfaces for process control. OPC offers interoperability between gauges, databases, programmable logic controllers (PLCs), distributed control systems (DCSs) and remote terminal units (RTUs).

Organic Compounds

Chemicals that contain carbon. Volatile organic compounds vaporize at room temperature and pressure. They are found in many indoor sources, including many common household products and building materials.

Outdoor Air

Air taken from the external atmosphere and, therefore, not previously circulated through the system.

Outdoor Air Supply

HVAC term for air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as “Make-Up Air”.


Package Unit

This smaller heating or cooling unit is used for residential purposes and is usually located on the roof or in the attic of a house. It can be moved without disconnecting any refrigerant lines.


Particles are tiny substances measuring less than 100 microns in diameter. Indoor air can be filled with particles, some of which can be seen with the naked eye, such as dust and dirt. Others can only be seen with a microscope, such as bacteria and viruses, which typically measure only one micron or smaller in diameter. All of these airborne particles can have significant impacts on occupants’ health and comfort.

Particulate Matter

A state of matter in which solid or liquid substances exist in the form of aggregated molecules or particles. Airborne particulate matter is typically in the size range of 0.01 to 100 micrometers.


Fine liquid or solid particles such as dust, smoke, mist, fumes, and fog found in air and emissions.


A metering device used in heating, ventilation, and air conditioning (HVAC) systems. They are often made of brass and have a fixed bore and a hole in the center that allows refrigerant to pass through.


HVAC term for an air compartment connected to a duct or ducts.

Positive Pressure

Condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.

Pressure, Static

In flowing air, the total pressure minus velocity pressure. The portion of the pressure that pushes equally in all directions.

Pressure, Total

In flowing air, the sum of the static pressure and the velocity pressure.

Pressure, Velocity

In flowing air, the pressure due to the velocity and density of the air.

Preventive Maintenance

(Or, Preventative Maintenance) Regular and systematic inspection, cleaning, and replacement of worn parts, materials, and systems. Preventive maintenance helps to prevent parts, material, and systems failure by ensuring that parts, materials and systems are in good working order.

Programmable Thermostat

A type of thermostat that allows the user to program into the devices’ memory a pre-set schedule of times (when certain temperatures occur) to turn on HVAC equipment.


A hydrocarbon gas, C3H8, occurring in crude oil, natural gas, and refinery cracking gas. It is used as a fuel, a solvent, and a refrigerant. Propane liquefies under pressure and is the major component of liquefied petroleum gas (LPG).


Two quantities are said to be proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio. Proportion also refers to the equality of two ratios.

Proportional Band

The change in input required to produce a full range of change in the output due to the proportional control action. Or simply, it is the percent change of the input signal required to change the output signal from 0% to 100%.

Proportional Integral Derivitave

A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems. A PID controller attempts to correct the error between a measured process variable and a desired setpoint by calculating and then outputting a corrective action that can adjust the process accordingly and rapidly, to keep the error minimal.


A communications protocol is the set of standard rules for data representation, signaling, authentication and error detection required to send information over a communications channel.

PSI (pounds per square inch)

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)

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)

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


The analysis of atmospheric conditions, particularly moisture in the air.


Polyvinyl chloride; a type of plastic.


R-22 Refrigerant

The old standard for residential air conditioners, now being phased out by the U.S. EPA. Lennox offers dry-charged units for those who still have R-22 compatible systems.

R-410A Refrigerant

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

Radiant Barrier

In HVAC a thin, reflective foil sheet that exhibits low radiant energy transmission and under certain conditions can block radiant heat transfer; installed in attics to reduce heat flow through a roof assembly into the living space.

Radiant Ceiling Panels

Ceiling panels that contain electric resistance heating elements embedded within them to provide radiant heat to a room.

Radiant Energy

Energy that transmits away from its source in all directions.

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. The whole floor is evenly heated. Thus, the room heats from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.

Radiant Heat Transfer

Radiant heat transfer occurs when there is a large difference between the temperatures of two surfaces that are exposed to each other, but are not touching.

Radiant Heating System

HVAC term for a heating system where heat is supplied (radiated) into a room by means of heated surfaces, such as electric resistance elements, hot water (hydronic) radiators, etc.


A room heat delivery (or exchanger) component of a hydronic (hot water or steam) heating system; hot water or steam is delivered to it by natural convection or by a pump from a boiler.

Radiator Vent

A device that releases pressure within a radiator when the pressure inside exceeds the operating limits of the vent.


Situation that occurs when the air being exhausted from a building is immediately brought back into the system through the air intake and other openings in the building envelope.


A storage tank for holding liquid refrigerant that is located after the condenser in a refrigeration system.

Reciprocating Compressor

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

Recirculated Air

Air removed from the conditioned space and used for ventilation, heating, cooling, humidification, or dehumidification.


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 evaporate and absorb heat.

Refrigerant Charge

The amount of refrigerant in a system.

Refrigerant Lines

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


The process of the absorption of heat from one location and its transfer to another for rejection or recuperation.

Refrigeration Capacity

A measure of the effective cooling capacity of a refrigerator, expressed in Btu per hour or in tons, where one (1) ton of capacity is equal to the heat required to melt 2,000 pounds of ice in 24 hours or 12,000 Btu per hour.

Relative Humidity

A measure of the percent of moisture actually in the air compared with what would be in it if it were fully saturated at that temperature. When the air is fully saturated, its relative humidity is 100 percent.

Return Air

Air that is returned to a heating or cooling appliance from a heated or cooled space.

Return Duct

The central heating or cooling system contains a fan that gets its air supply through these ducts, which ideally should be installed in every room of the house. The air from a room will move towards the lower pressure of the return duct.


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.

Seasonal Energy Efficiency Ratio (SEER)

A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season.

SEER (Seasonal Energy Efficiency Ratio)

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.

Self-Contained System:

A package unit.


Any device that receives a signal or stimulus (such as heat, pressure, light or motion etc.) and responds to it in a distinctive manner.


Part of the thermostat which indicates the desired indoor temperature.

Setback Thermostat

A thermostat that can be set to automatically lower temperatures in an unoccupied house and raise them again before the occupant returns.


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.


A collection of computer programs, procedures and documentation that perform some tasks on a computer system.

Specific Humidity

The weight of water vapor, per unit weight of dry air.

Split System Air Conditioner

HVAC term for an air conditioning system that comes in two to five pieces: one piece contains the compressor, condenser, and a fan; the others have an evaporator and a fan. The condenser, installed outside the house, connects to several evaporators, one in each room to be cooled, mounted inside the house. Each evaporator is individually controlled, allowing different rooms or zones to be cooled to varying degrees.

Spine Fin

The aluminum outdoor coil made of tiny spines. This creates a greater surface area, which improves heat exchange efficiency. This technology is patented.

Stand-by Heat Losses

A term used to describe heat energy lost from a water heater tank.

Static Pressure

Condition that exists when an equal amount of air is supplied to and exhausted from a space. At static pressure, equilibrium has been reached.

Storage Water Heater

A water heater that releases hot water from the top of the tank when a hot water tap is opened. To replace that hot water, cold water enters the bottom of the tank to ensure a full tank.

Supply Duct

HVAC term for the duct(s) of a forced air heating/cooling system through which heated or cooled air is supplied to rooms by the action of the fan of the central heating or cooling unit.


Tankless Water Heater

A water heater that heats water before it is directly distributed for end use as required; a demand water heater.

Temperature Zones

In HVAC individual rooms or zones in a building where temperature is controlled separately from other rooms or zones.


A unit of heat containing 100,000 British thermal units(BTU).


A thermidistat is a device that monitors indoor temperature, humidity levels and automatically adjusts your heating or cooling system to maintain desired levels.


A wall mounted device that monitor and controls the output of an HVAC system.

Thermostatic Expansion Valve

A device that creates a constant evaporator temperature by regulation of refrigerant flow through the system.

Ton (Air Conditioning)

A unit of air cooling capacity; 12,000 Btu per hour.

Touch Screen

A display which can detect the presence and location of a touch within the display area. The term generally refers to touch or contact to the display of the device by a finger or hand. Touchscreens can also sense other passive objects, such as a stylus.


A device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer (for example, pressure sensors).


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.



This refers to the resistance of heat flow through building materials


Underwriters Laboratories.

Upflow Furnace

Describes an air handler or furnace that is installed in an upright position and circulates air through the side or bottom and out through the top. Typically used in basements, closets and attic installations.

Unit Ventilator

HVAC term for a fan-coil unit package device for applications in which the use of outdoor- and return-air mixing is intended to satisfy tempering requirements and ventilation needs.



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

Vapor Retarder

A material that retards the movement of water vapour through a building element (walls, ceilings) and prevents insulation and structural wood from becoming damp and metals from corroding. Often applied to insulation batts or separately in the form of treated papers, plastic sheets, and metallic foils.

Variable Air Volume System (VAV)

A HVAC system that has a stable supply-air temperature, and varies the air flow rate to meet the temperature requirements. Compared to CAV systems, these systems waste less energy through unnecessarily-high fan speeds. Most new commercial buildings have VAV systems.

Variable Refrigerant Flow (VRF)

Also known as variable refrigerant volume (VRV), is an HVAC technology invented in Japan by Daikin company in 1982. Like ductless minisplits, VRFs use refrigerant as the cooling and heating medium.

Variable Speed Motor

Ideal for high-efficiency air conditioners, a 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 most quiet operation, highest energy efficiency and least stress on mechanical parts.


A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.

Vent Damper

HVAC term for a device mounted in the vent connector that closes the vent when the heating unit is not firing. This traps heat inside the heating system and house rather than letting it draft up and out the vent system.

Vent Pipe

A tube in which combustion gases from a combustion appliance are vented out of the appliance to the outdoors.

Vented Heater

A type of combustion heating appliance in which the combustion gases are vented to the outside, either with a fan (forced) or by natural convection.


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

Ventilation Air

Defined as the total air, which is a combination of the air brought inside from outdoors and the air that is being re-circulated within the building. Sometimes, however, used in reference only to the air brought into the system from the outdoors; this document defines this air as “outdoor air ventilation.”

Ventilation Rate

The rate at which indoor air enters and leaves a building. Expressed in one of two ways: the number of changes of outdoor air per unit of time (air changes per hour, or “ach”) or the rate at which a volume of outdoor air enters per unit of time (cubic feet per minute, or “cfm”).


Is the derived unit for electrical potential and electromotive force.


The force pushing electrical current along wires and cables.



A unit of electrical power. It is equal to the flow of one amp at a potential difference of one volt, and does the same amount of work as 1 joule.


In HVAC caulking and weather-stripping to reduce air infiltration and exfiltration into/out of a building.


A material used to seal gaps around windows and exterior doors.

Wet Bulb Thermometer

A thermometer that measures the relative humidity in the air.



In HVAC an area within the interior space of a building, such as an individual room(s), to be cooled, heated, or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.


The combining of rooms in a structure according to similar heating and cooling patterns. Zoning requires using more than one thermostat to control heating, cooling, and ventilation equipment.