ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
absolute air filter

filter having an efficiency of 99.90% or higher, capable of dealing with particle sizes down to 0.01 micrometer, a HEPA filter. See HEPA filter.

absolute humidity

In a mixture of water vapor and dry air, the ratio of the mass of water vapor to the volume occupied by the mixture.

absolute pressure

a positive value of a pressure when the datum pressure is absolute zero.

absolute temperature

temperature as measured above absolute zero.

absolute vacuum

space totally void of matter (theoretical).

absolute viscosity
absolute zero temperature

zero point on an absolute temperature scale. See Kelvin temperature, Rankine temperature.


that substance absorbed by an absorbent.


material that, due to an affinity, extracts one or more substances from a liquid or gaseous medium with which it is in contact and which changes physically or chemically, or both, during the process. Calcium chloride is an example of a solid absorbent, while solutions of lithium chloride, lithium bromide, and the ethylene glycols are examples of liquid absorbents.


(1) device containing fluid, or other material, for absorbing refrigerant vapor or other vapors. (2) part of the solar collector receiving the incident radiation energy and transforming it into thermal energy. It may possess a surface through which energy is transmitted to the transfer fluid, however, the transfer fluid itself can be the absorber.

absorber area

the area of the absorber medium if both transfer fluid and solid surfaces jointly perform the absorbing function.

absorber spray pumps

recirculate absorbent solution over the absorber tube bundle to ensure adequate wetting of the absorber surfaces. These pumps are not found in all equipment designs.

absorber surface

(1) absorbed portion of the radiant energy striking a surface. (2) ratio of the radiant flux absorbed by a body to that incident upon it. Compare to reflectance.


(1) a process whereby a porous material extracts one or more substances from an atmosphere, a mixture of gases, or a mixture of liquids. (2) absorption of acoustical energy by acoustical materials and air. (3) transformation of radiant energy to a different form of energy by interaction with matter.

absorption chiller

absorption chillers differ from mechanical vapor compression chillers because they utilize a thermal or/and chemical process to produce the refrigeration effect necessary to provide chilled water. There is no mechanical compression of the refrigerant taking place within the machine, as occurs within more traditional vapor compression type chillers.

absorption coefficient

ratio of the radiant flux absorbed by a body to that incident upon it.

absorption hygrometer (chemical hygrometer)

(1) chemicals impregnated into small paper cards that change color with specific relative humidities. (2) instrument in which the relative humidity is determined from the absorption of water vapor by a hygroscopic material.

absorption refrigerating system

refrigeration is created by evaporating a refrigerant in a heat exchanger (evaporator) with the vapor then absorbed by an absorbent medium from which it is subsequently expelled by heating at a higher partial vapor pressure (in a generator) and condensed by cooling in another heat exchanger (condenser).


absorbed portion of the radiant energy striking unit area of a substance. Compare to absorptance.


the vector quantity that specifies the time rate of change of velocity.

acceleration due to gravity

rate of increase in velocity of a body falling freely in a vacuum, its value varies with latitude and elevation. The International Standard, derived from the value at sea level and 45 deg latitude, is 9.806 65 meters per second per second (m/s2), or 32.174 feet per second per second (ft/s2).


a transducer that converts a mechanical input acceleration to an electrical output that is proportional to the input acceleration.

acceptable indoor air quality

air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.

acceptable indoor environment

an environment that has been determined to be acceptable and suitable for the purposes of the intended occupancy according to the process that defines acceptability and by the individuals involved in this process who make the evaluations, assessments, or judgments that are part of the process. These individuals decide what is acceptable for them, whether they are occupants, operators, owners, or visitors.

acceptable performance

a component or system able to meet specified design parameters under actual load.

acceptable thermal environment

an environment that a substantial majority of the occupants would find thermally acceptable.

acceptable vulnerability

the level of vulnerability that has been deemed by the decision maker to be acceptable based on the level of risk, the potential consequences, and other factors.


a formal action, taken by a person with appropriate authority (which may or may not be contractually defined) to declare that some aspect of the project meets defined requirements, thus permitting subsequent activities to proceed.

acceptance angle

the angular zone within which radiation is accepted by the receiver of a concentrator. Radiation is said to be accepted because radiation incident within this angle reaches the absorber after passing through the aperture.

acceptance representative

an entity identified by the owner who leads, plans, schedules, and coordinates the activities needed to implement the building acceptance testing activities. The acceptance representative may be a qualified employee or consultant of the owner. The individual serving as the acceptance representative shall be independent of the project design and construction management, though this individual may be an employee of a firm providing those services.

access control

a method for regulating or restricting access to network resources or a physical space.

access door

movable panel mounted in a surface of an enclosure in order to permit inspection of the inside.

access panel

accessories intended to permit access into ducts, they are positioned in proximity to all those internal parts which require inspection and/or maintenance, such as fire dampers.


as applied to equipment and components, equipment or components where close approach is not prevented by locked doors, elevation, or other barriers or obstructions.

accidental release

unanticipated emission of a regulated substance or other extremely hazardous substance into the ambient air from a stationary source.


(1) apparatus to store cold by accumulation of ice on a coil. (2) pressure vessel connected to more than one circuit of a pneumatic system to obtain the average pressure of the connected circuits. (3) vessel for storing low pressure side liquid refrigerant, also known as a low pressure receiver, surge drum, surge header, or surge tank.


(1) degree of freedom from error, that is, the degree of conformity to truth or to a rule. Accuracy is contrasted with precision (e.g., four place numbers are less precise than six place numbers). (2) the ability of an instrument to indicate or record the true value of a measured quantity. The error of indication, which is the difference between the indicated value and the true value of the measured quantity, expresses the accuracy of an instrument.

acetylene welding

welding using an acetylene gas torch for fusing a selected metal (welding rod) in such a position as to fill the space where a junction is to be made.

acoustic anemometer

device to sense velocity of air at a point by use of the Doppler effect on the velocity of sound. It may also sense temperature.

acoustic environment

characteristics of a room that determine the qualities of sound therein, relative to hearing.

acoustical Doppler effect

change in pitch of a sound observed when there is relative motion between source and observer.

acoustical thermometer

A thermometer functioning via the measurement of the speed of sound in a gas. Used for low temperatures.

acoustically isolated duct

ductwork for which, in all frequency bands of interest, the breakout sound level is at least 10 dB less than the transmitted sound level of the terminal unit under test.


(1) characteristics of a room that determine the qualities of sound therein, relative to hearing. (2) science of the production, transmission, and effects of sound.

activated alumina desiccant

form of aluminum oxide that absorbs moisture.

activated carbon

carbon, usually in the form of granules, treated to enhance its surface area and consequent ability to adsorb gases through a highly developed pore structure.

active chilled beam

convector with integrated air supply where primary air plus induced air pass through the cooling coil(s). Cooling medium is generally water.

active power

the product of the voltage across a branch of an alternating current circuit and the component of the electric current that is in phase with the voltage.

active tracer gas release

controlled release of a tracer gas by a pressurized system or pump. (Term is used in ventilation rates measurement.)


device, either electrically, pneumatically, or hydraulically operated, that acts as a motor to change the position of movable devices such as valves or dampers.

acute toxicity

the adverse health effect(s) from a single, short-term exposure, as might occur during an accidental release of refrigerants or other toxic chemicals.

acute-toxicity exposure limit (ATEL)

the refrigerant, or chemical, concentration limit and intended to reduce the risk of acute toxicity hazards in normally occupied, enclosed spaces. ATEL values are similar to the immediately dangerous to life or health (IDLH) concentrations set by the National Institute of Occupational Safety and Health (NIOSH). ATELs include explicit, additional components for cardiac sensitization and anesthetic effects, but they do not address flammability. The lowest of the ATEL, 50,000 ppm by volume, or 10% of the lower flammability limit, therefore, provides a conservative approximation to IDLH concentrations when needed for refrigerants without adopted IDLH values.

adapted plants

plants that reliably grow well in a given habitat with minimal attention from humans in the form of winter protection, pest protection, water irrigation, or fertilization once root systems are established in the soil. Adapted plants are considered to be low maintenance but not invasive.

adaptive model

a model that relates indoor design temperatures or acceptable temperature ranges to outdoor meteorological or climatological parameters.


set of revisions to a document, contract, or standard in the form of a supplement.


an extension or increase in floor area or height of a building outside of the existing building envelope.

adiabatic compression

Compression (of a gas) during which no heat is exchanged with the surroundings.

adiabatic efficiency

(1) efficiency with which work is done with respect to heat gains or losses. (2) (indicated efficiency) ratio of the work absorbed in compressing a unit mass of refrigerant in an ideal compressor (isentropic process) to the work absorbed in compressing the same mass in an real compressor. See isentropic process.

adiabatic expansion

expansion of a fluid during which no heat is exchanged with the surroundings.

adiabatic process

thermodynamic process during which no heat is extracted from or added to the system.

adiabatic saturation

evaporating water into air without external gain or loss of heat. Sensible heat in both air and water becomes latent heat in evaporated vapor. The air is cooled and humidified.

adiabatic saturation temperature

the temperature at which water (liquid or solid) at temperature t, by evaporating into moist air at dry-bulb temperature t and humidity ratio W, can bring air to saturation adiabatically at the same temperature t while total pressure p is constant. Also known as thermodynamic-wet-bulb temperature.

adjustable-frequency drive (AFD)
adjusted net total capacity

the gross sensible capacity less the actual fan power. (Also see gross sensible capacity.)

adopting authority

the agency or agent that adopts a standard.


material that has the ability to cause molecules of gases, liquids, or solids to adhere to its surfaces without changing the material physically or chemically. Certain commercially available solid materials, such as silica gel, activated carbon, and activated alumina, have this property.


(1) process in which fluid molecules are concentrated on a surface by chemical or physical forces or both. (2) surface adherence of a material in extracting one or more substances present in an atmosphere or mixture of gases and liquids, unaccompanied by physical or chemical change.

adsorption isotherm

a curve obtained by plotting, at a constant temperature, the quantity of adsorbate against the concentration of the substance in the original gas or solution.

Adsorption, Chemical (Chemisorption)

Binding of a contaminant to the surface of an adsorbent by forces with energy levels approximately those of a chemical bond. Chemisorption is an irreversible process.


ASHRAE Advanced Energy Design Guide


exposing a substance or area to the circulation of air.

aerodynamic excitation

time varying loads acting on the blades of a fan due to nonconformities of the airflow. Note: spatial nonuniformities of airflow, which are steady in time, give rise to harmonic excitation at frequencies which are integer multiples of the rotation rate of the fan. Time excitations of the airflow give rise to random excitation. Turbulence of the airflow gives rise to random excitation.


small particles (solid or liquid) suspended in air (e.g., dust, fumes, fog, and smoke). The diameter of the particles may vary from micrometers (formerly micron) down to less than 0.01 micrometer. See fume.

age of air

time of passage of air from one point to another within an indoor space.


The formation of a larger airborne particle by the collision of two or more smaller particles. Agglomeration takes place when the attractive forces between the particles are greater than the kinetic energy of collision.

aggregated pricing node

An aggregated pricing node is a location in the electric grid where the locational marginal price is calculated by a weighted average of one or more underlying pricing nodes.


device to create turbulent motion in a fluid, usually inside a vessel.

agricultural land

land that is, or was within ten years prior to the date of the building permit application for the building project, primarily devoted to the commercial production of horticultural, viticultural, floricultural, dairy, apiary, vegetable, or animal products

air atomizing burner

burner in which the oil is atomized by compressed air which is forced into and through one or more streams of oil, breaking the oil into a fine spray.

air bound

condition where the introduction of air causes either a flow restriction or a malfunction of the system.

air change

a measure of the amount of air moving into and out of a space because of leakage or mechanical ventilation. One air change is a volumetric flow of air equal to the cubic content of the space. For example, if a space has a cubical content of 10,000 cubic feet and the ventilation rate is 1000 cfm, 0.1 (1000/10,000) air change is occurring every minute, or 6 (60 × 0.1) air changes occur per hour.

air change effectiveness

Ratio of the room mean age of air for an ideal plug (piston) flow pattern compared to the room mean age of air for the real flow at a test location.

air change rate

volume of air supplied to and removed from a space, via mechanical systems or through the building enclosure, per unit of time divided by the volume of the space, using the same units for volume such that the unit is inverse time.

air changes

expression of the amount of air movement or air leakage into or out of a building in terms of the number of building volumes or room volumes exchanged.

air changes per hour (ACH)

ventilation airflow divided by room volume. It indicates how many times, during one hour, the air volume from a space is replaced with outdoor air.

air circulation

motion of air, either natural or induced.

air cleaner

device used to remove airborne impurities from air. Compare to filter. See also precipitator.

air cleaning

the use of equipment that removes particulate, bioaerosols (pathogens), and gaseous contaminants (including odors) from air.

air conditioner

assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion. Compare to air-conditioning system.

air conditioning

the process of treating air to meet the requirements of a conditioned space by controlling its temperature, humidity, cleanliness, and distribution.

air contaminant

unwanted airborne constituent that may reduce acceptability of the air.

air curtain

(1) controlled stream of air moving across the height and width of an opening with sufficient velocity and volume to reduce the infiltration or transfer of air from one side of the opening to the other and to inhibit insects, dust, and debris from passing through. (2) In a refrigerated display cabinet, air flow designed to go from the air discharge toward the air return, thereby limiting both heat and mass transfers between the cabinet's net volume and the surrounding environment.

air density

the mass per unit volume of the air. See standard air.

air device

a diffuser, grille, register, or transfer grille used for controlling air patterns or air distribution or for preventing line of sight through an opening while allowing airflow through. See air terminal device or diffuser.

air diffuser

see diffuser.

air diffusion

(1) distribution of the air in a space, called the treated space, in a manner to satisfy certain specified conditions such as air change rate, pressure, cleanliness, temperature, humidity, air velocity, and noise level, in a specified zone within this treated space, which is called the occupied zone. It is usually achieved by means of air terminal devices which form the common boundaries between the treated space and the air-distribution system. (2) the introduction of air into a building space for the purpose of providing acceptable velocity and temperature distribution in the occupied zone.

air diffusion component

there are three main categories of components: air-terminal devices (ATDs), complementary accessories to air terminal devices, fixing accessories for air-terminal devices.

air discharge coefficient

ratio of the net area at vena contracta of air flowing through an orifice to the total free area of the opening.

air dispersion systems

any diffuser system designed to, both, convey air within a room, space, or area and diffuse air into that space while operating under positive pressure. Systems are commonly constructed of, but not limited to, fabric or plastic film.

air distribution

transportation of a specified airflow to or from the treated space, by ducts or plenums. Air-treatment devices can be added to the distribution system for the purpose of treating the air (e.g., cleaning, heating, cooling, humidifying or dehumidifying, etc.)

air distribution component

there are three main categories of components: elements of distribution (components for the purpose of ensuring a correct distribution of the air), air terminal units (ATUs) [equipment inserted into or added to the ends of ducts for the purpose of controlling one or more of various parameters such as velocity, pressure, flow rate, and temperature (also see air terminal unit)], and accessories of distribution (components ensuring the fitting and fixing in place of the elements of distribution and their inspection and maintenance).

air drop

vertical distance between the base of an air outlet and the bottom of the airstream at the end of the air throw. Note: drop occurs from the natural expansion of the airstream.

air duct

a tube or conduit used for conveying air. The air passages of self-contained systems are not air ducts.

air dumping

rapid falling of cold air that occurs when a variable-air volume (VAV) box or other device reduces airflow and the supply air leaves the diffuser at very low velocity.

air economizer

duct and dampers arrangement with an automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather.

air eliminator

(1) in a hydronic system, a fitting or tank mounted in the piping system that separates entrained air from the water and discharges the air to an air vent. (2) in a steam system, a device that closes if either steam or water is present in the vent body and opens when air or noncondensables reach it.

air entrainment

capture of part of the surrounding air by an airstream discharged from an outlet, sometimes called secondary air motion.

air exchange

exchange of outdoor air with the air already in a building can be divided into two broad classifications: ventilation and infiltration.

air extraction cooker hood

an assembly that hangs above the stove or cooktop in the kitchen and contains a skirt or capture panel to contain the rising gases or effluent plume, one or more grease filters, and a fan or tangential blower for forced ventilation used to remove airborne grease, combustion products, smoke, odors, heat, and steam from the air by a combination of filtration and evacuation of the air outside of the building. Air extraction cooker hoods may be ducted (or vented) application, or ductless (or recirculating) application. Ductless or recirculating hoods use particulate and gaseous filtration (activated charcoal) to clean the air to an acceptable quality for returning to the room. Hoods may include lighting. See also cooker hood, Hood, Type I Hood, Type II Hood.

air films

interior and exterior air surface film coefficients for winter and summer design conditions.

air filter bypass

unfiltered air that passes through the air-handling unit (AHU) filter installation but remains unfiltered because it bypasses the installed air filters.

air infiltration

the uncontrolled inward airflow through openings in the building envelope caused by the pressure effects of wind, the effect of differences in indoor and outdoor air density, or both (cfm) [m3/s].

air inlet

device or opening through which air is withdrawn from or discharged into a conditioned space (grilles, registers, diffusers, and slots may be used as air inlets).

air leakage

1) the flow of air through the building envelope caused by a specified pressure difference, a measure of airtightness, cfm at fixed pressure (m3/s at fixed pressure). 2) undesirable or unwanted leakage of air from within a component within an air-distribution system that could include such items as ducts, air terminal devices, and AHUs.

air leakage factor

airtightness expressed as the air leakage rate per unit envelope or unit area.

air leakage rate

air leakage of a component or components subjected to air pressure.

air lock

compartment whose purpose is to control air exchange into or out of a conditioned space. Two individual closures usually are used to restrict air transfer by keeping one of them closed.

air main

(1) pipe carrying air to the laterals supplying ice cans contained in a freezing tank. (2) supply line from air compressor or central pressure reducing station to branch feeder in a pneumatic control installation. (3) tube carrying the supply of compressed air to the components of a pneumatic control system.

air mass

(1) any combination of outdoor and transfer air intended to replace exhaust air and exfiltration. (2) totality of gas molecules in a defined volume of air.

air motor

(1) air-operated device that is used primarily for opening or closing pneumatic valves and dampers. (2) device that converts compressed air into mechanical force. (3) pneumatic operator, a final control device that assumes a position as directed by an input pressure signal.

air outlet

device or opening through which air is discharged into a ventilated space.

air permeance

Ka is the time rate of air transfer through a unit surface of a porous membrane or layer induced by a unit air pressure difference over that layer. Expressed in units of lbm/ft2·s·in., Hg [kgm/(m2·s·Pa), kg/(m·s·Pa), or kg/(s·Pa)].

air pollutant

any material in the atmosphere that affects persons and their environment (pollutants include materials such as liquids, solids, aerosols, gases, and odors).

air pollution

result of the presence of air pollutants in the atmosphere.

air power

(1) (theoretical), power required to drive a fan or blower as though there were no losses in the fan or blower (100% efficiency). (2) operational power required to move air at a given rate of flow against a given resistance. The ratio of air power to input power of a fan or blower is termed efficiency.

air recirculating kitchen hood

kitchen hood containing filters to remove contaminants, after which the treated air is recirculated to the room.

air separation (air fractionation)

separation of the various components of air through distillation at very low temperature.

air separation unit

device used to remove air from another fluid stream. See also purge.

air shutter

an adjustable device for varying the amount of primary air entering atmospheric or powered burners.

air speed

the rate of air movement at a point, without regard to direction.

air speed, average (Va)

the average air speed surrounding a representative occupant. The average is with respect to location and time. The spatial average is for three heights as defined for average air temperature ta. The air speed is averaged over an interval not less than one and not more than three minutes. Variations that occur over a period greater than three minutes shall be treated as multiple different air speeds.

air splitter

blade or blades fitted across an air duct to divide the airstream into a number of streams in parallel. Should not be used in current engineering practice due to excessive pressure drop and noise considerations.

air spread

divergence of an airstream after it leaves an outlet.

air stratification

the layering of air within a space due to density differences caused by temperature distribution of the air.

air system balancing

adjusting airflow rates through air distribution system devices, such as fans and diffusers, by manually adjusting the position of dampers, splitter vanes, extractors, etc., or by using automatic control devices, such as constant-air-volume or variable-air-volume (VAV) boxes.

air temperature

the temperature of the air measured at a test point.

air terminal
air terminal device

any device (e.g., grille, register, diffuser) placed in an opening to a room, through which controlled air enters or leaves. Component of the air-distribution system which has the purpose of achieving the predetermined movement of air into or from a treated space.

air terminal unit

(1) an air-distribution assembly consisting of inlet duct connection(s) and outlet duct connection(s) whose purpose fulfills (either manually or automatically) one or more of the following functions: controls the rate of the airflow, controls the velocity or pressure and/or temperature of the air, mixes primary streams of different temperatures or humidities, or mixes within the device primary air with air from the treated space. (2) an air terminal unit may be composed of automatic or manual dampers, filters valves, heating or cooling coils, sound attenuation, nozzles, or fan assemblies. See also diffuser terminal.

air throw

horizontal or vertical axis distance an airstream travels after leaving an air outlet before the stream velocity is reduced to a specific terminal value. The rated throw will be a function of a predetermined terminal velocity.

air transport factor

ratio of the rate of useful, sensible heat removal from the conditioned space to the energy input to the supply and return fan motor(s), expressed in consistent units and within designated operating conditions.

air treatment

process by which the state of the air is modified with respect to various properties such as temperature, moisture content, dust content, bacterial count, gas, and vapor contents.

air valve

device to control volume and flow in air distribution. See also damper.

air velocity

rate of motion of air in a given direction, measured as distance per unit time. See also velocity.

air vent

manual or automatic device for removing air from circulating hot or chilled water systems, also used for removing air from steam systems. See air eliminator.

air washer

A unit spraying, atomizing, or down-flowing water (or any type of solution, e.g., dehydrating) into the air stream and capable of heating, cooling, humidifying, or dehumidifying the air according to the temperature, hygrometric, or concentration characteristics of the fluids present. Can be used for the purpose of removing particulate matter from the airstream See scrubber.

air-blast cooling

cooling by forced circulation of air at high velocity. Compare to jet cooling.

air-cleaning system

a device or combination of devices applied to reduce the concentration of airborne contaminants such as microorganisms, dusts, fumes, respirable particles, other particulate matter, gases, vapors, or any combination thereof.

air-conditioner capacity

useful net, available refrigerating capacity of an air conditioner for removing sensible and latent heat from the space being conditioned.

air-conditioning installation

combination of all components required to provide air conditioning.

air-conditioning process

in enclosed spaces, combined treatment of the air to control, as specified, temperature, relative humidity, velocity of motion, and radiant heat energy level, including consideration of the need for removal of airborne particles and contaminant gases. Some partial air conditioners that may not accomplish all of these controls are selected for their capability to control specific phases of air treatment.

air-conditioning system

assembly of equipment for air treatment to simultaneously control its temperature, humidity, cleanliness, and distribution to meet the requirements of a conditioned space. See air conditioner.

air-cooled air conditioner

air conditioner whose refrigerating system has an air-cooled condenser.

air-cooled condenser

a refrigerant condenser in which heat removal is accomplished entirely by heat absorption by air flowing over condensing surfaces. See also condenser, desuperheater, double-pipe condenser (tube-in-tube condenser), evaporative condenser, shell-and-tube condenser.

air-cycle refrigeration

a refrigeration cycle consisting of four stages: compression of air, cooling the air down to ambient temperature, expansion of the air, and heating of the cold air by heat absorption in the space to be cooled.

air-delivery rate

the air volume flow rate per unit area of the entire floor space being conditioned.

air-diffusing ceiling

modular air-terminal device designed to diffuse air to the treated space from a pressurized plenum through holes or slots in the ceiling surface or the supporting framework. Also see air terminal device.

air-diffusion performance index (ADPI)

a single number rating of the air-diffusion performance of a mixing system at specified supply-air delivery rate, temperature, moisture content, and space cooling load. ADPI is based on air speed and effective draft temperature.

air-distribution envelope

surface or locus of points of equal velocity which describe an air-distribution profile.

air-handling unit (AHU)

Assembly consisting of sections containing a fan or fans and other necessary equipment to perform one or more of the following functions: air circulation, filtration, heating, cooling, heat recovery, humidifying, dehumidifying and mixing of air, and necessary control functions.

air-side economizer
air-to-air energy recovery effectiveness

ratio of actual heat transfer to the thermodynamically limited maximum heat transfer possible in a counterflow exchanger of infinite transfer area. Note: effectiveness may be stated as total, sensible, or latent when the ratio defined above uses these heats as the actual and maximum possible heat transfer quantities. Compare air-to-air energy recovery with system efficiency.

air-to-air energy recovery system efficiency

ratio of the apparent heat recovered to the sum of the thermodynamically limited maximum possible in a counterflow heat exchanger of infinite heat transfer area; plus, all external energy inputs including, but not limited to, fan energy, auxiliary heaters, cross leakage, and casing loss. Note: This ratio adjusts heat exchanger effectiveness for auxiliary energy inputs of the energy recovery system.

air-to-air heat exchanger

exchanger that transfers heat from an exhaust airstream to a separated supply airstream. Note: fixed plate, rotary wheels, heat pipes, runaround coil loops, and shell and tube condensers are the most common types.

air, exhaust
air, makeup

see makeup air.

air, outdoor
air, recirculated
air, standard
air, supply

see supply air.

air, transfer
air, ventilation
air/cloth ratio

value used in dust collector calculations to measure air velocity through the bag filter media. Note: calculated by dividing active volumetric airflow by the effective area of the cloth media.

air/gas ratio

ratio of the air volume to the gas volume. A specified ratio is necessary to achieve a desired character of combustion.

air/mass ratio

ratio of the mass of atmosphere in the actual earth to sun path to the mass which would exist at sea level if the sun were directly overhead.

air/media ratio

value used in dust collector calculations to measure air velocity through non-cloth bag filter media (e.g., paper cartridges). Note: calculated by dividing active volumetric airflow by the effective area of the cloth media.

airborne droplet nuclei

particles are released when an infected host coughs or sneezes, droplet nuclei are formed when mucus coating these particles evaporates and the virus becomes airborne. Also known as quanta.

airborne infection isolation (AII) room

rooms in which care is administered to patients who have, or are suspected of having, an infectious disease. AII rooms are used to protect the population of a health care facility by placing potentially infectious patients in quarantine.

airborne particles

impurities as solid or liquid particulate matter from whatever source.

airborne sound

sound that reaches the point of detection by radiation through the air.


(1) movement of air usually within boundaries (such as ducts). (2) the volume of air per unit time.

airflow rate (Q)

the volume of standard air per unit of time that moves past a given plane, expressed in cubic feet per minute (cfm) or liters per second (L/s).

airflow resistance

deterrent (due to friction, change of direction, etc.) to the passage of air within an air-distribution system and/or equipment.

airflow retarder

material or construction that adequately impedes transmission of air under specified conditions. Compare to structural barrier, water vapor retarder.


a cross sectional blade shape of a fan type that is used to optimize flow to reduce turbulence.


natural ventilation by window opening.

airtight construction

construction in which the building envelope is designed with a continuous air barrier.


qualitative term describing the integrity of the building envelope relative to air permeation, the resistance of the building envelope to the flow of air and entrained moisture. Compare to air infiltration.

AK factor

the effective area of an air terminal device equal to the measured airflow rate divided by the velocity reading of a particular instrument used in a prescribed manner.


signal, either audible or visual, or both, that alerts an operator to an off-normal condition which requires some form of corrective action.

alarm acknowledgment

the process of indicating that a human operator has seen and responded to an event notification.

alarm point

that point in the range of a variable which is the threshold of an off-normal condition.


ratio of reflected solar radiation from a surface and the incident solar radiation. See also reflectance.


minute, freshwater plants that form a scum on the surfaces of recirculated water apparatus, interfering with fluid flow and heat transfer.


any substance inhibiting the growth of algae.


prescribed set of well-defined rules or processes for the solution of a problem in a finite number of steps (e.g., a full statement of an arithmetical procedure for evaluating sine x to a stated precision).


sum of bicarbonate, carbonate, and hydroxide ions in water. Other ions, such as borate, phosphate, or silicate, can also contribute to alkalinity.


a synthetic hydrocarbon composed of a benzene ring attached to one or more saturated hydrocarbon chains.

allocation device

a device that monitors parameters used to determine the net HVAC energy consumed in conditioning an individual unit.

alpha-value (a-value)

the ratio of path pressure drop, including fully open control damper pressure drop, to the pressure drop across the fully open damper at design flow.


a replacement or addition to a building or its systems and equipment, routine maintenance, repair, and service or a change in the building’s use classification or category shall not constitute an alteration.

alternate on-site sources of nonpotable water

water from on-site sources (i.e., air-conditioning condensate, stormwater, etc.) that is collected and treated to acceptable levels for nonpotable uses.

alternating current

electric current in an electrical circuit that periodically reverses polarity. See also frequency.

alternative energy sources

nondepletable sources alternative to energy derived from combustible waste or heat recovery processes.

ambient air

Air surrounding a building or object, the source of outdoor air brought into a building, etc. (Usually outdoor air or the air in an enclosure under study.)

ambient air conditions

characteristics of the environment. For example, temperature, relative humidity, pressure, and motion.

ambient noise

all noise associated within a given environment at a given time, including noise from the sound source of interest (e.g., background noise plus the particular sound of interest). Compare to background noise.

ambient pressure

(1) environmental pressure in which a device operates. (2) pressure of the medium surrounding a device. (3) the pressure of the surroundings relative to zero absolute pressure.

ambient temperature

(1) The temperature (usually of air, but could be water or earth as well) surrounding a given object, measured at a location undisturbed or unaffected by the temperature of the object. (2) For an air-conditioning or refrigeration system, temperature of the medium to which heat is dissipated (or absorbed in the case of a heat pump system).

ammonia refrigerant

anhydrous ammonia (NH3).


ability of a wire to carry electric current safely without undue heating.


(1) circuit used to increase the power, voltage, and/or current level of a signal. (2) device that enables an input signal to control power from a source independent of the signal and thus be capable of delivering an output that bears some relationship to, and is generally greater than, the input signal. (3) device whose output is an enlarged reproduction of the essential features of an input signal and that draws power from a source other than the input signal.

analog data

data represented in a continuous form, as contrasted with digital data represented in a discrete (discontinuous) form, an analog of the variable represented.

analog display

(1) display of a physical variable in a continuous form. (2) display of analog data values.

analog indication

analog display (e.g., a meter or gage) which shows an analog value to an operator.

analog input

(1) a verifying quantity (e.g., pressure, voltage, or temperature) which can have any value between a minimum and a maximum, used as the initiating part of a control system. (2) input of a physical variable in a continuous form, such as a voltage input to a voltmeter.

analog output

(1) continuous variable used to represent another (e.g., in temperature measurement, an electric voltage or current output represents temperature input). (2) output that is continuously variable and represents a physical variable such as a voltage, current, or pressure output.

analog to digital converter

device that converts a signal that is a function of a continuous variable into a representative number sequence.

analog transmission

(1) method by which analog values are transferred from the sensing location to a controlling location. (2) sending of a continuously variable signal from one point to another.

analytical solution

mathematical solution of a model that has a deterministic result for a given set of parameters and boundary conditions.


device in the high side of an absorption system for increasing concentration of refrigerant in the vapor entering the rectifier or condenser.

anechoic termination

device placed at the end of a test duct to prevent excessive reflection of the sound waves back into the test duct where they would interfere with the waves to be measured.


device to sense and measure velocity of airflow at a point.

aneroid barometer

a barometer in which a change of atmospheric pressure relative to a vacuum bends a metallic surface connected to a pointer.

aneroid capsule

thin, disc-shaped box or capsule, usually metallic, partially evacuated and sealed, held extended by a spring, that expands and contracts with changes in atmospheric or gas pressure.

anesthetic effect

loss of the ability to perceive pain and other sensory stimulation.

angle of discharge

the largest included angle between center lines of principal jets of the primary airstream.

angle of incidence

the angle between the solar beam and the normal to the aperture plane of the solar collector.

angle valve

valve in which the inlet and outlet are at an angle, usually 45° or 90°.

aniline point

the minimum temperature at which a lubricant is soluble in aniline, a solvent for hydrocarbons. It is used to estimate the aromatic/olefin content in a lubricant.


(1) ion that is negatively charged (e.g., chloride, silicate, sulfate). Compare to cation. (2) negatively charged ion of an electrolyte that migrates toward the anode influenced by an electric potential gradient.


(1) process involving controlled heating and subsequent controlled, generally slow, cooling. Applied usually to induce ductility in metals. (2) treatment intended to remove internal stresses, alter mechanical or physical properties, produce a definite microstructure, and remove gases.

annual fuel-utilization efficiency (AFUE)

the ratio of annual output energy to annual input energy, which includes any nonheating season pilot input loss and, for gas- or oil-fired furnaces or boilers, does not include electric energy.

annual heating load

the heating load for the entire one-year simulation period (e.g., for hourly simulation programs, this is the sum of the hourly heating loads for the one-year simulation period).

annual incident unshaded total solar radiation

sum of direct solar radiation and diffuse solar radiation that strikes a given surface for the entire one-year simulation period when no shading is present (e.g., for hourly simulation programs, this is the sum of the hourly total incident solar radiation for the one-year simulation period).

annual mean zone air temperature

the average zone air temperature for the one-year simulation period (e.g., for hourly simulation programs, this is the average of the hourly zone air temperatures for the one-year simulation period).

annual sensible-cooling load

sensible-cooling load for the entire one-year simulation period (e.g., for hourly simulation programs, this is the sum of the hourly sensible cooling loads for the one-year simulation period).

annual transmitted solar radiation

sum of direct solar radiation and diffuse solar radiation that strikes a given surface for the entire one-year simulation period when no shading is present (e.g., for hourly simulation programs, this is the sum of the hourly total incident solar radiation for the one-year simulation period.)

annular flow

A form of gas/liquid two-phase flow in a pipe where the gas forms the core and the liquid flows annularly past the internal walls of the pipe.


positive electrode in an electrolytic system, such as is applied in cathodic protection. The electrode at which oxidation or corrosion occurs or from which the current is transmitted to the electrolyte. Compare to cathode.


American National Standards Institute


resulting from or produced by human activities


substance produced by a living organism (e.g., fungi, bacteria) that inhibits the growth of, or kills, another living organism. See also bactericide.

anticipating control

control methodology that is actuated faster than normal to produce a smaller differential of the controlled variable.


typical additives that scavenge oxygen-containing species to prevent further breakdown of the lubricant or refrigerant.

antisiphon valve

valve or mechanical device that eliminates siphon flow.

antiwear/extreme pressure additive

typical additives that improve the lubrication when circumstances of boundary lubrication (lubricant film break through) are present.

aperture area

the maximum projected area of a solar collector through which the unconcentrated solar radiant energy is admitted.

aperture plane

the projected plane at or above the solar collector through which the unconcentrated solar radiation is admitted.


testing facility or laboratory and instrumentation.

apparatus dew point (ADP)

the effective coil surface temperature when there is dehumidification. This is the temperature to which all the supply air would be cooled if 100% of the supply air contacted the coil. On the psychrometric chart, this is the intersection of the condition line and the saturation curve, where the condition line is the line going through entering air conditions with slope defined by the sensible heat ratio ([gross sensible capacity]/[gross total capacity]). (Also see gross sensible capacity and gross total capacity.)

apparent load

a value that can be calculated for a system based on the quantity of the fluid flow, the average temperature of the entering fluid, and the average temperature of the space.

apparent power

product of the volts and amperes of a circuit. This product generally is divided by 1000 and designated in kilovolt-amperes (kVA). It comprises both real and reactive power.

apparent solar time

time based on the apparent angular motion of the sun across the sky, with solar noon the time the sun crosses the meridian of the observer.

apparent temperature

temperature of an object as determined from the measured radiance.

apparent thermal conductivity

measured property of a material or assembly of materials, heat flows by a combination of conduction, convection, radiation, and latent heat exchange and may depend on orientation, direction, or both. The specific test conditions (i.e., sample thickness, orientation, environment, environmental pressure, surface temperature, mean temperature, temperature difference, and moisture distribution) should be reported with the values. The symbol K is used to denote the lack of pure conduction or to indicate that all values reported are apparent. Materials with a low apparent thermal conductivity are called insulation materials.

apparent volumetric efficiency

on an indicator card, the ratio of suction line length to stroke.

application part-load value (APLV)

part-load value based on operation at actual design conditions. Typically used in system design and specification.

application protocol-control information

information exchanged between application-entities, using presentation services, to coordinate their joint operation (ISO 9545).

application rating

a rating based on tests performed at application rating conditions (other than standard rating conditions).

application-specific controller (ASC)

digital controller dedicated to a specific application, such as a VAV box or water-source heat pump.


(1) in a water cooling tower or evaporative cooling device, the difference between the average temperature of the circulating water leaving the device and the average wet-bulb temperature of the entering air. (2) in heat exchangers, the temperature difference between the leaving fluids.


acceptable to the authority having jurisdiction.

approved agency

an established and recognized agency regularly engaged in conducting tests or furnishing commissioning services, where such agency has been approved.

approved source

an independent person, firm, or corporation, approved by the code official, who is competent and experienced in the application of engineering principles to materials, methods, or systems analyses.

approximate lethal concentration (ALC)

the concentration of a substance, such as a refrigerant, that is lethal to even a single test animal when tested by the same conditions as for an LC50 test.


thermostat designed for use in water.


geologic unit that is capable of yielding groundwater to a well in sufficient quantities to be of practical use.

arc welding

group of welding processes wherein coalescence of metal is produced by heating with an electric arc or arcs, with or without the application of pressure, and with or without filler metal.

Archimedes principle

a body immersed in a fluid undergoes an apparent loss in weight equal to the weight of the fluid it displaces.


designation reserved, usually by law, for a person professionally qualified and duly licensed to perform architectural services, including, but not necessarily limited to, analysis of project requirements, creation and development of project design, preparation of drawings, specifications and bidding requirements, and general administration of the construction contract.

architect engineer

individual offering professional services as both architect and engineer.

area factor (Ak)

a multiplying factor that adjusts the unit power density (UPD) for spaces of various size to account for the impact of room configuration on lighting power utilization. Also see Ak Factor.

arithmetic-mean temperature difference

in a parallel flow or counterflow heat exchanger, the arithmetic mean of the temperature differences between the two fluids at both ends of the exchanger.

aromatic content

the amount of aromatic hydrocarbon fraction contained in an oil.

aromatic hydrocarbon

a compound containing carbon and hydrogen and having a molecular structure of one or more closed ring structures with the carbon atoms joined by double bonds.

artificial intelligence (AI)

capability of a device to perform functions that normally are associated with human intelligence, such as reasoning, learning, and self-improvement. See also expert system; knowledge base system; neural-network computers.

as-built records

documents that represent the actual installed conditions, equipment, and systems, such as drawings, computer graphics, equipment data sheets, operation manuals, maintenance manuals, and the training program and video media.

aseptic environment

a sterile environment.


International, diverse organization representing building system design and industrial processes professionals around the world. (Formally known as American Society of Heating, Refrigerating and Air-Conditioning Engineers.)


International, diverse organization representing building system design and industrial processes professionals around the world. (For the US Internal Revenue Service (IRS), known as the American Society of Heating, Refrigerating and Air-Conditioning Engineers.)

ASHRAE arrestance

quantifies filter efficiency by mass of particles removed or arrestance and is generally only used to measure performance of prefilters or low efficiency filters—efficiencies measured by weight give little indication of their performance for the smallest, lightest particles (the most respirable and hazardous). 

ASHRAE atmospheric dust spot efficiency

measure of the ability of a device to remove the staining portion of atmospheric dust from the test air.

ASHRAE built environment

1. Human-made spaces, such as homes, schools, offices, manufacturing facilities, social recreation settings, or microenvironments, such as cars, buses, trains, subways, and airplanes. 2. places and spaces created or modified by people, including buildings, parks, and transportation systems.

ASHRAE synthetic-arrestance dust

compounded test dust used for arrestance measurement and for loading filters.

aspect ratio

(1) in any rectangular configuration, the ratio of the longer dimension to the shorter. (2) ratio of the length to width of a rectangular air duct.


This is a genus that includes several hundred species of mold. Some species can cause paranasal sinus infections or Aspergillosis. Aspergillus is of particular concern in health care facilities because many patients already have compromised immune systems. If HVAC systems are not properly maintained or designed, mold growth may be likely.

aspirated psychrometer

psychrometer having mechanical means for rapidly circulating air to be tested over dry and wet bulbs.


production of movement in a fluid by suction created by fluid velocity. See also Venturi.


portion of an envelope component represented by an arrangement and connection of building construction materials with a specific thermal transmittance or thermal conductance.


A logical entity with measurable and reportable consumption, e.g., an asset may be a physical device with its own meter or the main meter at the service delivery point of a service location.

ASTM test

a test conducted according to an ASTM International standard test procedure.

athermour barrier

substance or space that does not allow the passage of radiant heat.

atmospheric burner

gas burner in which air for combustion is supplied by natural draft and the inspirating force created by gas velocity through orifices.

atmospheric condenser

condenser in which the pipes in open air are cooled by water flowing over them.

atmospheric dust

particulate matter naturally occurring in the air. See also air contaminant.

atmospheric freeze drying

process in which the solid phase of the solvent is sublimed at atmospheric pressure.

atmospheric pressure

standard atmospheric reference pressure (assumed sea level) is defined by the International Civil Aeronautics Organization (ICAO) as 101.325 kPa. In I-P units, the value is approximately 14.696 psi, or 29.921 inches of mercury at 32°F.

atmospheric tower

(also called natural draft cooling tower), air movement through a cooling tower by aspiration or natural convection.


to create a fine spray from a liquid.


the decrease in the sound level between the source and the receiver from various mechanisms, such as geometrical divergence, atmospheric absorption, and building structures.

attic and other roofs

all other roofs, including roofs with insulation entirely below (inside of) the roof structure (i.e., attics, cathedral ceilings, and single-rafter ceilings), roofs with insulation both above and below the roof structure, and roofs without insulation (excluding metal building roofs).

attic fan

exhaust fan to exhaust air near the top of a building while air, generally cooler, is forced (drawn) in at lower levels.

attic ventilation

introduction of cool, outdoor air into an attic by exhausting its warm air to the outdoors.


(of a controller), ratio of effect on a manipulated variable of one input signal as compared to that of another.

authority having jurisdiction

the agency or agent responsible for enforcing a standard.

authorized personnel

Persons who, by virtue of their training and job description, have been specifically granted permission by the owner to enter a restricted area, perform restricted tasks, or both.


self acting, operating by its own mechanism when actuated by some nonmanual influence, such as a change in current strength, pressure, temperature, or mechanical configuration.

automatic changeover

changeover from one mode of operation to another without operator intervention (e.g., a thermostat that changes from heat to cool without need for manual operation of levers or setpoints).

automatic control device

a device capable of automatically controlling devices without manual intervention.


(1) implementation of process by automatic means. (2) investigation, design, development, and application of methods of rendering processes automatic, self moving, or self controlling. (3) theory, art, or technique of making a process more automatic.

auxiliary air

unconditioned or partially conditioned supply or supplemental air delivered to a laboratory at the laboratory fume hood to reduce room air consumption.

auxiliary devices

(also known as controls), equipment such as relays and switches to manipulate signals.

auxiliary energy
auxiliary fuel

fuel used in an auxiliary thermal source.

auxiliary power

power input to devices that are not integral to the operation of the vapor compression cycle, excluding power input to integrated pumps (if present) used for liquid in either the evaporator or the condenser. Auxiliary power includes devices such as, but not limited to, oil pumps, refrigerant pumps, control power, fans, and heaters.

auxiliary thermal source

a source of thermal energy, other than solar, used to provide the service water heating, usually in the form of electrical resistance heat or thermal energy derived from combustion of fossil fuels.

available energy

energy in the form of shaft work or in a form completely convertible to shaft work by ideal processes.

average age of air

average of local mean ages of air measured throughout an indoor airspace.

average air outlet speed (Vo)

the time-averaged speed of the air from each individual supply air outlet.

average arrestance

for an air cleaning devices with efficiencies less than 20% in the size range of 3.0–10.0 µm, the average value of the arrestances made on the device during the loading test, weighted by the amounts of dust fed to the device during each, incremental, dust-loading step.

average ASHRAE arrestance

the average value of the arrestances made on a single filter during the loading test, weighted by the dust fed to the filter between successive arrestance measurements.

average ASHRAE dust-spot efficiency

the average value of the dust spot efficiencies made on a single filter during the loading test, weighted by the dust fed to the filter during the intervals between successive dust spot tests.

Avogadro’s law

under the same conditions of pressure and temperature, equal volumes of all gases contain equal numbers of molecules.

axial fan

fan that moves air in the general direction of the axis about which it rotates.

axial flow compressor

turbocompressor in which the compressed fluid generally flows in a direction parallel to the axis of rotation.

axonometric drawing

perspective drawing showing plan and partial elevations on the same drawing.


a mixture of liquids whose vapor and liquid phases in equilibrium have identical compositions (the boiling point is constant).

azeotropic blend

a blend containing two or more refrigerants whose equilibrium vapor and liquid phase compositions are the same at a given pressure. At this pressure, the slope of the temperature versus composition curve equals zero, which mathematically is expressed as (dt/dx)p = 0, which, in turn, implies the occurrence of a maximum, minimum, or saddle point temperature. Azeotropic blends exhibit some segregation of components at other conditions. The extent of the segregation depends on the particular azeotrope and the application.

azeotropic point

temperature at which a liquid mixture boils and produces a vapor having the same composition as the liquid.

azeotropic refrigerant

a blend that contains two or more refrigerants whose equilibrium vapor phase and liquid phase compositions are the same at a given pressure. The temperature of an azeotropic refrigerant remains constant as it evaporates or condenses at constant pressure (compare to zeotropic refrigerant).

azeotropic temperature

the temperature at which the liquid and vapor phases of a blend have the same mole fraction of each component at equilibrium for a specified pressure.