A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 378 terms in this directory beginning with the letter A.
Associated Air Balance Council

American Automatic Control Council

American Association of Health Care Consultants

American Boiler Manufacturers' Association

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
see [[dynamic 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
see [[solar collector absorber]].

(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]].

See [[alternating current]].

Air Conditioning Contractors of America

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.

A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

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.

American Conference of Governmental and Industrial Hygienists

American Concrete Institute

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.

American Ceramic Society

American Congress on Surveying and Mapping

activated alumina desiccant
form of aluminum oxide that absorbs moisture.

activated carbon
form of carbon made porous by special treatment which allows it to be capable of adsorbing various odors, gases, and vapors. It may be coated or treated with chemicals to improve the removal of specific gases or odors.

activated shelf life
The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

activation voltage(s)
The voltage(s) at which a charge controller will take action to protect the batteries.

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.

Americans with Disabilities Act

ADA Accessibility Guidelines for Buildings and Facilities

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.

Air Diffusion Council

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 set point
A feature allowing the user to adjust the voltage levels at which a charge controller will become active.

adjustable-frequency drive (AFD)
see [[variable-frequency drive]].

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.

[[air diffusion performance index]]

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.

Association of Energy Engineers

Architectural Engineering Institute

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]].

American Fan and Bearing Manufacturers' Association

See [[adjustable-frequency drive (AFD)]].

American Foundrymen's Society

American Gas Association

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

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

American Gear Manufacturers Association

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

American Hospital Association

American Health Care Association

See [[authority having jurisdiction]].

American Institute of Architects

See amperage interrupt capability.

American Institute of Consulting Engineers

American Institute of Chemical Engineers

American Industrial Hygiene Association

American Institute of Industrial Engineers, Inc.

American Institute of Plant Engineers

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 rate
airflow in volumetric units per hour divided by the volume of the space on which the air change rate is based (normally expressed in air changes per hour).

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 reduces the concentration of airborne contaminants, such as microorganisms, dusts, fumes, respirable particles, other particulate matter, gases, odors, and/or vapors in air.

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, and/or vapours in 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 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 mass (sometimes called air mass ratio)
Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

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
any device or opening for supplying air to a space (such as a diffuser, a grille, or a register), through which air is discharged into a conditioned space. See [[air terminal device]] and [[diffuser]].

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 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
see [[testing, adjusting, and balancing]].

air temperature
the temperature of the air measured at a test point.

air terminal
see [[air terminal unit]].

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-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 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: circulating, filtration, heating, cooling, heat recovery, humidifying, dehumidifying, and mixing of air. Is usually connected to an air-distribution system.

air-side economizer
see [[air 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 are the most common types.

air, exhaust
see [[exhaust air]].

air, makeup
see [[makeup air]].

air, outdoor
see [[outdoor air]].

air, recirculated
see [[recirculated air]].

air, standard
see [[standard air]].

air, supply
see [[supply air]].

air, transfer
see [[transfer air]].

air, ventilation
see [[ventilation air]].

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 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]].

American Institute of Steel Construction

Association of Iron and Steel Engineers

American Iron and Steel Institute

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 (α-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.

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

alternating current (AC)
A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

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).

Air Movement and Control Association International, Inc.

ammonia refrigerant
anhydrous ammonia (NH3).

amorphous semiconductor
A non-crystalline semiconductor material that has no long-range order.

amorphous silicon
A thin-film, silicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. See also single-crystal silicon an polycrystalline silicon.

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

amperage interrupt capability (AIC)
direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.

ampere (amp)
A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

ampere hour meter
An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

ampere-hour (Ah/AH)
A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

(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 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.

ancillary services
Services that assist the grid operator in maintaining system balance. These include regulation and the contingency reserves: spinning, non-spinning, and in some regions, supplemental operating reserve.

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 solar savings
The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

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

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.

antireflection coating
A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

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.

Air Pollution Control Association

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.

American Pipe and Fittings Association

American Public Health Association

American Petroleum Institute

[[application part-load value]]. See [[part-load value]].

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 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.

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.

American Public Works Association

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.

Air-­Conditioning and Refrigeration Institute

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.

See photovoltaic (PV) array.

array current
The electrical current produced by a photovoltaic array when it is exposed to sunlight.

array operating voltage
The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

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.

Acoustical Society of America

American Society of Civil Engineers

American Society of Certified Engineering Technicians

American Society for Engineering Education

aseptic environment
a sterile environment.

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

American Society of Lubricating Engineers

American Society of Mechanical Engineers International

American Society for Nondestructive Testing

American Society of Plumbing Engineers

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.

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]].

American Society of Quality Control, Inc.

American Society of Safety Engineers

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

American Society for Testing and Materials

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

Architectural and Transportation Barrier Compliance Board

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 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.

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.

autonomous system
See stand-alone system.

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
see [[auxiliary thermal source]].

auxiliary fuel
fuel used in an auxiliary thermal source.

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.

The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

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.

American Welding Society

American Water Works Association, Inc.

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.

azimuth angle
The angle between true south and the point on the horizon directly below the sun.