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

(also known as turbulator or turbulence promoter), device inserted in flooded evaporator tubes to prevent the evaporator from becoming oil bound or the refrigerant liquid from becoming quiescent at a pressure lower than its boiling point.


(1) a process employing heat recovery equipment or heat exchangers. (2) device that, on proper variable sensing, initiates control signals or actions to conserve energy. A control system that reduces the mechanical heating and cooling requirement.

economizer, fluid

a system by which the supply air of a cooling system is cooled indirectly with a fluid that is itself cooled by heat or mass transfer to the environment without mechanical cooling. Examples of commonly used fluids are water, glycol mixtures, and refrigerants.

eddy current

(1) electric current induced in the body of a conductor when that conductor either moves through a nonuniform magnetic field or is in a region where there is a change in magnetic flux. (2) localized areas of fluid turbulence known as eddies give rise to persistent vortices.

eddy current test

nondestructive test in which the change of impedance of a test coil brought close to a conducting specimen indicates the eddy currents induced by the coil, thereby indicating certain properties or defects of the specimen (e.g., thickness).

eddy flow
effective air change rate

the constant outdoor air change rate that would result in the same average pollutant concentration over the same period of time as actually occurs under varying conditions.

effective compressor work

network input absorbed at the shaft of a compressor, irrespective of time.

effective draft temperature

a calculated temperature difference that combines air temperature difference and measured air speed at each test point.

effective efficiency

(1) overall efficiency. (2) ratio of the work absorbed in conveying and compressing a unit mass of refrigerant in a perfect compressor and the work absorbed, per unit mass of refrigerant, by the shaft of an actual compressor.

effective emittance of an air space

the combined effect of emittances from the boundary surfaces of an air space, where the boundaries are parallel and of a dimension much larger than the distance between them.

effective freezing

time required from a given initial temperature to freeze a product and to lower its temperature at the thermal center to a given value.

effective length of a duct

dimension by which a straight duct contributes to the length of an air-distribution installation.

effective length of a fitting

dimension by which a duct fitting contributes to the length of an air-distribution installation.

effective power input

average electrical power input to the equipment within a defined interval of time, in watts (i.e., the sum of: the power input for operation of the compressor, excluding additional electrical heating devices, the power input of all control and safety devices of the equipment, and the proportional power input of the conveying devices for the transport of the heat transfer media through the heat pump only. Also, indoor-side and outdoor-side pumps, whether internal or external, whether provided with the equipment or not).

effective temperature
effective temperature difference

difference between the room air temperature and the supply air temperature at the outlet in the room.


actual energy transfer divided by the maximum possible transfer.

effectiveness ratio of a heat exchanger

ratio of the actual heat transfer to the thermodynamically limited maximum heat transfer possible in a counterflow exchanger of infinite transfer area.


(1) for filters: (1 – penetration). Penetration and efficiency are usually expressed as percentages or decimal fractions. (2) general: the ratio of the energy output to the energy input of a process or a machine.

efficiency ratio

ratio of the energy output to the energy input of a process or a machine.


in a fluid-carrying pipe, a device with a reduced diameter to increase the fluid velocity sufficiently to induce a lower static pressure at that point to draw another fluid into the line.


material that can be stretched markedly and repeatedly and, immediately on release of the stress, returns to its approximate original length.

elbow (ell)

pipe or duct fitting to make an angle between connected pipes or ducts. If angle is not specified, a 90° angle is implied.

electric contact thermometer

thermometer designed to indicate temperature by closing in succession a series of electrical contacts.

electric current

movement or flow of charge in an electrical circuit, measured in amperes.

electric field (+-) particle air filter

particle air filter in which the airstream is passed through a nonionizing high voltage field to polarize the electrons on the surface of the particle. Collected on Grounded Polarized Media. May or may not be MERV rated.

electric generating capability

maximum load that a generating unit, generating station, or other electrical apparatus can carry under specified conditions for a given period without exceeding approved limits of temperature and stress ratings.

electric load factor (ELF)

average load divided by peak load for a specified time period.

electric noise

random electrical signals, introduced by circuit components, natural disturbances, or other and unrelated circuits, which tend to degrade the performance of a control system.

electric power

time rate of generating, transferring, or using electric energy. Expressed in watts (W) or kilowatts (kW).

electric power factor (PF)

(1) factor equal to the cosine of the phase angle between voltage and current, by which the product of voltage and current is multiplied to convert volt amperes to power in watts. (2) ratio of real power (kW) to apparent power (kVA) at any given point and time in an electrical circuit. Generally expressed as a percentage.

electric power load factor

(1) comparison of electricity use to electric demand for a given time period, calculated by dividing electricity usage in kilowatt hours (kWh) by the product of electric demand in kilowatts (kW) times the hours in the period. (2) ratio of average load in kilowatts supplied during a designated period to the peak or maximum load in kilowatts occurring in that period.

electric power load shedding

control method of stopping selected power uses to avoid overloading power supply or to avoid demand charges.

electric power on-site generation

generation of any electrical energy on customer’s property, with or without use of recoverable heat.

electric resistance

the property of an electric circuit or of any object used as part of an electric circuit that determines for a given circuit the rate at which electric energy is converted into heat or radiant energy and that has a value such that the product of the resistance and the square of the current gives the rate of conversion of energy.

electric resistance thermometer

temperature measuring and display instrument in which an electric resistance varies as a function of the temperature.

electric space heating

permanently installed electric heating as the principal source of space heating throughout an entire dwelling or business establishment.

electrical bus

common portion of one or more conductors used for transmitting signals or power from one or more sources to one or more destinations.

electrical conductance

reciprocal (opposite) of resistance. The current carrying capacity of any wire or electrical component.

electrical demand

see demand.

electrical enclosure

case enclosing electrical equipment and wiring that is designed expressly to prevent (1) a person from accidentally contacting uninsulated energized parts, (2) burning or molten materials from contacting adjacent combustible materials or falling onto combustible materials, (3) conductive or combustible materials from dropping on uninsulated energized parts, and (4) mechanical abuse of electrical equipment not designed or approved to withstand the intended normal use without such additional enclosure.

electrical energy

energy derived from either moving or stationary electric charge, measured in joules. Measured in kilowatt hours (kWh) in the electric utility industry.

electrical fault

short circuit, either line-to-line or line-to-ground.

electrical feeder

main distribution line (usually three phase) supplying electric energy within an electric service area or subarea.

electrical induction

process of producing electron flow by the relative motion of a magnetic field across a conductor.

electrical insulation

material having high electrical resistivity and, therefore, suitable for separating adjacent conductors in an electric circuit or preventing possible future contact between conductors.

electrical resistance (O)

opposition that limits the amount of current that can be produced by an applied voltage in an electrical circuit, measured in ohms.

electrical resistor

(1) electrical component that adds resistance to an electrical network. (2) material used to produce heat or reduce voltage by passing a current through the material.

electrical volume resistivity

electrical resistance of a substance contained between flat, parallel electrodes, multiplied by the area of the electrodes and divided by the thickness of the substance.

electrically erasable programmable read only memory (EEPROM)

a type of nonvolatile memory used to store small amounts of data that must be saved when power is removed. Data is erasable in socket (requires no batteries).

electric-power baseload generation

large, steady electric load of an electric system produced by the largest, most efficient generating facilities within the system. Note: these facilities are operated to the greatest extent possible to maximize system mechanical and thermal efficiency and to minimize system operating costs.

electric-power cogeneration generation

any of several processes that either use waste heat from generation of electricity to satisfy thermal needs or process waste heat in the steam generation of electricity.


the application of novel, energy-efficient electric technologies as alternatives to fossil-fueled or nonenergized processes


solution through which an electric current can flow.

electrolytic brine

any brine capable of causing chemical decomposition of one of two dissimilar metals by electrolysis or galvanic corrosion.

electrolytic hygrometer

device that indicates relative humidity from the electrical resistance of a hygroscopic salt.

electromagnetic radiation

electromagnetic waves and, especially, the associated electromagnetic energy.

electromotive force (emf)

electric potential difference, measured in volts, between the terminals of any device used as a source of electrical energy (i.e., to supply an electric current).

electronic ballast

a ballast constructed using electronic circuitry.

electronic protocol implementation conformance statement

a specific standardized (BACnet) code or data set unique to each component or equipment panel and communicated between the component and the building management system.

electronic switch

semiconductor device designed to operate without mechanical contacts (e.g., SCR).

electropneumatic device

one that converts an electric signal to a pneumatic signal.

electrostatic filter

filter in which the airstream is passed through a high-voltage ionizing field to impart an electrical positive charge to particles, then collecting them on electrically negative plates. See also precipitator.

electrostatic ionizing particle air filter

filter in which the airstream is passed through a significant electric voltage ionizing field to impart an electrical positive charge to particles, then collecting them on electrically negative plates or grounded surfaces. See also precipitator.

electrostatic precipitator

device for removing dust from the air by inducing an electric charge on the dust particles.

elevation drawing

two-dimensional graphic representation of the design, location, and dimensions of a project, or parts thereof, seen in a vertical plane viewed from a given direction.

embedded fan

a fan that is part of a manufactured assembly where the assembly includes functions other than air movement.

embodied carbon emissions

the total greenhouse gas emissions arising from the manufacturing, transportation, installation, maintenance, and disposal of an asset (i.e., building)

embossed plate evaporator

energy management and control system. See energy management system (EMS).

emergency relief valve

manually operated safety valve for discharge of refrigerant in case of fire or other emergency.

emergency standby generation

The provision of capacity deployed by the balancing authority to meet NERC and regional reliability organization contingency requirements.


(electromagnetic interference), generated at radio frequencies inside systems

emissive power

see emittance.


radiation property of a material, evaluated with its surface optically smooth and clean and of sufficient thickness to be opaque.


ratio of the radiant flux emitted by a physical surface to that emitted by a blackbody at the same temperature and under the same conditions.


relatively stable suspension of small but not colloidal particles of a substance in a liquid with the suspended particles undissolved.

enclosed liquid cooler

(1) apparatus for cooling fluid out of contact with the atmosphere. (2) within a pressure-tight vessel, a heat exchanger consisting of an assembly of tubes containing the liquid to be cooled.

enclosed space

a volume substantially surrounded by solid surfaces such as walls, floors, roofs, and openable devices such as doors and operable windows.

endothermic reaction

one in which heat must be supplied to further the reaction. Compare to exothermic reaction.


(1) capability for doing work. (2) capacity for producing an effect. Having several forms, which may be either stored or transient, and can be transformed from one into another. Forms include thermal (heat), mechanical (work), electrical, radiant, and chemical. (3) time rate of work being done.

energy audit

identification and documentation of gross energy usage during a calendar period using any of several means such as source, department, product, equipment, and cost.

energy availability

maximum fraction of energy in a system that, in the presence of a medium at a given temperature T0, can be converted into useful work during a process at the end of which the system temperature is T0.

energy cascading

multiple use of an energy stream for successive needs.

energy concept

(also known as total energy system), a single fuel source provides all energy services such as electricity, heating, and cooling.

energy conservation

more effective use of energy resources. Energy conservation seeks to reduce energy invested per unit of product output, service performed, or benefit received through waste reduction. Energy conservation and energy use reduction are not synonymous.

energy conservation measure (ECM)

installation of equipment, subsystems, or systems or modification of equipment, subsystems, systems, or operations for the purpose of reducing energy and/or demand (and, hence, energy and/or demand costs).

energy cost

the total cost for energy, including such charges as base charges, demand charges, customer charges, power factor charges, and miscellaneous charges.

energy cost allocation systems (ECA systems)

systems providing a means to apportion approximate energy consumption to individual dwelling units in a multiple-occupancy residential building, based on one or more parameters controlled by the resident.

energy cost budget

the annual energy cost for the budget building design.

energy efficiency ratio (EER)

(1) ratio of net cooling capacity in Btu/h to total rate of electric input in watts under designated operating conditions. (2) ratio of the net total cooling capacity to the effective power input at any given set of rating conditions, in watts per watt.

energy emissions

The pollution emissions associated with generating a quantity of electrical energy.

energy factor

measure of overall efficiency in terms of energy output compared to energy consumption over a 24-hour usage cycle.

energy information provider

A business that provide energy supply or demand information to customers or authorized third parties.

energy level

state of a physical system with a certain defined energy.

energy management control systems (EMCS)

a computer/processor-based hardware and software system with sensors, control devices, and all the necessary components that perform some or all of the following functions: measures conditions related to the use of various forms of energy by HVAC systems, controls these conditions at selected setpoints, monitors and/or controls the energy use, provides status reports on the HVAC system's performances, and provides information for the management of a building's environment, and/or its energy efficiency and/or HVAC system maintenance. Also see energy management systems.\n

energy management functions

apply to the efficient and effective use of energy in buildings.

energy management system

A system used to monitor and control the energy consuming devices in a building. Within this standard, \"energy management system\" always refers to a customer/facility energy management system and not a utility energy management system. (EIS Alliance Use-Cases).

energy management system (EMS)

a system of computer applications used by building engineering staff to monitor, control, and optimize the building's operating performance (e.g., energy consumption, occupant comfort levels). EMS optimizes building operating performance through supervisory control programs that utilize core BMS functionality.

energy recovery system

heat exchanger assembly for transferring heat between two isolated fluid sources. The recovery system may be of air-to-air design or a closed loop hydronic system design. The system will include all necessary equipment such as fans and pumps, associated ducts or piping and all controls (operating and safety), and other custom-designed features.

energy savings

the reduction in use of energy from the pre-retrofit baseline to the post-retrofit energy use, once independent variables (such as weather or occupancy) have been adjusted for.

energy service company (ESCO)

an organization that designs, procures, installs, and possibly maintains one or more energy conservation measures (ECMs) at an owner’s facility or facilities.

energy services interface

An abstract representation of a bi-directional communication interface between devices within a facility and an external electric service provider. This interface enables exchange of information including pricing, energy consumption, demand response interactions, load forecasts, energy production forecasts, and weather information.

energy sources

conventionally, fossil fuels, hydro, nuclear, solar, geothermal, and wind.

energy storage system

(1) system that has to be operated during on-peak as well as off-peak periods. (2) system wherein the load demand is met by a combination of stored thermal energy and an energy conversion device.

energy supplier

An entity that delivers electricity to end use customers. Examples include investor-owned utilities, municipal utilities, and private companies.

energy transmittance ratio

ratio of the energy flow rate traversing a body to the energy flow rate it receives by radiation. (Also known as transmission factor or transmissivity).

energy use intensity (EUI)

an expression of the annual energy used or calculated to be used by a building or building space per unit of gross floor area. Expressed in MBtu/ft2·yr (watts-hours/m2·yr).

energy value target

energy use value to which a building is designed.

energy-cost allocation (ECA)

the allotment of energy costs to residents at an aggregate cost sum, which does not exceed the actual costs paid to purchase and rebill that energy.


prime mover, device for transforming fuel or heat energy into mechanical energy.

engine bleed air

air extracted from the compressor stages of gas-turbine propulsion engines and auxiliary power units.


the discipline, art, and profession of acquiring and applying technical, scientific, and mathematical knowledge to design and implement materials, structures, machines, devices, systems, and processes that safely realize a desired objective or invention.

enhancement factor

water vapor pressure enhancement factor, f, is the ratio of the actual partial pressure, pwvs, of saturated water vapor in moist air to the vapor pressure of pure H2O, e, at the same temperature. [f = pwvs(p, t)/e(t)]. This factor accounts for the nonideal behavior of saturated water vapor in air due to (a) the effect of intermolecular forces (gas imperfections), (b) the effect of pressure on the properties of the condensed phase, and (c) the effect of dissolved gases.

entering dry-bulb (EDB) temperature

the temperature that a thermometer would measure for air entering the evaporator coil. (E.g., for a draw-through fan configuration with no heat gains or losses in the ductwork, EDB equals the indoor dry-bulb temperature).

entering wet-bulb (EWB) temperature

the temperature that the wet-bulb portion of a psychrometer would measure if exposed to air entering the evaporator coil. For a draw-through fan with no heat gains or losses in the ductwork and no outdoor air mixed with return air, this would also be the zone air wet-bulb temperature. For a similar configuration, but when outdoor air is mixed with return air, EWB equals the mixed-air wet-bulb temperature. For mixtures of water vapor and dry air at atmospheric temperatures and pressures, the wet-bulb temperature is approximately equal to the adiabatic saturation temperature (temperature of the air after undergoing a theoretical adiabatic saturation process). The wet-bulb temperature given in psychrometric charts is really the adiabatic saturation temperature.


(also known as heat content), thermodynamic quantity equal to the sum of the internal energy of a system plus the product of the pressure volume work done on the system. H = E + pv, where H = enthalpy or total heat content, E = internal energy of the system, p = pressure, and v = volume. (Compare to specific enthalpy.)

entrainment flow

(also known as conventional mixing). Systems with ceiling-based supply air diffusers and return air grilles are common examples of air-distribution systems that produce entrainment flow.

entropy ratio

ratio of the heat absorbed by a substance to the absolute temperature at which it was added.

entry loss

loss in pressure caused by a fluid stream flowing into a pipe, duct, hood, or vessel.


the boundary surface of points of equal terminal velocity that describe the air-diffusion profile. Envelope is also referred to as isovel.

envelope component

major section of the entire envelope, such as the opaque walls above grade, ceilings, slabs, floors, glazings, doors, or walls below grade.

envelope floor

that lower portion of the building envelope, including opaque area and fenestration, that has conditioned or semiheated space above and is horizontal or tilted at an angle of less than 60°F (15°C) from horizontal but excluding slab-on-grade floors. For the purposes of determining building envelope requirements, the classifications are defined as follows: (a) mass floor: a floor with a heat capacity that exceeds (1) 7 Btu/ft²·°F (39.7 watt h/m² °C) or (2) 5 Btu/ft²·°F (28.4 watt h/m²·°C) provided that the floor has a material unit mass not greater than 120 lb/ft³ (1924.6 kg/ m³), (b)  steel joist floor: a floor that (1) is not a mass floor and (2) that has steel joist members supported by structural members, (c) wood framed and other floors: all other floor types, including wood joist floors.


all of the elements over which a designer has no control and that affect a system and its input or output.

environmental chamber

(also known as climatic chamber), enclosed space designed to create a particular environment.

environmental conditions

natural or controlled conditions of air and radiation prevailing around a person, an object, a substance, etc.

environmental product declaration (EPD)

quantifies environmental information on the life cycle of a product to enable comparisons between products fulfilling the same function

environmental systems

systems that primarily use a combination of mechanical equipment, airflow, water flow, and electrical energy to provide heating, ventilating, air conditioning, filtration, humidification, and dehumidification for the purpose of human comfort or process control of temperature and humidity.


complex organic substance (such as diastase, pepsin, etc.), capable of transforming other compounds by catalytic action, a soluble ferment.

equal friction method duct sizing

method in which ducts are sized so that their frictional resistance per unit length is constant.


(1) general: piping arrangement to maintain a common liquid level or pressure between two or more chambers. (2) refrigeration: piping arrangement on an enclosed compressor to equalize refrigerant gas pressures in crankcase and suction, also a device for dividing the liquid refrigerant between parallel low side coils, also a piping arrangement to divide the lubricating oil between the crankcases of compressor operating in parallel or in tandem. (3) steam: piping arrangement to equalize steam header pressure and condensate return pressure in a steam boiler.

equalizer tank

receiver fitted between the compressor suction line and the return lines of several evaporators in parallel.

equalizing damper

device to maintain constant downstream static pressure or volume flow.

equation of state

thermodynamic expression relating the volume, pressure, and temperature of a given substance.

equation of time

difference between true solar time and local mean time as the result of earth’s orbit about the sun. The difference varies throughout the year, reaching a maximum of 16 minutes during the first week of November. See true solar time.


the steady-state condition during which the fluctuations of variables being measured remain within stated limits.

equilibrium-point dryness (EPD)

the water content of a liquid refrigerant after being in contact with a specific drier at a particular temperature long enough to reach an equilibrium state. EPD is expressed in milligrams of water per kilogram of refrigerant (ppm).

equipment capacity

the manufacturer's rated capacity at the defined rating point for HVAC equipment. Equipment capacity should be adjusted for altitude and other effects.

equivalent diameter

diameter of a circle having the same area as another geometric shape.

equivalent direct radiation (EDR)

thermal radiation expressed in terms of square foot of steam radiator surface emitting 240 Btu/hr. The metric equivalent is 757 watts/m². Note: this term is generally not used in SI.

equivalent evaporation

amount of water a boiler would evaporate, in pounds per hour (kilograms per second), if it received and vaporized feedwater at 212°F (100°C) and standard atmospheric pressure. (Used for comparison of different boilers.)

equivalent length

the length of a straight section of pipe or duct causing the same pressure drop as actually occurs within the system at the same flow rate.

erasable programmable read-only memory (EPROM)

field erasable via exposure to ultraviolet light and reprogrammable (requires no batteries).


(1) incorrect step, process, or result. (2) part of an erratum due to a particular identifiable cause (e.g., a truncation error as contrasted with a mistake). In a restricted sense, an error is that deviation due to unavoidable random disturbances or to the use of finite approximations to what is defined by an infinite series. (3) an error that persists and is usually due to the particular instrument or technique of measurement.

essential facility

building or portion of a building serving one of the following functions: \na) hospitals and other health care facilities having surgery or emergency treatment facilities\nb) fire, rescue, and police stations and emergency vehicle garages\nc) designated earthquake, hurricane, or other emergency shelters\nd) designated emergency preparedness, communication, and operation centers and other facilities required for emergency response\ne) power-generating stations and other public utility facilities required as emergency backup facilities for other essential facilities\nf) structures containing highly toxic materials where the quantity of the material exceeds the maximum allowable quantities\ng) aviation control towers, air traffic control centers, and emergency aircraft hangars, or \nh) buildings and other structures having critical national defense functions

ethylene glycol

clear, colorless liquid used to depress the freezing point of water for use as a secondary coolant in HVAC&R systems. Inhibitors are required to control corrosion caused by ethylene glycol solutions.


instrument for volumetric measurement and analysis of gases.


instrument for assessing comfort environment in terms of operative temperature.


mixture of substances whose solid and liquid phases in equilibrium have identical composition. Such a mixture has a minimum freezing point.

eutectic ice

frozen eutectic mixture used as cold accumulation.

eutectic plate

thin, rectangular container containing eutectic mixture, generally with a coil to be connected to a refrigerating system in order to freeze the mixture.

eutectic point

freezing temperature of a liquid mixture that produces a solid phase of the same composition.

eutectic solution

mixture that melts or freezes, normally at a specific temperature, with constant composition. The mixture's melting point is usually the lowest possible for mixtures of given substances. See also cryohydrate.

evacuated thermal insulation

(also known as vacuum insulation), insulation consisting of double walls, with the space between them (with or without packing) evacuated.

evaporating temperature

temperature at which a fluid vaporizes at a given pressure.


change of state from liquid to vapor.

evaporation design wet-bulb temperature

the outdoor wet-bulb temperature used in conjunction with the mean coincident dry-bulb temperature, often used for the sizing of evaporative systems such as cooling towers.

evaporation loss

water evaporated into the atmosphere as the condenser water is cooled, typically in a cooling tower.

evaporation rate

mass of substance evaporated per unit time.

evaporative condenser

condenser in which the removal of heat from the refrigerant is achieved by the evaporation of water from the exterior of the condensing surface, induced by the forced circulation of air and sensible cooling by the air.

evaporative cooler

a cooler that cools indoor air by moisture evaporation, thereby lowering its dry-bulb temperature and raising its wet-bulb temperature, all at a constant energy (adiabatic) level. See also evaporative cooling.

evaporative cooling

two methods using evaporating water to cool air: (1) direct, which is adiabatic and humidifies the air, and (2) indirect, which is not adiabatic and cools the air being treated without adding moisture.

evaporative cooling unit

cools air by moisture evaporation, thereby lowering its dry-bulb temperature and raising its wet-bulb temperature, all at a constant energy level.

evaporative equilibrium

the condition attained on a wet-bulb instrument when the wetted wick has reached a stable and constant temperature.


heat exchanger, usually of the shell-and-tube type, over which liquid refrigerant is dripped or sprayed and evaporated.

evaporator holdover

ability of an evaporator to stay cold after evaporation stops.

evaporator pressure regulator

automatic valve or control device used to maintain the pressure, and thereby the temperature, in an evaporator above a predetermined minimum.

evaporator unit

assembly of refrigerating compressor, evaporator, and necessary accessories attached to one base.

excess air

in combustion, percent of air greater than that required theoretically to completely oxidize the fuel.


(1) armature current that induces the magnetic field in a rotating electric machine. (2) external force applied to a system that causes the system to vibrate. See also torsional excitation.


property of a system that is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. Exergy is a combination property of a system and its environment because unlike energy, it depends on the state of both the system and environment. The exergy of a system in equilibrium with the environment is zero. Exergy is neither a thermodynamic property of matter nor a thermodynamic potential of a system.


(1) leakage of indoor air out of a building through similar types of openings. Like natural ventilation, it is driven by natural and/or artificial pressure differences. (2) uncontrolled outward air leakage from conditioned spaces through unintentional openings in ceilings, floors, and walls to unconditioned spaces or the outdoors caused by pressure differences across these openings due to wind, inside-outside temperature differences (stack effect), and imbalances between supply and exhaust airflow rates.

exhaust air

air that must be removed from a space due to contaminants, regardless of pressurization

exhaust opening

opening through which air is removed from a conditioned space.

exhaust sound power level

sound power that is transmitted from an exhaust terminal inlet back to the room (counter to the airflow).

exhaust system

a system consisting of one or more fans that remove air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths through the building envelope.

exhaust terminal unit

terminal unit for regulating exhaust or return airflow.

exhaust terminal-unit total leakage

total amount of the air in cubic feet per minute (litres per second) drawn through the casing and a fully closed damper/valve into the airstream of an exhaust terminal unit at a given outlet pressure.

existing building

a building or portion thereof that has been in operation and normal use for at least 12 consecutive months following the date of initial occupancy, the date of the certificate of occupancy, or occupancy class change, whichever is later.

existing building commissioning process

a quality-focused process for attaining the current facility requirements (CFR) of an existing facility and its systems and assemblies being commissioned. The process focuses on planning, investigating, implementing, verifying, and documenting that the facility and/or its systems and assemblies are operated and maintained to meet the CFR, with a program to maintain the enhancements for the remaining life of the facility.

Existing-Building Commissioning (EBCx) Process

a quality-focused process for attaining the CFR of an existing building and/or its systems and assemblies. The process focuses on planning, investigating, implementing, verifying, and documenting that the facility and/or its systems and assemblies are operated and maintained to meet the CFR, with a program in place to maintain the enhancements for the remaining life of the facility.

exit air

air with acceptable quality that may be removed from the space for pressurization control.


total radiant flux that leaves unit area of a surface. The sum of the radiant flux emitted and reflected by the surface plus any radiant flux transmitted through that surface. Compare to radiosity.

exothermic reaction

chemical reaction that releases more energy (heat) than is required to start it. The combustion reaction (burning) is an example, as are fission and fusion reactions. Compare to endothermic reaction.

expander cycle

(also known as work extraction cycle or work cycle), thermodynamic cycle in which the working fluid transfers mechanical energy to some outside system (e.g., in a mechanical expander).


increase in volume of a substance.

expansion bend

bend, usually a loop, put into a pipe run to relieve stresses induced by expansion and contraction from temperature changes.

expansion coefficient

change in length per unit length or the change in volume per unit volume per degree change in temperature.

expansion joint

device in a structure, a pipe run, etc. that can by linear compensation accept variation of length from expansion or contraction due to temperature changes.

expansion stroke

part of the piston stroke between the top dead center and the opening of the suction valve.

expansion tank

partially filled tank for the accommodation of volume expansion of a fluid, typically water. Compare to compression tank.

expansion valve

controlling device for automatically regulating the flow of refrigerant into a cooling unit, actuated by changes in evaporator pressure. The basic response is to regulate evaporator pressure.

expansion-valve capacity

refrigerating effect in watts, Btu/h, or tons (12,000 Btu/h) produced by the evaporation of refrigerant passed by the valve under specified conditions.

expansion-valve superheat

difference between the temperature of the external bulb and the corresponding system refrigerant saturation temperature at the bulb location.

expansion-valve superheat change

change in superheat of a thermostatic expansion valve required to open the valve a predetermined amount.

expendable refrigerant

(1) liquefied gas (generally liquid nitrogen) whose vapor is lost after change of phase. (2) solid carbon dioxide (dry ice).


a systematic approach for collecting information on a physical apparatus or system to determine the nature of its operation.

expert system

computer programs that attempt to provide an answer to control problems where normally one or more human experts would need to be consulted by using previous and current data to assemble a knowledge base and use internal procedures.  Expert systems are a form of artificial intelligence.

extended surface

(also known as augmented surface), increase in area of a heat transfer surface by addition of fins, discs, micro channels or other means.

exterior building envelope
exterior lighting power allowance

the maximum lighting power in watts allowed for the exterior of a building.

exterior-imaging survey

generation of thermograms of outside surfaces of a building. Compare to interior imaging survey.

external defrosting

defrosting using heat from external sources.

external equalizer

in a thermostatic expansion valve, a connection from a selected point in the low-pressure part of the circuit to the system pressure-sensing side of the actuating element such that the selected point pressure is transmitted to the actuating element (e.g., diaphragm or bellows).

external heat

heat generated from sources outside the conditioned area. This heat from gas, oil, steam, electricity, or solar sources supplements internal heat and internal process heat sources. Recovered internal heat can reduce the demand for external heat.

external interrupt

device or switch designed to disconnect load from a power source or distribution center under an overload or short-circuit condition.

external interrupt switch

interrupt generated by an alarm point going into alarm. See also interrupt device.

external resistance
external static pressure loss
extinction coefficient

the proportionality constant K in Bouguer’s Law ((dI) = (I·K·dx)) where I is the local intensity of solar radiation within a medium and x is the distance the radiation travels through the medium.

extract air

airflow leaving a treated space. Compare with return air.

eyebrow hood

an eyebrow hood is mounted directly to the face or top of an appliance above the opening(s) or door(s) from which effluent is emitted, overhanging the front of the opening(s) to capture the effluent. Mounting height is fixed.