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
I-P units (inch-pound units)

units using inches, pounds, and other designations. Examples are: foot, Btu, horsepower, gallon. As compared to SI unit system.

ice builder

refrigerated coils immersed in a tank of water used for forming ice and to provide ice water.

ice harvester

machine that collects ice on a cooling surface, then delivers it to storage or use in a process.

ice maker

automatic ice-making machine in which ice is formed, harvested, and stored.

ice point

temperature at which water freezes under normal atmospheric pressure, 14.696 psig, 32°F (101.325 kPa, 0°C).

ice regelation

(1) refreezing of the water film that forms on or within ice when the pressure from applied material is sufficient to reduce the freezing point to below the ice temperature at that location. (2) refreezing phenomenon: for ice, after removal of pressure induced by contacting materials (such as ice skates, wheels, wires, and bars) water refreezes to ice.

ice slurry (liquid ice)

suspension of ice crystals in a secondary coolant.

ice storage

thermal storage system used for chilling processes or for comfort cooling that uses primarily the latent heat of phase change from ice to water. Ice is formed during periods of low refrigerating demand for delivery of cooling during periods of high refrigerating demand. Compare to cool storage and cold storage.

ice surfacing

treatment to smooth the ice surface.

ice tray

container for freezing water into ice.

ice water

(1) melted ice. (2) water chilled, as with ice.

ice-bank control

control of the thickness of a bank of ice for off-peak cool storage, also for use with milk coolers and other refrigerating cabinets in which refrigerating capacity is stored by means of ice.

ice-bank cooler

water cooler in which ice is allowed to collect on the evaporator tubes.

ice-bank evaporator

evaporator immersed in water. Ice will form on evaporator.

ice-bank tank (ice-buildup tank)

water-cooling tank in which ice is allowed to build up on the evaporator tubes to provide a reserve for cooling.

ice-cooled equipment

equipment that uses metals or plastic as a direct heat transfer medium and that uses ice as a primary source of refrigeration.

ice-core sucking and filling system

combination of a pump, distributing piping, hose, and device, by which impure water may be withdrawn from the unfrozen core of an ice block, and, by connecting the source of water supply, the core may be refilled with pure water.

ice-making capacity

actual productive mass output of an ice-making plant in a given time.

ice-making temperature

temperature range in which most freezing takes place [i.e., about 25°F to 30°F (4°C to 1°C) for water]. Also called zone of maximum crystal formation.

ice-making water forecooler

device for cooling water before it is frozen in an ice maker.

ice-melting equivalent

latent heat absorbed by ice at 32°F (0°C) in liquefying into water phase at 32°F (0°C). For precise data, 143.5 Btu/lb. (334.4 kJ/kg), for definition of ton, 144 Btu/lb (2.326 kJ/kg).

ice-on-coil thermal storage

container (tank) in which ice is formed on tubes or pipes.




(1) induced draft. (2) Also, i.d., inside diameter.

ideal gas

gas whose internal energy and enthalpy depend solely on temperature and, which is defined by pressure times specific volume divided by temperature, is a constant for a unit mass (pv/T = R). Also called perfect gas.


indoor environment quality

ignition temperature (ignition point, kindling temperature)

temperature at which a combustible material will unite with oxygen in the atmosphere and combustion will commence.


luminous (or photometric) irradiance.

Illuminance (lx)

The total luminous flux, incident on a surface, per unit area. Expressed in lx = lm/m².

imaging line scanner

apparatus that scans in a single dimension and is moved perpendicular to the scan direction to produce a two-dimensional thermal map of the scene.

immediately dangerous to life or health (IDLH)

the maximum concentration from which unprotected persons are able to escape within 30 minutes without escape-impairing symptoms or irreversible health effects.

impact filter

filter in which air is submitted to sudden changes of direction and speed to induce deposit of heavier particles on an adhering surface.


total restriction (resistance and reactance) that an electrical circuit presents to the flow of an alternating current, specifically, the ratio of voltage to current and the phase angle between them.


(also known as rotor or wheel), rotating part of a device (fan, blower, compressor, or pump) that moves fluid.

impeller backplate

(also known as impeller hub disc or impeller hub plate), disc rotating with the axle to which the impeller blades of a centrifugal fan are fixed.

impeller reaction

ratio of the variation of the fluid pressure in the impeller to the total variation of pressure in the device.

impeller tonal noise frequency

tonal noise related to the blade pass frequency of the impeller. Impacted by the number of blades and the rotational speed of the impeller. There will be a primary tone frequency and harmonics that are multiples of the primary frequency (2×, 3×, etc.)

incidence angle

angle defined by the intersection of a line normal to a surface and a ray that strikes that surface.

incident angle

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

incoming electrical service point

the point of connection between the facilities of the serving utility and the premises wiring, also known as the point of demarcation between where the serving utility ends and the premises wiring begins, as defined by the US National Electrical Code (NFPA 70).

incomplete combustion

burning with an insufficient supply of air so the burning substance is only partially consumed and could have been burned further with additional air supply. See stoichiometric reaction.

indicated compressor work

work equivalent of indicator card area for a reciprocating compressor or engine. See also indicator diagram.

indicated horsepower

power capacity as measured by means of an indicator diagram.

indicated work

work as measured by an indicator diagram, independent of time.

indicating thermometer

thermometer designed for the visual display of temperature.

indicator diagram

(also known as indicator card), pressure volume diagram tracing a gas in a compressor or engine cycle in terms of pressure and volume displaced. See also volumetric efficiency.

indicator diagram reexpansion line

curve on an indicator card representing the pressure, the total volume relationship of clearance fluid during the initial portion of the return stroke of the piston prior to the opening of the suction valve.

indirect cooling system

system in which a liquid such as brine or water, cooled by the refrigerant, is circulated to the material or space to be refrigerated or is used to cool air so circulated.

indirect emissions

greenhouse gas emissions that are a consequence of the activities of the reporting entity, but occur at sources owned or controlled by another entity

indirect evaporative cooler

(1) a heat and mass transfer device used to sensibly cool a primary airstream without addition of moisture, by means of an evaporatively cooled secondary airstream. Since the secondary air provides wet-bulb depression, it represents a heat sink to the primary air. (2) an indirect evaporative cooling device consisting of an indirect evaporative-cooling heat exchanger, a means of delivering and distributing water to the wet passages of the heat exchanger, a basin for collecting water, a recirculating water pump, and the piping that connects the basin and the water distribution system.

indirect evaporative cooler with integrated heat exchanger

an indirect evaporative cooling device with integrated primary (dry) and secondary (wet) air passages in a single sensible and evaporative heat exchanger.

indirect evaporative cooling unit

a packaged, semi-packaged, or component indirect evaporative cooling unit. The term cooling unit is also used interchangeably for evaporative cooling unit or evaporative cooler.

indirect fired

machines using steam or hot liquids as a heat source.

indirect heat exchanger

(also known as water heater), heat exchanger taking heat from steam or water rather than heat directly.

indirect ice contact

(also known as internal melt), ice storage system using a method of heat exchange in which ice in containers is formed and melted by a circulating secondary coolant enclosed in a pipe or tube.

indirect load management

subfunction of power system distribution automation that implements economic incentives to induce user action, including automation provided by the user. Characterized by user local control of loads in response to economic incentives applied. Also called passive load management.

indirect refrigerating system

system in which a secondary coolant, cooled or heated by the refrigerating system, is circulated to the air or substance to be cooled or heated.

indirect system

refrigerating system in which secondary coolant, cooled by a refrigerant, is circulated to the material or space to be cooled or is used to heat or cool circulated air.

indirect-fired generator

usually of the shell-and-tube type with the absorbent solution either flooded or sprayed outside the tubes and the heat source (steam or hot fluid) inside the tubes.

indirect-fired heater

one in which combustion products do not come into contact with the material to be heated, heating of the material is accomplished by radiation or conduction from the heated surface.

indirect-heating system

one in which a fluid, such as air, is circulated to the material or space to be heated or is used to heat air so circulated.

indirectly conditioned space

an enclosed space within a building that is not a heated space or a cooled space, which is heated or cooled indirectly by being connected to adjacent spaces, provided 1. the product of the U-factors and surface areas of the space adjacent to connected spaces exceeds the combined sum of the product of the U-factors and surface areas of the space adjoining the outdoors, unconditioned spaces, and to or from semiheated spaces (e.g., corridors) or 2. that air from heated or cooled spaces is intentionally transferred (naturally or mechanically) into the space at a rate exceeding 3 ach (e.g., atria).

indoor air

the air in an enclosed occupiable space.

indoor air quality (IAQ)

attributes of the respirable air inside a building (indoor climate), including gaseous composition, humidity, temperature, and contaminants. See also indoor environment quality (IEQ). See also sick building syndrome.

indoor air volume

the entire air volume of a space or building in which the ventilation air is distributed, including ductwork and plenums. The volume of indoor furnishings, equipment, and occupants must be subtracted from the gross indoor volume that is based on interior dimensions of the space or building.

indoor coil

the heat exchanger that removes heat from or adds heat to the conditioned space.

indoor environment quality (IEQ)

a perceived indoor experience of the building indoor environment that includes aspects of design, analysis, and operation of energy efficient, healthy, and comfortable buildings. Fields of specialization include architecture, HVAC design, thermal comfort, indoor air quality (IAQ), lighting, acoustics, and control systems. See indoor air quality (IAQ).

indoor side

that part of the system that removes heat from or adds heat to the indoor airstream.

induced air temperature

air temperature of the internally induced airflow. The mixed airstream resultant temperature of the mix of room air temperature stream with supply airstream.

induced airflow

air that is drawn into a terminal by means of induction and discharged through the terminal outlet

induced draft

fan-induced movement of hot gases from the heat-absorbing equipment.

induced-draft water-cooling tower

type of mechanical draft tower in which one or more fans are located in the air outlet to induce airflow through the air inlets.


entrainment of room air by the jet action of a primary airstream.

induction ratio

ratio of total (primary plus secondary) air to primary air.

induction terminal

terminal that supplies varying proportions of primary and induced air to satisfy the indoor load (excludes fan-powered terminals).

induction unit

one that uses nozzles or the velocity of the primary air source to induce a flow of secondary air to be mixed with the primary air.

induction unit

a device that uses nozzles or the velocity of the primary air source to induce a flow of secondary air to be mixed with the primary air.

inductive load

alternating current load in which current lags voltage.

Industry Foundation Classes

a vendor-neutral, open-data-exchange specification. It is an object-oriented file format developed for the building industry and is commonly used in building information modeling to facilitate interoperability between software platforms.


uncontrolled inward air leakage to conditioned spaces through unintentional openings in ceilings, floors, and walls from unconditioned spaces or the outdoors, caused by the same pressure differences that induce exfiltration.

infiltration degree days

a measure of the severity of the climate as it relates to infiltration (°F day [°C day]).

infiltration factor

the ratio of the building infiltration load with the distribution effects included to the load with the distribution system effects excluded.

infiltration parameter

that portion of unconditioned outside air drawn into the heated space as a consequence of loss of conditioned air through the exhaust system of a furnace or boiler (expressed as a decimal).

information technology equipment (ITE)

ITE includes computers, data storage, servers, and network/communication equipment.

informative appendix

an appendix that is not part of the standard but is included for information only.

infrared detector

device that transduces the infrared irradiance incident on it into some other form of energy.

infrared emittance

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

infrared imaging system

apparatus that converts the two-dimensional spatial variations in infrared radiance from any object surface into a two dimensional thermal map of the same scene in which variations in radiance are displayed in gradations of gray tone or color.

infrared noise

equivalent temperature difference measure of the (electric) noise of an infrared imaging system or line scanner, the target-to-background temperature difference between a low-spatial-frequency blackbody target and its blackbody background at which a ratio of one is obtained between the peak-to-peak signal and RMS noise at the output of the detector processing electronics of the sensor in question.

infrared radiation (IR)

range of electromagnetic radiation wavelengths greater than those of visible light and shorter than those of microwaves, generally between 0.8 micrometer and 1 millimeter. IR originates from either incandescent or nonincandescent hot bodies or from flames. The energy is used as a means of direct heat transfer from the source to the object(s) to be heated without materially heating the intervening air.

infrared resolution

measure of the capability of an infrared thermal sensing device to distinguish the apparent radiance temperature difference between two blackbodies near the same temperature.

infrared sensing device

wide class of instruments used to display and/or record information that is proportional to or equivalent to the thermal radiation from any object surfaces viewed by the instrument. The instrumentation varies from simple spot radiometers, (which measure only one spot or area) to two dimensional thermal imagers (which provide photographic quality pictures and which map the scene radiosity).

inherent motor protection

device buried in the motor winding to detect the winding temperature and interrupt the supply line when the allowable winding temperature is exceeded.

inherent regulation

property of a system to reach a steady state after a disturbance, without the intervention of a control equipment.


chemical substance that reduces the rate of corrosion, scale formation, fouling, or slime production.

initial resistance

the pressure loss of the device operating at a specified airflow rate with no dust load, expressed in Pa (in. of water).

inlet area

the gross inside area measured in the plane(s) of the inlet connection(s). For converging inlets without connection elements, the inlet area shall be considered to be that where a plane, perpendicular to the airstream, first meets the bell mouth or cone.

inlet Reynolds number

in thermal storage, volume flow rate leaving a diffuser per unit length divided by the kinematic viscosity of the incoming fluid.

input rating

fuel burning capacity of an appliance in British thermal units per hour (Btu/h) [kilowatts (kW)] as specified by the manufacturer. Appliance input ratings are based on sea-level operation and need not be changed for operation up to 2000 ft (600 m) altitude.

input/output device

equipment that introduces data into or receives data from a data communications system.

insert system

a combined appliance where refrigerant tubing is inserted directly into the water heater, usually through one of the ports on the water heater, the inserted tubing is the desuperheater.

insertion loss

reduction in sound pressure level caused by installation of an element in the sound path.

insertion thermostat

(1) thermostat that has an element that is inserted directly into the airstream of a duct or pipe. (2) thermostat with the sensing member in the sensed variable but with the controlling element outside.

inside dimension

inside diameter, width, height, or cross-sectional diagonal.


solar radiation incident on the solar collector. See instantaneous irradiance.

inspection authorities

person(s) designated by governing codes to inspect installed systems for compliance with regulations.

instantaneous efficiency

the ratio of the energy removed by the transfer fluid per unit of collector area to the total solar radiation incident on the collector per unit area (aperture or gross) during a test period for which the condition of the test corresponds to the steady state or quasi-steady state.

instantaneous heater

a self-contained packaged water-heating device that is capable of providing a continuous supply of hot water at a predetermined temperature without any storage capacity. Energy sources can be fuel fired (natural gas/propane), electric, or steam fired. The term is typically utilized for domestic water-heating applications, but pool, spa, and radiant heating systems are also an application. Another term that is used on smaller heaters is point-of-use heaters. Some manufacturers allow for multiple unit installations when the need for larger capacity is required.

instantaneous irradiance

the rate at which solar radiation is incident on a unit surface area in unit time, measured in Btu/(h·ft2) [W/m2].

instantaneous thermal efficiency

the amount of energy removed by the transfer fluid per unit of gross collector area during the specified time period divided by the global total solar radiation incident on the collector per unit area during the same test period, under steady state or quasi-steady state.

instantaneous water heater

a material separating a body from others to reduce or retard the transfer of energy (heat), electricity, or sound.

integral control mode

circuit that contributes to the output of the controller an amount equal to the integral of the error signal over time, multiplied by the integral gain.

integral diffuser air terminal

diffuser with the features of an air terminal. Air is modulated by outlet or inlet dampers.

integral draft diverter

a device that is an integral part of a furnace or boiler and is designed to (a) provide for the exhaust of the products of combustion in the event of no draft, back draft, or stoppage beyond the draft diverter, (b) prevent a back draft from entering the furnace, and (c) neutralize the effect of stack action of the chimney or gas vent upon the operation of the furnace.

integral fin

surface extension formed from the parent material at time of manufacture.

integrated heater

a boiler operated in conjunction with an indirectly fired storage water heater or an external storage tank in which domestic water, heated by the boiler, is stored. Part-load value based on operation at standard ARI conditions, typically used for ARI rating purposes.

integrated part-load value (IPLV)

a single number part-load efficiency figure of merit calculated per the method described in the current version of ANSI/AHRI Standard 550/590.

integrated seasonal coefficient of performance (ISCOP)

a seasonal efficiency number that is a combined value based on the formula listed in AHRI Standard 920 of the two COP values for the heating season of a direct expansion dedicated outdoor air system (DX-DOAS) unit water or air source heat pump, expressed in W/W.

integrated seasonal moisture removal efficiency (ISMRE)

a seasonal efficiency number that is a combined value based on the formula listed in AHRI Standard 920 of the four dehumidification moisture removal efficiency (MRE) ratings required for DX-DOAS units, expressed in lb of moisture/kWh.

integrated system

(1) more than one building system, such as lights and air distribution, combined into a common design. (2) system in which many subsystems of a building are combined into a single package (e.g., fire, security, clock, and HVAC).


a device using signals from temperature and flow sensors through time for computing thermal energy transferred.

intelligent end point

A system that internally measures, collects, and analyzes its energy cost/consumption.

intercooler (interstage cooler)

apparatus for cooling compressed gas or vapor between two compression stages.

interior imaging survey

generation of thermograms of portions of a building as viewed from the building interior. Compare to exterior-imaging survey.

interior lighting power allowance

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

interior zone

any space not affected by exterior loads.

intermediate atmospheric vent backflow preventer

small back-pressure- and back-siphonage-type backflow prevention device designed to operate under continuous pressure, for use where certain quasi contaminants are involved.

intermediate pressure (interstage pressure)

pressure prevailing between stages of multistage compression.

intermittent burner ignition

ignition source that is automatically ignited or energized when an appliance is called on to operate and that remains continuously ignited or energized during each period of main burner operation (ANSI Standard Z 21.13).

intermittent ignition burner

burner for which ignition is supplied initially to start, not continually, and at timed intervals so long as combustion is required.

internal discharge makeup air

makeup air delivered directly to the interior of an exhaust hood such that it is exhausted without entering the occupied space. Sometimes this kind of makeup air is called short-circuit makeup air.

internal energy

sum of all kinetic and potential energies contained in a substance due to the states of motion and separation of its several molecules, atoms, and electrons. It includes sensible heat (vibration energy) and that part of latent heat represented by the increase in energy during evaporation.

internal equalizer

in a thermostatic expansion valve, an integral internal part or passage whereby the actuating element (e.g., diaphragm or bellows) is exposed to pressure leaving the valve.

internal gross volume

the volume as determined from internal dimensions of the container with no allowance for the volume of internal parts.

internal heat

total passive heat generated within the conditioned space. It includes heat generated by lighting, computers, business machines, occupants, and mechanical and electrical equipment such as fans, pumps, compressors, and transformers.

internal process heat

heat from industrial activities and sources such as wastewater, boiler flue gas, coolants, exhaust air, and some waste materials. This heat is normally wasted unless equipment is included to extract it for further use.

internal static pressure loss
internal vibration isolation

in rotating equipment, isolation of all moving parts is within the unit that supports the rotating device.

interpretation of a standard

written explanation of the meaning of specific provisions, as determined by the issuing body in response to an inquiry.

interrupt device

device to disconnect a load from a power source or distribution center under an overload or short circuit condition (e.g., a fuse, disconnect switch, etc.) This interrupt is not an integral part of the operating equipment.

interrupted burner ignition
interruptible load

electrical or gas loads that by contract can be interrupted by the supply system in the event of a capacity limit in the supplying system.

interruptible rate

price rate, normally covering reduced pricing for a supply of electricity, that can be interrupted at the utility’s option, either instantaneously as required or with advance notice.

interstitial space

space between two zones, rooms, or floors of a building.

intrinsic electric barrier

component or circuit that limits the energy available in a hazardous location to a level that is incapable of igniting the hazardous atmosphere.

inverted bucket steam trap

see steam trap.

inverted bucket trap (IBT)

steam trap in which the inlet is channeled to the bottom of the trap body so that the condensate enters underneath the inverted bucket. A small hole in the top of the bucket helps discharge incondensable gases and entrapped air. The outlet from the trap is at the top and, as long as the trap is filled with steam, it floats in the condensate and keeps the outlet closed. The valve opens when the trap fills with condensate, sinks, and discharges the condensate into the return line.

ionizers, air cleaner

process creates various types of reactive oxygen species (ROS) using ions created by heat, electrical discharge, radiation, or chemical reaction. ROS may then reduce or deactivate bioaerosols like pathogens.

IRIS damper

sectorized blades.


(1) at a point of a surface, quotient of the flux incident on an element of a surface containing the point, divided by the area of that element, measured in watts per square meter. (2) radiant flux density. See also solar irradiance.

isenthalpic expansion

expansion of a fluid during which no heat or mechanical energy is exchanged with the surroundings, the enthalpy remains constant.

isentropic process

(also known as reversible adiabatic), thermodynamic change at constant entropy.


ISO/draft International Standards


(1) line at one barometric pressure. (2) line graph of state representing an isobaric process.


(also known as isopiestic), thermodynamic change at constant pressure.


thermodynamic change at constant volume.


graph that shows the variation of one quantity with another, for example, the variation of pressure with temperature when the volume of the substance is held constant.

isokinetic sampling

sampling in which the flow in the sampler inlet is moving at the same velocity and direction as the flow being sampled.

isolated combustion system

an installation in which a furnace or boiler is installed indoors and all combustion and ventilation air is admitted through grilles or ducts from outdoors and (all such air) does not communicate with air in the conditioned space.

isolated system

in thermodynamics, a system that can interchange neither matter nor energy with its surroundings.

isolation devices

devices that isolate HVAC zones so that they can be operated independently of one another. Also, a device, such as a service valve, that allows maintenance of a portion of a system. Isolation devices include, but are not limited to, separate systems, isolation dampers/valves, and controls providing shutoff at terminal boxes.


one of a group of substances with the same ultimate chemical composition but different molecular structure.

isometric drawing

form of three-dimensional projection in which all of the principal planes are drawn parallel to corresponding established axes and at true dimensions.


state of constant relative humidity.


graph of a state line that represents one temperature.


process at constant temperature.

isothermal leak

a leak that occurs at a fixed temperature—i.e., under conditions such that thermal equilibrium is maintained with the surroundings at a specified fixed temperature.

issues and resolution log

a formal and ongoing record of problems or concerns and their resolution that have been raised by members of the Cx Team during the course of the Commissioning Cx Process.

issues log

a formal and ongoing record of problems or concerns—and their resolution—that have been raised by members of the commissioning team during the course of the commissioning process.

iterative procedure

process that repeatedly executes a series of operations until some prescribed condition is satisfied.