ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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equipment or materials to which a label, symbol, or other identifying mark of an organization, acceptable to the authority having jurisdiction, has been attached. This organization is concerned with product evaluation and maintains periodic inspection of the production of labeled equipment or materials. By labeling the equipment or materials, the manufacturer indicates compliance with appropriate standards or performance in a specified manner.


facility acceptable to the local, national, or international recognized authority having jurisdiction and which provides uniform testing and examination procedures and standards for meeting design, manufacturing, and factory testing requirements.

laboratory fume hood

box-like structure enclosing a source of potential air contamination, with one or more open or partially open sides into which air is moved for the purpose of containing and exhausting air contaminants. A laboratory hood is generally used for bench scale laboratory operations but does not necessarily involve the use of a bench or table.

labyrinth seal

device consisting of a series of grooves, fins, etc., machined into or fitted onto a shaft, piston, packing piece, etc. to prevent leakage of a fluid.


(1) delay attributable to the transport of material or the finite rate of propagation of a signal or condition. Note: also termed transport lag and transportation lag. (2) delay in action of the sensing element of a control device due to the time required for the sensing element to reach equilibrium with the variable being controlled (i.e., temperature lag, flow lag). (3) in thermal insulation, preformed, narrow blocks attached around large pipes or vessels by wires or bands. (4) retardation of an output with respect to the causal input.

Lambert’s law

(1) illumination of a surface by a light ray varies as the cosine of the angle of incidence between the normal to the surface and the incident ray. (2) luminous intensity in a given direction radiated or reflected by a perfectly diffusing plane surface varies as the cosine of the angle between that direction and the normal to the surface.

laminar flow

having airflow nominally in one direction.

lap weld

weld formed by lapping two pieces of metal and then pressing or hammering, applied particularly to the longitudinal joint produced by a welding process for tubes or pipe, in which the edges of the skelp are beveled or scarfed so that when they are overlapped, they can be welded together.

latent cooling capacity

the rate, expressed in Btu/h (W), at which the equipment removes latent heat (reduces the moisture content) of the air passing through it under specified conditions of operation.

latent cooling effect

that portion of the cooling effect that results in water vapor condensation in the air circulating through the equipment.

latent heat

the change in enthalpy associated with a change in humidity ratio, caused by the addition or removal of moisture.

latent heat load

(also known as moisture tons or wet tons), cooling load required to remove latent heat.

latent heat of condensation

quantity of heat released on change of unit mass of saturated vapor to saturated liquid with no change in temperature, measured in Btu/lbm (J/kg).

latent heat of fusion

quantity of heat required to change a unit mass of ice to water at 32°F (0°C) temperature, measured in Btu/lbm (J/kg).

latent heat of vaporization

quantity of heat required to cause a change of state of a substance from a saturated liquid to a saturated vapor with no change in temperature, measured in Btu/lbm (J/kg).

latent storage

use of a phase change of a medium for storing heating or cooling capacity. See also ice storage.

lazy airflow

airflow within the fume hood is described as lazy when smoke remains on the work surface without smoothly flowing to the back baffle.


(1) in gravity warm air heating, the duct running horizontally from the furnace to the riser or stack. (2) in plumbing, a water conductor from the roof to the storm drain.

leak detection

any operation designed to reveal a leak in a system.


(1) volumetric flow rate required to maintain a constant static pressure in the test section. (2) the amount of air interchanged between the room side and outdoor side through a unit as a result of construction features, faulty sealing techniques, temperature differential, or height differential. See infiltration.

leakage class

divisions of airtightness based on normalized leakage.

least squares

estimate in statistics and econometrics. The method of estimation that involves the choice of an estimate such that the sum of the squares of the deviations of the data from the estimate is a minimum.

LeChatelier’s principle

when an external force is applied to a system at equilibrium, the system adjusts so as to minimize the effect of the applied force.

Lenz’ law

an electromotive force (emf) induced in a conductor is always in such a direction that the current it would produce would oppose the change that causes the induced emf.

Lewis number

dimensionless number equal to thermal diffusivity divided by mass diffusivity. Symbol Le, Le, or NLe.

licensed contractor

person or entity certified by governmental authority (where required by law) to engage in construction contracting.

life-cycle assessment

the process of evaluating a component, product, assembly, building, etc. and its development from the moment of extraction of raw materials, transportation, processing, manufacturing, use, recyclability, and disposal and assigning a value or assessment of its cumulative and ultimate social, environmental and economic costs, benefits, and impacts. This is often referred to as a cradle-to-grave or cradle-to-cradle assessment.

life-cycle cost

cost of equipment over its entire life including operating, maintenance, and repair/replacement cost. May also include decommissioning cost.


vertical distance that fluid must be pumped to reach a specified height.

lift-check valve

valve with the mechanism lifting vertically from its seat. Lift-check valves are primarily used in vertical piping. See check valve.

light transmission

(1) the amount of incident light passed through a glazing system. (2) the percentage of incident light passed by the translucent sampling filters (targets) used in the dust spot test.

lighting fixture

the component of a luminaire that houses the lamp or lamps, positions the lamp, shields it from view, and distributes the light. The fixture also provides for connection to the power supply, which may require the use of a ballast.

lighting power allowance, exterior

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

lighting power allowance, interior

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

lighting power density (LPD)

the lighting power per unit area of a building, space, or outdoor area expressed in W/ft2 (W/m2).

lighting system

a group of luminaires circuited or controlled to perform a specific function.

lighting, general

lighting that provides a substantially uniform level of illumination throughout an area. General lighting shall not include decorative lighting or lighting that provides a dissimilar level of illumination to serve a specialized application or feature within such area.

limit control

control device used to limit the desired maximum or minimum state of the controlled variable or to provide an alarm if those limits are exceeded.

limit stat

controller designed to provide limit control.

limit stop

physical stop or device that prevents an operator from adjusting the setpoint of a controller beyond a maximum or minimum setting, often for safety.

limit switch

control to limit some function. Once tripped, can be automatically reset or can require manual reset.

line voltage thermostat

device that acts to automatically control the temperature of a room or space in which it is installed by switching directly to the control device.

linear regression

finding a straight line that best fits the data points, commonly by use of the least squares technique.

linear swell

percentage increase in the length of a test specimen, usually an elastomer, as a result of exposure to refrigerants or oils.

line-break motor protection

(1) control device that automatically opens the line connections to a motor when a predetermined unsafe temperature exists in the windings of the motor. (2) electrical motor protective action that causes the power line to the motor to be interrupted on protective requirement.

liner system (LS)

a continuous vapor barrier liner installed below the purlins and uninterrupted by framing members.


change of state to liquid, generally used instead of the term condensation in cases of substances that are ordinarily gaseous.

liquefied petroleum gas (LPG)

mixed hydrocarbon gases stored under pressure in liquid form.


state of matter intermediate between crystalline substances and gases in which the volume of a substance, but not the shape, remains relatively constant.

liquid composition

the concentration (mass percent) of components present in the liquid phase of a refrigerant blend that is in equilibrium with its vapor phase.

liquid cooler

(1) a factory-made assembly of elements in which the heat is transferred from the liquid to the refrigerant, causing the refrigerant to evaporate and the liquid to be cooled. (2) heat exchanger designed to cool liquids.

liquid feed

connection between the pumping unit outlet and the evaporator inlet.

liquid line

tube or pipe carrying the refrigerant liquid from the condenser or receiver of a refrigerating system to a pressure-reducing device.

liquid- or suction-heat exchanger

superheater or desuperheater.

liquid overfeed system

system feeding an evaporator with refrigerant at a rate to make the exit vapor quality less than one.

liquid overfeed system evaporator

(also known as liquid circulation), evaporator feed system whereby refrigerant liquid is flashed to saturated suction pressure and temperature in an accumulator and is then fed by a mechanical pump or by refrigerant vapor pressure to the evaporators. This liquid is normally fed at a rate greater than the evaporation rate for the refrigerant to ensure wetting of the entire evaporator surface for better heat transfer.

liquid pocket

(also known as liquid slug), accidental trapping of liquid refrigerant at any point in the system.

liquid receiver

a vessel, permanently connected to a refrigerating system by inlet and outlet pipes, for storage of liquid refrigerant.

liquid sample

a sample taken from the liquid phase of a refrigerant blend.

liquid state

one of the three states of matter characterized by limited freedom of molecules and by substantial incompressibility. See also liquid.

liquid transmission

in applications where the sensing point is always at a higher temperature than the rest of the system, vapor pressure is transmitted by a column of liquid.

liquid trap

device for collecting liquid refrigerant at a given point in a refrigerating circuit.

liquid, mechanical vacuum pump

vacuum pump in which the flow of the gas is induced by a vaned rotor and a mass of liquid projected by centrifugal force against the stator.

liquid/vapor mixture

liquid and gaseous phases of a single substance or azeotrope, usually at saturation.

liquid-charged thermostat

vapor thermostat charged with such an amount of liquid that a certain quantity of liquid remains in the bulb whatever the operating conditions may be.

liquid-cooled unit

a condensing unit in which the condensing media is a liquid (for example, water).

liquid-expansion steam trap

trap with an adjustable bellows and steel valve head located downstream of the steam system. It operates on temperature rise, with the setpoint adjustable from the outside. It discharges at a fixed temperature for protection from freezing and for high-capacity venting requirements.

liquid-immersed transformer

a transformer in which the core and coils are immersed in an insulating liquid.

liquid-injection valve

valve which controls the introduction of liquid refrigerant (e.g., into the intercooler in a multistage compression system).

liquid-line filter

a device for removing and retaining solid contaminants from the liquid line of a refrigeration system.

liquid-line filter drier

a filter containing a desiccant capable of removing moisture and other dissolved contaminants in the refrigerant stream.

liquid-refrigerant injection

a method of internally cooling the compressor mechanism or lubricant or the reduction of discharge temperature by introducing saturated or subcooled discharge-side liquid refrigerant into the compressor or condensing unit. Liquid-refrigerant injection mass flow rate is not taken into account when calculating compressor or condensing unit efficiency, capacity, or volumetric efficiency.


solution used in absorption refrigeration.


equipment or materials included in a list published by an approved, nationally recognized testing laboratory, inspection agency, or other organization concerned with product evaluation which maintains periodic inspection of production of listed equipment or materials and whose listing states either that the equipment or material meets nationally recognized standards or has been tested and found suitable for use in a specified manner.

lithium bromide

an absorption system where water (R-718) is the refrigerant and lithium bromide (LiBr) is the absorbent.


liquefied natural gas


(1) amount of heat per unit time imposed on a system by the required rate of heat addition or removal. (2) energy absorbing device. (3) material, force, torque, energy, or power applied to or removed from a system or element. See also capacity.

load diversity ratio

in a thermal storage system, the ratio of average daily load to maximum capacity. A 100 kW chiller meeting an average load of 75 kW would have a diversity of 0.75.

load factor

ratio of actual mean load to a maximum load or maximum production capacity in a given period. See electric power load factor, cooling load factor, heating load factor.

load leveling

charging a thermal storage system at a constant rate during a complete cycle.

load management

deliberate local control or influencing of user loads to affect the time of use of power or energy such as gas, steam, or electric. Compare with direct-load management.

load profile

summary of thermal or other energy loads in a system over a period of time. Note: for example, a common load profile on a peak design day for thermal storage designs would show hourly system load requirements for 24 hours.

loading dust

a compounded, synthetic dust used for air cleaner capacity and efficiency testing.

local mean air velocity

magnitude of the time-averaged vector of velocity at a point of an airstream.

locked rotor current

(also known as locked rotor amperage [LRA]), steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating current motors) applied to the motor.

logical decision

choice or ability to choose between alternatives. This amounts to a yes or no answer to a question of equality or relative magnitude.

logical flowchart

detailed statement of the work order in terms of logic or built-in operations and characteristics of a specified machine. Concise symbolic notation is used to represent the information and describe the input, output, arithmetic, and logical operations by use of a standard set of block symbols. A coding process normally follows a logical flowchart.

logical instruction

instruction that carries out a logical operation, such as AND, OR, NOR.


(1) electric circuit containing a complete, continuous path, as in a feedback loop. (2) sequence of instructions that is repeated until an exit condition prevails.

loss coefficient

a dimensionless fluid resistance coefficient having the same value in dynamically similar streams (i.e., streams with geometrically similar stretches, equal Reynolds numbers, and equal values of other criteria necessary for dynamic similarity). The loss coefficient represents the ratio of total pressure loss to velocity pressure at the referenced cross section - C = ?Pt / pvt where C = total pressure loss coefficient, dimensionless, ?pt = total pressure loss, in. of water (Pa), pvi = velocity pressure at referenced cross section i, in. of water (Pa).

loss of charge protector

(1) device that is capable of discerning the loss of refrigerant charge in a refrigerating machine and of reacting to protect the apparatus from motor burnout. (2) splitting of a condensate line to permit vapors to pass above and condensate to pass below an obstruction.


device consisting of an assembly of parallel sloping vanes, intended to permit the passage of air while providing a measure of protection against environmental influences. Also spelled louvre.

low temperature

the temperature range for maintaining product in a frozen state in refrigeration applications.

lower flammability limit (LFL)

the minimum concentration of a substance that is capable of propagating a flame through a homogeneous mixture of the substance and air under specified test conditions.

low-limit controller

limited fill thermal element, located in the mixed air portion of an HVAC unit, which opens an electrical circuit if any one foot portion of the element reaches its setpoint (usually 35°F [2°C]). It usually shuts down the HVAC unit and/or initiates an alarm. Formerly referred to as freeze stat.

low-pressure boiler

electric, gas, or oil-burning appliance designed to supply low-pressure steam or hot water for space-heating application. A low-pressure steam boiler operates at or below 15 psig [103 kPa (gage)] steam pressure, a hot-water boiler operates at or below 160 psig [1100 kPa (gage)] water pressure and 250°F (120°C) water temperature.

low-pressure control

(1) electric switch and pressure-responsive element connected into the suction side of a refrigerating system to control the operation of the system. (2) pressure-responsive device actuated directly by refrigerant vapor pressure in the low side.

low-pressure float valve

float-type expansion valve operated by changes in liquid level on the low-pressure side.

low-pressure induction unit
low-pressure induction unit
low-pressure receiver

(1) sometimes referred to as an accumulator or surge drum, this vessel acts as the separator for the mixture of vapor and liquid returning from the evaporators, the constant refrigerant level is usually maintained by control devices. (2) vessel on the low side of a refrigerating system into which liquid refrigerant can be collected and used to supply evaporators.

low-pressure refrigerant system

system whose gage pressure at room temperature (74°F [23.3°C]) is near or less than atmospheric pressure. Common low-pressure refrigerants include R-11 and R-123.

low-pressure safety cut-out

(also known as suction pressure safety cut-out), switch designed to stop the compressor motor when the suction pressure reaches a predetermined low value.

low-pressure side

(1) low side or suction side. (2) portion of a refrigerating system operating at approximately the evaporator pressure.

low-pressure stage

compression stage from a low to an intermediate pressure level.

low-temperature brazed joint

gastight joint obtained by the joining of metal parts with metallic mixtures or alloys that melt at temperatures below 1500°F (800°C) but above 800°F (427°C). Compare to soldered joint.

low-temperature district heating

use of fluids less than 200°F (50°C) (in round numbers) for district-heating supply, permitting use of energy sources formerly discarded.

low-temperature hot-water system

hydronic system intended for operation at temperatures below 200°F (93°C). Compare to high-temperature hot-water system.

low-velocity air terminal device

air terminal device that is designed for thermally controlled ventilation (e.g., displacement flow applications). Also see air terminal device.

lubricant circulation rate

the ratio of the mass of lubricant circulating through a refrigerant system to the total mass of refrigerant and lubricant flowing through the system at a specified set of operating conditions.

lubricated plug valve

see plug valve.


a complete lighting unit consisting of a lamp or lamps together with the housing designed to distribute the light, position and protect the lamps, and connect the lamps to the power supply.


(also known as photometric), referring to a radiometric quantity, luminous indicates the weighted average of the spectral radiometric quantity, with the product of the photopic spectral luminous efficiency functions and the standard solar spectral irradiance distribution being the weighting function.