ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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(1) object point that is directly beneath an aircraft flying an airborne imaging survey. (2) point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer.


North American Industry Classification System, as published by the Office of Management and Budget, 1997.

nameplate rating

full-load, continuous rating of a generator, prime mover, or other equipment under specified conditions, as designated by the manufacturer and usually indicated on an attached plate. Compare to capacity.

naphthenic oil

a petroleum-lubricating oil fraction in which naphthenic ring-type hydrocarbon structures predominate.

nationally recognized laboratory

see laboratory.

natural air circulation

(1) air circulation caused by air movement or pressure differences. (2) air circulation caused by differences of density induced by differences of temperature and/or humidity.

natural convection

heat transmission by movement of a fluid, caused by density differences. Usually created by a thermal heat source or heat sink.

natural convection cooling

(1) cooling a fluid by natural convection over the cooling surface. (2) cooling a space by means of air cooled by natural convection.

natural draft

airflow resulting from the difference between atmospheric density and some lower density, chimney effect.

natural gas

naturally occurring mixture of hydrocarbon and nonhydrocarbon gases found in porous geologic formations beneath the earth’s surface, often in association with petroleum. The principal constituent is methane.

natural ventilation

movement of air into and out of a space primarily through intentionally provided openings (such as windows and doors), through nonpowered ventilators, or by infiltration.

natural-draft burner

burner that depends primarily on the natural draft created in the chimney or venting system to induce the air required for combustion into the burner.

natural-draft water-cooling tower

one in which air movement is dependent upon the difference in density between the entering air and internal air. As the heat of the water is transferred to the air passing through the tower, the warmed air tends to rise and draw in fresh air at the base of the tower. See hyperbolic tower.

naturally stratified storage

thermal storage in which temperature stratification is achieved and maintained by density differences alone and not by mechanical separators.

near azeotropic

a zeotropic blend with a temperature glide sufficiently small that it may be disregarded without consequential error in analysis for a specific application.

near field

the sound field close to a sound source (between the source and the far field) where the instantaneous sound pressure and particle velocity are not in phase. Measurements in this region typically do not correlate to sound power measured at a further distance, and the boundary of this field is typically one to two dimensions of the source away.

near resonance

operation at a frequency near, but not exactly equal to, the resonance frequency.


to supply very fine water spray or steam into an airstream or enclosure.

needle bearing

type of roller bearing in which rod-like rollers have small diameters relative to their lengths.

needle valve

(1) valve having a very small hole in the seat, fitted with a needle-shaped plunger. (2) valve in which the obturator is a stem ending in a conical point which mates with a corresponding valve seat.

negative air pressure

(1) in a building, pressure lower than pressure outdoors. Note: as the negative pressure increases, outdoor air is drawn in through any openings in the building envelope. (2) in building spaces, pressure lower than pressure in adjacent spaces or rooms.

negative-rated operating pressure

tested maximum negative pressure at which a duct is rated.

net capacity

(1) (equipment effective capacity), maximum load that a machine, apparatus, device, or system is capable of carrying under service conditions. (2) capacity (volume) of a room after deducting the loss of space due to coils, columns, air ducts, dunnage, and other dedicated space required to provide air circulation.

net cooler refrigerating capacity

rate of heat removal from a fluid flowing through a cooler (air, water, brine, etc.) at stated conditions, the difference in specific enthalpies of the cooling fluid entering and leaving the cooler. In case frosting occurs within the cooler, the latent heat of fusion and the subcooling heat of the ice (frost) must be added in determining the net cooler refrigerating capacity.

net demand

Total demand minus total supply. Net demand is determined at the facility interconnection point, billing meter, or internal location, and may be positive or negative.

net effective filtering area

the total area in the device on which dust collects. For devices using fibrous media, it is the net upstream area of the media exposed to airflow measured in the plane or general surface of the media. Net effective area excludes the area blocked by sealants, flanges, or supports. In electronic air cleaners, it is the total exposed surface of those electrodes available for dust precipitation, including the ionizing section but excluding supports, holes, and insulators. Net effective filtering area is measured in m2 (ft2) to three significant figures.

net energy

the sum of the purchased energy minus sold or exported energy.

net exhaust flow rate

the exhaust flow rate for a hood minus any internal discharge makeup airflow rate.

net latent cooling effect

the total useful capacity of the air conditioner for removing water vapor from the space to be conditioned.

net occupiable area

the floor area of an occupiable space defined by the inside surfaces of its walls but excluding shafts, column enclosures, and other permanently enclosed, inaccessible, and unoccupiable areas. Obstructions in the space such as furnishings, display or storage racks, and other obstructions, whether temporary or permanent, are considered to be part of the net occupiable area.

net positive suction head (NPSH)

minimum head at the pump inlet to prevent the liquid being pumped from flashing into a vapor at that temperature and pressure and potentially causing the pump to cavitate. There are 2 types of NPSH values: net positive suction head available (NPSHA) and net positive suction head required (NPSHR). NPSHA is actual or available head at the pump impeller and should be greater than the vapor pressure of operating fluid at the operating temperature. NPSHR is the value stated by the pump manufacturer that is the minimum required head at the pump impeller. NPSHA should be greater than NPSHR. Values of NPSH are expressed as head in units of feet (kPa).

net refrigerating effect

(1) (brine cooler) product of the mass rate of water or brine flow and the difference in enthalpy of the entering and leaving water or brine, expressed in heat units per unit of time. It is expressed also by the total refrigeration effect less the heat leakage losses. (2) (condensing unit capacity) rate at which heat is removed from outer media by a refrigerant in the low-pressure side or by the difference in total enthalpy between refrigerant liquid leaving the unit and the total enthalpy of the refrigerant vapor entering it. (3) (packaged air conditioners) rate at which heat is removed from the airstream, as measured entering the cooling coil and leaving the unit. (4) rate at which heat is removed by the primary refrigerant from the cooling medium (secondary coolant) that is used to transmit the refrigerating effect.

net refrigeration capacity

that portion of the total refrigeration capacity of a liquid cooler that produces useful cooling. This is the product of the mass flow rate of liquid, specific heat of the liquid, and the difference between entering and leaving liquid temperatures expressed in energy units per unit of time. It is represented also by the total refrigeration capacity less the heat leakage rate

net sensible capacity

the gross sensible capacity less the default rate of fan heat assumed by the manufacturer, this rate of fan heat is not necessarily the same as for the actual installed fan.

net total capacity

the gross total capacity less the default rate of fan heat assumed by the manufacturer, this rate of fan heat is not necessarily the same as for the actual installed fan (see adjusted net total capacity). (Also see gross total capacity.)

net total cooling capacity

total cooling capacity with fan power adjustment

net total cooling effect

the refrigeration capacity available for space and product cooling. It is equal to the gross total cooling effect less the heat equivalent of energy required to operate the cooler.

net usable volume

the volume of interior usable space intended for refrigerated storage or display, specifically consisting of the usable interior volume within the claimed load limit boundaries. Any of this volume occupied by evaporator coils, fan grilles, ducts (including any space intentionally made unusable by fences and grilles), or any other significant interior protrusions are excluded from the net usable volume. For cases normally equipped with shelves, the front edge of the shelf is assumed to be the front-load limit boundary. To be consistent, shelves and other display devices are not treated as significant interior protrusions. The volume occupied by shelves and other display devices is not subtracted from the net usable volume.

net zero carbon emissions

achieved when anthropogenic emissions of greenhouse gases to the atmosphere are balanced by anthropogenic removals over a specified period


a process used to maintain order and provide access to the communication medium.

networked guest room control system

a control system, accessible from the hotel/motel front desk or other central location, that is capable of identifying rented and unrented rooms according to a timed schedule, and is capable of controlling HVAC in each hotel/motel guest room separately.

neural network

computer circuits patterned after the complex interconnections among nerve cells in the brain.

neutral pressure level

(also known as neutral zone) building height at which there is no difference between inside and outside building static air pressure.

new building

any building or portion thereof that is not an existing building.

Newton’s law of cooling

the rate of heat flow out of an object by both natural convection and radiation is proportional to the temperature difference between the object and its environment and to the surface area of the object.

Newton’s laws of motion

(1) change of motion is proportional to force applied and takes place in the direction of the line of action of the force. (2) every body continues in its state of rest or of uniform motion in a straight line, except insofar as it may be compelled to change that state by action of some outside force. (3) to every action there is always an equal and opposite reaction.

night cover

removable cover placed over an open-top refrigerated display case during overnight periods in order to reduce energy transfer with the environment.

night setback thermostat

manual or automatic reset of temperature control point of a thermostat, usually coupled with a start-up time for restoration of desired daytime temperature level.

nitrogen capacity

the volumetric flow rate, L/s (cfm), equivalent to the mass flow rate of dry nitrogen that would be passed for a specified inlet pressure if discharge had been to standard atmospheric pressure of 101.325 kPa (14.696 psi) absolute.

nitrogen purge

procedure in which nitrogen gas is bled into piping as it is welded, brazed, or soldered to displace gases within the piping. The nitrogen displaces the air from inside the pipe, minimizing oxidation and resultant scale formation.

no-frost refrigerating system

(1) system in which all the refrigerated surfaces in the cabinet are defrosted by an automatic defrost system. (2) use of a secondary coolant sprayed on evaporator surfaces to prevent formation of frost, water absorbed in the coolant is removed by distillation.

nodal line

a line of zero response on the fan such that blades on opposite sides of the line vibrate in opposite phase.


(1) ambient background sound when equipment is not in operation. (2) any disagreeable or undesired sound.

noise criteria curve (NC curve)

a series of curves of octave band sound spectra in a system for rating the noisiness of an occupied indoor space, an actual octave band spectrum is compared with this set of curves to determine the NC level of the space. See also room criterion curve (RC curve).

noise reduction (NR)

difference between the average sound pressure levels, or sound intensity levels, of two spaces. Usually those two spaces are two adjacent rooms called, respectively, the source room and the receiving room.

nominal capacity

(1) the capacity recorded and reported by a given test. (2) the capacity reported by the manufacturer for a specified device.

nominal formulation

the bulk manufactured composition of the refrigerant blend, which includes the gas and liquid phases.

nominal freezing

for a given product of specified dimensions and at an initial uniform temperature of 32°F (0°C), the time it takes for the thermal center to reach 14°F (–10°C).

nominal motor efficiency

the median efficiency occurring in a population of motors of the same manufacturer and rating.

nominal size of an air terminal

nominal value of dimensions of the prepared opening (duct) into which the air terminal device is to be fitted.

nominal size of duct and fitting

reference dimension used for designation, calculation, and application of duct and fitting.

nominal storage capacity

a theoretical capacity of the thermal storage device, which in many cases is greater than the usable storage capacity. This measure should not be used to compare usable capacities of alternative storage systems.

nominal time constant

the indoor air volume of a space or building divided by the rate of outdoor air supply, the nominal time constant also equals the average age of air exiting from the space or building. The reciprocal of the nominal time constant is called the nominal air change rate.

non-self-renewable air cleaners

devices that are designed to operate through a specified performance range and then be manually cleaned or reconditioned by equipment not included in the air cleaner itself. An example is the washable expanded metal filter.


see manual.


a synonym for the preferred term zeotropic. See zeotropic.

nonconcentrating collector

a solar collector in which the absorber heat flux is not greater than the solar irradiance across the aperture area.

noncondensable gas

gas that cannot be liquefied at normal operating pressures and temperatures.

noncondensable separation and storage tank

located in the absorber tube bundle or external to the absorber/evaporator vessel.

noncyclic ice maker

continuous ice maker with simultaneous water supply, freezing, and harvesting phases in the ice-making operation.

nondepletable energy

forms of energy (excluding minerals) derived from incoming solar radiation, including energy from photosynthetic processes, energy from resulting phenomena including wind, waves, and tides and lake or pond thermal differences, and energy from the internal heat of the earth and nocturnal thermal exchanges. See renewable energy.

nondepletable energy sources

sources of energy (excluding minerals) derived from incoming solar radiation (including photosynthetic processes), from phenomena including wind, waves and tides, lake or pond thermal differences, and energy derived from the internal heat of the earth (including nocturnal thermal exchanges).

nondestructive test (NDT)

procedure that evaluates equipment fitness and integrity without permanently altering physical state or arrangement.

nonessential lighting

lighting that would not impair the safety nor materially interfere with the occupancy of an area if it were turned off. Examples include decorative, advertising (except simple identification of structures or addresses), and multiple lamp groupings resulting in illumination levels greater than accepted standards for the purpose (as for aisles, corridors, streets, and highways).

nonimaging survey

generation of a set of apparent temperature measurements of building surfaces (usually the interior) obtained with a nonimaging thermal sensing device.

nonisothermal jet

jet in which the primary air temperature differs from the mean space temperature.

nonlaminar flow workstation

one without uniform airflow patterns and velocities.

nonlinear impedance

(1) impedance that cannot be represented by a first order mathematical equation. (2) impedance that varies with applied voltage.

nonpositive displacement compressor

a compressor in which the increase in vapor pressure is attained without changing the internal volume of the compression chamber.

nonrenewable energy

energy derived from a fossil-fuel source.

nonreturn damper

device that allows air to flow only in a predetermined direction. See backdraft damper.

nonreturn valve

check valve.

nonswinging door

roll-up, metal coiling, sliding, and any other door that is not a swinging door.

nonuniform flow

fluid flow varying in velocity across the plane perpendicular to the flow.

nonvolatile refrigerant

one that remains a liquid during the process of absorbing heat within the air cooler. See secondary coolant (secondary fluid).

normal operating conditions

a point of operation representing normal usage of the equipment.


adjustment of the results of a model due to changes in baseline assumptions (nonindependent variables) during the test or post-retrofit period.

normalization parameter

a factor or parameter that allows for the normalization of energy use by the factor or parameter (e.g., Btu/student·yr or kWh/widget·yr) chosen to allow for expression of energy performance.

normalized leakage

dimensionless value calculated from the leakage area, building height, and floor area that describes the relative airtightness of the building envelope.

normally closed device

(1) controlled device that closes when the control signal is off. (2) device that returns to the closed position when power is off.

normally open device

device that returns to the open position when power is off.

normative appendix

integral parts of the mandatory requirements of the standard, which, for reasons of convenience, are placed after all other normative elements.

north oriented

facing within 45 degrees of true north (northern hemisphere).


short tube with a taper or constriction used to speed up or direct a flow of fluid.

nozzle outlet

discharge opening suitably shaped to produce a jet of air or other fluid of regular form and given throw.


(1) no requirement. (2) noise reduction.

nucleate boiling

boiling in which bubble formation is at the liquid/solid interface rather than from external or mechanical devices.

numerical control
Nusselt number

dimensionless number, used in calculation of forced convection, that gives a measure of the ratio of total heat transfer to conductive heat transfer, equal to the heat transfer coefficient times a characteristic length divided by the thermal conductivity.