ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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gage (gauge)

(1) instrument for measuring pressure, flow, level, or other factors. (2) arbitrary scale of measurement for sheet-metal thickness and wire and drill diameters.

gage glass

device for showing liquid level in a tank, boiler, or other storage container.

gage pressure (gauge pressure)

the difference between absolute pressure and ambient or surrounding pressure. It may be negative or positive.

galvanic corrosion

corrosion resulting from the contact of two dissimilar metals in an electrolyte or from the contact of two similar metals in an electrolyte of nonuniform concentration.

garment insulation (Iclu)

the increased resistance to sensible heat transfer obtained from adding an individual garment over the nude body. Expressed in clo units.


state of matter in which substances exist in the form of nonaggregated molecules and which, within acceptable limits of accuracy, satisfy the ideal gas laws, usually a highly superheated vapor. See state.

gas constant

constant of proportionality appearing in the equation of state of an ideal gas, equal to the absolute pressure of the gas times its molar volume divided by its absolute temperature. Also known as the gas law constant.

gas constant coefficient

R in the perfect gas relation pV = mRT.

gas enrichment

increasing the heat content of a gas by mixing it with a gas of higher heat content.

gas metal arc welding (GMAW)

arc-welding process wherein coalescence is produced by heating with an arc between a continuous filler metal (consumable) electrode and the work. Note: shielding is obtained entirely from an externally supplied gas or gas mixture.

gas tungsten arc welding (GTAW)

arc-welding process wherein coalescence is produced by heating with an arc between a single tungsten (nonconsumable) electrode and the work. Note: shielding is obtained from a gas or gas mixture. Pressure may or may not be added.

gas turbine

(1) electric generating station in which the prime mover is a gas turbine. (2) prime mover in which gas, under pressure or formed by combustion, is directed against a series of turbine blades, the energy in the expanding gas is converted into mechanical energy supplying power at the shaft. Also called a combustion turbine or a simple cycle turbine.

gas welding

group of welding processes wherein coalescence is produced by heating with a gas flame or flames, with or without the application of pressure, and with or without the use of filler metal.

gas/air mixture

lean mixture: gas/air mixture of which the air content is more than adequate for complete combustion, and the resultant combustion gases will contain an excess of oxygen. Rich mixture: gas/air mixture of which the air content is not sufficient for complete combustion and the resultant combustion gases will contain burnable but unburned hydrocarbons.

gaseous state

one of the three basic states of matter, characterized by the greatest freedom of molecules and the lack of any inherent fixed shape or volume.

general lighting

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.

generally accepted engineering standard

a specification, rule, guide, or procedure in the field of engineering, or related thereto, recognized and accepted as authoritative.

geothermal energy

direct or indirect utilization of available heat in the ground.


see packing.


the absolute value of the difference between the starting and ending temperatures of a phase-change process by a refrigerant within a component of a refrigerating system, exclusive of any subcooling or superheating. This term usually describes condensation or evaporation of a zeotrope.

global irradiance

(1) (also known as hemispherical irradiance), quantity of solar energy incident on a unit surface area in unit time through a unit hemisphere above the surface, expressed in Btu/h·ft2 (W/m2). (2) the global (or hemispheric) solar radiant energy is the quantity of solar energy incident upon a unit surface area in unit time through a unit hemisphere above the surface, expressed in W/m2 (Btu/[h·ft2]).

global warming potential (GWP)

an index developed to provide a simplified means of describing the relative ability of a chemical compound to affect radiative forcing, if emitted to the atmosphere, over its lifetime in the atmosphere, and thereby to affect the global climate. Radiative forcing reflects the factors that affect the balance between the energy absorbed by the earth and the energy emitted by it in the form of longwave infrared radiation. The GWP is defined on a mass basis relative to carbon dioxide. The GWP for a compound must be calculated up to a particular integrated time horizon, for example, 20, 100, or 500 years. The time horizon most widely accepted is 100 years.

Graetz number

dimensionless number used in calculation of streamline flow, equal to the mass flow rate of a fluid times its specific heat at constant pressure, divided by the product of its thermal conductivity and a characteristic length. Also spelled Gratz number. Symbol Gz, Gz, or NGz.

Graham’s law

the rate of diffusion of a gas is inversely proportional to the square root of its density.

grain of moisture

convenient unit of measurement of water vapor. 1 grain = 1/7000 pound avoirdupois (historically, the average weight of a grain of wheat).


(1) a subsidy furnished by a central government to a local one to help finance a public project. (2) a financial subsidy given to an individual or institution for research, educational, or cultural purposes.

GRAS (generally recognized as safe)

according to US FDA/HHS

Grashof number

dimensionless number used in the calculation of free convection of a fluid caused by a hot body, equal to the product of the fluid’s coefficient of thermal expansion, the temperature difference between the hot body and the fluid, the cube of a typical dimension of the body, and the square of the fluid’s density divided by the square of the fluid’s dynamic viscosity. Also known as free convection number. Symbol Gr, Gr or NGr.

gravity circulating evaporator

organized evaporator with circulation induced by a static head of liquid refrigerant driving an opposing column of boiling refrigerant in the evaporator, sometimes referred to as a thermosiphon.

gravity circulating system

heating or refrigerating system in which heating or cooling fluid is circulated by the forces induced by the differences in densities of cooler and warmer fluids in the system or surrounding atmosphere.

gravity current

stream of incoming fluid that moves horizontally immediately adjacent to the upper or lower boundaries of a stagnant body of fluid. The stream moves due to density differences between the fluid in the stream and that in the stagnant body of fluid. See also Froude number.

grease duct

a containment system for the transportation of air and grease vapor. The system is designed and installed to reduce the possibility of the accumulation of combustible condensation, thus reducing the possibility of damage if a fire occurs within the ventilation system.

grease-laden exhaust

effluent from a cooking process that contains grease particles or grease vapor. This type of exhaust requires a rated hood (e.g., Type I, Type II).

grease-removal device

a listed device (such as, but not limited to, baffle filters, removable extractors, and stationary extractors) designed and installed to remove grease vapor and/or particles from the airstream, .


constructed and performing in such a manner as to prevent the passage of grease under normal operating conditions.

green energy

energy that is produced in a manner that has less of a negative impact on the environment than energy sources like fossil fuels, which are often produced with harmful side effects. Some “greener” types of energy are solar, wind, geothermal, and hydro energy. There are several more, even including nuclear energy, that are sometimes considered a green energy source because of its lower emissions relative to energy sources such as coal or oil.

greenhouse gas (GHG)

gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds. This property causes the greenhouse effect. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and ozone (O3) are the primary GHGs in the Earth’s atmosphere. Moreover, there are a number of entirely human-made GHGs in the atmosphere, such as the halocarbons and other chlorine- and bromine-containing substances, dealt with under the Montreal Protocol. Besides CO2, N2O and CH4, the Kyoto Protocol deals with the GHGs sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).


louvered or perforated covering for an opening in an air passage which can be located in a wall, ceiling, or floor.

grooved joint piping

coupling system that joins pipes end to end by engaging circumferential grooves in the pipe using split couplings and a gasket.

gross building envelope floor area

the gross floor area of the building envelope, excluding slab-on-grade floors.

gross capacity

load (uncorrected) that a machine, apparatus, device, or system is designed to deliver.

gross conditioned floor area

the gross floor area of a conditioned space.

gross floor area

the sum of the floor areas of all the spaces within the building with no deductions for floor penetrations other than atria. Gross floor area is measured from the exterior faces of exterior walls or from the centerline of walls separating buildings, but it excludes covered walkways, open roofed-over areas, porches and similar spaces, pipe trenches, exterior terraces or steps, roof overhangs, parking garages, surface parking, and similar features.

gross lighted floor area

the gross floor area of lighted spaces.

gross refrigerating capacity

total rate of heat removal from all sources by the evaporator of a refrigerating system at stated conditions. It is numerically equal to the system refrigerating effect.

gross roof area

the area of the roof measured from the exterior faces of walls or from the centerline of party walls.

gross semiheated floor area

the gross floor area of semiheated spaces.

gross sensible capacity

the rate of sensible heat removal by the cooling coil for a given set of operating conditions. This value varies as a function of performance parameters such as EWB, ODB, EDB, and airflow rate. (Also see sensible heat.)

gross solar collector area

the maximum projected area of the complete collector module, including integral mounting means.

gross total capacity

the total rate of both sensible heat and latent heat removal by the cooling coil for a given set of operating conditions. This value varies as a function of performance parameters such as EWB, ODB, EDB, and airflow rate. Also see sensible heat and latent heat.

gross wall area

the area of the wall measured on the exterior face from the top of the floor to the bottom of the roof.

ground coupling

in a thermal storage system or a heat pump, a closed loop of piping, plastic tubing, or ducting used as a heat exchanger between the ground (acting as a low-grade heat source or heat sink) and a circulating fluid.

ground fault

unintentional electrical connection to ground. Electrical ground plane voltage or any point electrically connected to the earth or ground.

ground-loop heat pump application

heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat source/heat sink.


water that exists within the zone of saturation. Water that occurs below the surface of the earth, also called subsurface water, to distinguish it from surface water such as lakes oceans and streams.

groundwater heat pump

water-to-water heat pump using water pumped from a well, lake, or stream functioning as a heat source/heat sink. Note: the temperature of the water is related to the climatic conditions and may vary from 40°F to 75°F (5°C to 25°C) for deep wells.

guide-vane assembly

assembly of adjustable guide vanes fitted to the inlet of a fan or turbocompressor. The guide vane's function is to modify the flow and the pressure/flow rate.

guillotine damper

isolation damper with a blade which is withdrawn from the duct area when the damper is fully open.