ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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U-factor converted into electrical terms for calculations in electric heating. The amount of heat flow, in watts per square foot per degree Fahrenheit temperature difference (W/ft2·F) between air on the inside and air on the outside of the building section (wall, floor, roof, or ceiling). For conversion, W = 0.293U.


that portion of the building envelope, including opaque area and fenestration, that is vertical or tilted at an angle of 60 degrees from horizontal or greater. This includes above and below-grade walls, between floor spandrels, peripheral edges of floors, and foundation walls.

wall section

cross section of wall, or a drawing of it, arranged to reveal thermal and moisture migration characteristics.

wall sleeve

opening in a wall having a shroud insert to accept a pipe, duct, or equipment installed in or through the wall.

wall-mounted canopy hood

those portions of the building envelope that are vertical or tilted at an angle of 30 degrees or less from the vertical plane. Above grade—all the exterior walls of any given story if 50% or more of the gross exterior wall area of the story is exposed to outside air. Below grade—all the exterior walls of any given story if more than 50% of the gross exterior wall area of the story is below grade. Mass wall—a wall constructed of concrete, concrete masonry, insulating concrete form (ICF), masonry cavity, brick (other than brick veneer), earth (adobe, compressed earth block, rammed earth), and solid timber or logs.

warm up

(1) increase in space temperature to occupied setpoint after a period of shutdown or setback. Also referred to as morning warm up. (2) period of time required for a space to return to normal temperature after a period of shutdown or setback.

warming-up allowance

addition to the capacity of a heating system (as calculated for heat loss) to provide quick warm up in the morning.

waste energy sources

any useful energy source which otherwise would be unused or left to the environment.

waste heat

(1) heat rejected from the building (or process) because its temperature is too low for economical recovery or direct use. (2) unused heat rejected from a system, usually a heat engine or combustion furnace, to its surroundings.


transparent, odorless, tasteless liquid, a compound of hydrogen and oxygen (H2O), containing 11.188% hydrogen and 88.812% oxygen by mass, freezing at 32°F (0°C), boiling near 212°F (100°C).

water column (wc)

(1) tubular column located at the steam and water space of a boiler to which protective devices, such as gage cocks, water gage, and level alarms are attached. (2) unit of pressure.

water contaminant

substance introduced into potable water thus creating a health hazard. Compare to water pollutant.

water cooling tower
water defrosting

defrosting in which water is sprayed or poured over the frosted surface.

water economizer

a system by which the supply air of a cooling system is cooled indirectly with water that is itself cooled by heat or mass transfer to the environment without the use of mechanical cooling.

water gage (wg)

(1) designation that water is the fluid in a manometer. (2) gage glass with attached fittings which indicates water level within a vessel. (3) unit of pressure (I-P units). See water column (wc).

water hammer
water heater

closed vessel in which water is heated by the combustion of fuels, electricity, or any other source and is withdrawn for use external to the system at pressures not exceeding 160 psig [1100 kPa (gage)], including the apparatus by which heat is generated, and all controls and devices necessary to prevent water temperatures from exceeding 210°F (99°C). See also boiler.

water heater low pressure

hot-water boiler having a volume not exceeding 120 gal (450 L), or a heat input not exceeding 200,000 Btu/h (60 kW), or an operating temperature not exceeding 250°F (120°C), to provide hot water to a system.

water loop heat pump application

water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat source/heat sink.

water manifold

water connection wherein one pipe communicates with several other lateral outlets through a common housing or fitting.

water pollutant

substance introduced into potable water, thus creating an objectionable condition, but not creating a health hazard. Compare to water contaminant.

water regulating valve

an automatic valve that controls the flow of cooling water through a condenser to maintain a set condensing pressure.

water rejection efficiency of a weather louver

efficiency of a weather louver at any air velocity under test conditions.

water system

(public water system), system operated as a public utility that supplies potable water to the service connection of the consumer’s water system. It is the primary component of a public water system.

water treatment

process that alters supply water so that it can be used for process or HVAC purposes without creating undue corrosion or scaling to the piping systems and other deleterious effects.

water tube boiler

boiler in which tubes contain water and steam, with heat applied to their outside surfaces.

water vacuum refrigerating system

uses a vacuum to boil water at the temperature desired, one in which evaporating water vapor is the refrigerant.

water vapor

water in the vapor or gas phase.

water vapor content

in a gas, the mass of water vapor per unit mass of dry air.

water vapor density

water vapor is lighter or less dense than dry air. At equivalent temperatures it is buoyant with respect to dry air.

water vapor flux

time rate of water vapor transfer through a unit area, mv, in lbm/ft2·s [kgm/(m2·K)]. Vapor flux is a vector.

water vapor migration

(vapor transfer, vapor transmission), transmission of water vapor through insulating or other material resulting from the difference in partial pressures on both sides.

water vapor resistivity

steady vapor pressure difference that induces unit time rate of vapor flow through unit area and unit thickness of a flat material (or construction that acts like a homogeneous body) for specific conditions of temperature and relative humidity at each surface. Vapor resistivity is the reciprocal of vapor permeability.

water vapor retarder

material or construction that adequately impedes the transmission of water vapor under specified conditions. Water vapor retarders have a water vapor permeance of less than 1.0 perm when tested in accordance with ASTM E 96.

water vapor transmission rate (WVTR)

steady-state vapor flow in unit time through unit area of a body, normal to specified parallel surfaces, under specific conditions of temperature and humidity at each surface.

water-formed deposit

any accumulation of insoluble material derived from water or formed by the reaction with water on surfaces in contact with it.

water-holding capacity

during a drying process, the ability of a substance to retain water.

waterside economizer

a heat exchanger that uses the condenser water side of the system for cooling without requiring the operation of the chiller. Also a coil on the air entering side of a heat pump or HVAC unit that uses condenser water flow to precondition the entering air when conditions are favorable. See also cooling.

waterside economizer cooling

economizer process that uses cooling tower-water directly or indirectly in the cooling coils, permitting the chiller to be shut down when the outside wet-bulb temperature is sufficiently low.


ability of an externally mounted air transfer device to resist water penetration.

watt meter

metering system capable of measuring the energy added to or extracted from an electric or fluid stream. Also called thermal energy meter, heat meter, or thermal meter. Compare to Btu meter.


in petroleum oils, a material, usually a solid hydrocarbon, that may separate on cooling of an oil refrigerant mixture.


(1) drip from frozen foods. (2) in buildings, a small opening that allows water to drain from within an assembly. They are located at the bottom of the object to allow for drainage. The weep hole must be sized adequately to allow for surface tension.

welded joint

a gastight joint obtained by the joining of metal parts in the plastic or molten state.

well ventilated

ventilation rated to sustain life and safety.

wet compression system

system of refrigeration in which some liquid refrigerant is mixed with vapor entering the compressor to cause the discharge vapor from the compressor to be saturated, rather than superheated.

wet return

(1) in a refrigeration system, where the connections between the evaporator outlets and the low pressure receiver through which the mixture of vapor and overfeed liquid is drawn. (2) in a steam system, where a return pipe carries condensate, the pipe is usually located below the level of the waterline in the boiler.

wet saturated steam

steam at the saturation temperature corresponding to the pressure and containing water particles in suspension. Compare to dry saturated steam.

wet ton (moisture ton)

latent heating or cooling load. See ton of refrigeration.

wet vapor

(1) opaque ice in which air or salts present in the raw water are trapped. (2) saturated vapor containing liquid droplets in suspension.

wet-bulb depression

the difference between the dry-bulb and wet-bulb temperatures at the same location.

wet-bulb temperature

the temperature indicated when a thermometer bulb is covered with a water saturated wick over which air is caused to flow at approximately 4.5 m/s (900 ft/min) to reach the equilibrium temperature of water evaporating into the air when the heat of vaporization is supplied by the sensible heat of the air.


those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation adapted for life in saturated soil conditions. This definition incorporates all areas that would meet the definition of “wetlands” under applicable federal or state guidance—regardless of whether they are officially designated, delineated, or mapped—including manmade areas that are designed, constructed, or restored to include the ecological functions of natural wetlands.

white noise

generated noise having uniform sound pressure at all frequences, generally between 125 and 4000 Hertz

white oil

a highly refined petroleum-based lubricant fraction that is essentially free of aromatic hydrocarbons, olefins, and heteroatoms (sulfur, nitrogen, or oxygen).

white room

see cleanroom.

whole-life carbon emissions

the total greenhouse gas emissions, including operational carbon emissions and embodied carbon emissions over the life cycle of an asset (i.e., building).

Wien’s displacement law

when the temperature of a radiating blackbody increases, the wavelength corresponding to maximum energy decreases in such a way that the product of the absolute temperature and the wavelength is constant.

Wien’s radiation law

intensity of radiation emitted by a blackbody per unit wavelength, at that wavelength at which this intensity reaches a maximum, is proportional to the fifth power of the temperature.

wind chill

the apparent temperature felt on exposed skin due to wind. The degree of this phenomenon depends on both air temperature and wind speed. The wind chill temperature (often popularly called the wind chill factor) is always lower than the air temperature for values where the wind chill formula is valid. In cases where the apparent temperature is higher than the air temperature, the heat index is used instead. Wind chill is always expressed as a temperature. Compare to chill factor, which is always expressed as time.

wind pressure

total force exerted on a structure by wind. See velocity pressure.

window air conditioner

room air conditioner designed for mounting in window.


erosion of a valve plug or seat due to very high fluid velocity, usually caused by prolonged operation in a nearly closed position.

Wobbe index

number which indicates interchangeability of fuel gases, obtained by dividing the heating value of a gas by the square root of its specific gravity.


(1) compare to energy, which is work per unit time. (2) mechanical work: the amount of energy transferred by a force. Units of mechanical work are ft·lb (J).

working fluid

medium evolving within a thermodynamic cycle.

working pressure range

the range of pressures the system is expected to experience during normal operation.

workplace environmental exposure level (WEEL)

an occupational exposure limit set by the American Industrial Hygiene Association (AIHA).

worst case of fractionation for flammability (WCFF)

the composition produced during fractionation of the worst case of formulation for flammability that results in the highest concentration of flammable components as identified in ANSI/ASHRAE Standard 34 in the vapor or liquid phase.