A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 77 terms in this directory beginning with the letter W.
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.

A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

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
see [[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
see [[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
see [[hydraulic shock]].

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 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.

The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

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]].

The shape of the phase power at a certain frequency and amplitude.

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

see [[water column (wc)]].

see [[workplace environmental exposure level]].

(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 shelf life
The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

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.

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]].

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]].

A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

window air conditioner
room air conditioner designed for mounting in window.

wire types
See Article 300 of National Electric Code for more information.

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).

Work function
The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.

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.

Water Pollution Control Federation