ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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state in which the fluid, usually gas, pressure is lower than atmospheric pressure.

vacuum breaker (vacuum cracking)

a device that allows an increase in the pressure in an enclosure under vacuum by admitting gas (air).

vacuum cooling (vacuum chilling)

cooling by vaporization under vacuum of part of the water contained in the material to be cooled.

vacuum diffusion pump

pump in which the motion of a gas is induced by its diffusion into the vapor phase of a working fluid in the pump.

vacuum freezing

freezing of a substance by lowering pressure to induce vaporization of a part of the solvent (usually water).

vacuum gage

gage for measuring pressures below atmospheric pressure.

vacuum pump

pump for exhausting a system, pump designed to produce a vacuum in a closed system or vessel.

vacuum pump (ion pump)

vacuum pump operated via the ionization of gases, the produced ions being transported by an electrical or magnetic field.

vacuum test

test to check the gas tightness of an uncharged system or component by drawing a vacuum on it.


device to regulate or stop the flow of fluid in a pipe or a duct by throttling.

valve area

area encountered by a fluid flowing through a valve.

valve body

shell or housing of a valve into which the mechanism for regulating flow is fitted.

valve cage

device for holding and locating a valve disc, poppet, etc., in relation to a valve seat.

valve cover

removable part that allows access to the valve mechanism.

valve disc

disc or button fixed to the end of a valve stem and which meets the valve seat on closure.

valve flutter (valve bounce)

uncontrolled oscillation of a compressor valve on its seat during opening or closing.

valve guard

part that limits or controls the opening of a valve.

valve lift

vertical distance through which a valve is raised to allow the passage of a fluid.

valve obturator

that part of a valve that closes off or stops fluid flow.

valve plate

plate on which the valves of a reciprocating or rotary compressor are located.

valve plug

(1) a movable part that provides a variable restriction in a port. (2) generic name for that part of a valve which mates with the seat to control or shut off flow.

valve port

opening for passage of a fluid in a valve.

valve seat

stationary portion of the valve which, when in contact with the movable portion, stops flow completely.

valve stem

spindle movable towards or away from a valve seat for guiding the valve head during closing or opening.

valve trim

internal parts (stem, plug, disc, seat, etc.) of a valve.

Van’t Hoff’s principle

if the temperature of interacting substances in equilibrium is raised, the equilibrium concentrations of the reaction are changed, so that the products of that reaction that absorb heat are increased in quantity, or if the temperature for such an equilibrium is lowered, the products that evolve heat in their formation are increased in amounts.

vane ratio

(1) in air-distributing devices, the ratio of the depth of vane to shortest opening width between two adjacent bars in a grille. (2) ratio of the chord length to the vane pitch.

vaneaxial fan

fan that moves air in the general direction of the axis about which it rotates, parallel to the shaft, and flow through the centrifugal fan wheel, impeller, or propeller is substantially radial. The fan is typically mounted in a round duct or tube to improve pressure characteristics and has vane-type straighteners to remove much of the swirl or spin of the air created by the fan wheel to improve fan efficiency. Fans can be either direct drive or belt driven.


(1) gas, particularly one near equilibrium with its liquid phase and one that does not follow the gas laws. The term is usually used instead of gas to refer to a refrigerant, or in general, to any gas below the critical temperature. (2) substance in the gaseous state that can also exist as a liquid or solid at normal atmospheric conditions. Compare to gas.

vapor barrier

use vapor retarder. See water vapor retarder.

vapor composition

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

vapor concentration

(also known as absolute humidity or water vapor density number), in a mixture of water vapor and dry air, the mass of water vapor in a specific volume of the mixture. Compare to relative humidity.

vapor content ratio

ratio of the amount of vapor present compared to the amount present at saturated conditions.

vapor control

(1) control device with a vapor charged power element. (2) method of control using the expansion and contraction forces of a contained vapor.

vapor jet

refrigerating cycle using an ejector to compress the refrigerant vapor from the evaporator to the condenser. The term steam jet is used when water is the refrigerant.

vapor lock

formation of vapor in a liquid line that reduces mass flow compared to the flow of the total liquid at the same pressure differential.

vapor lock device

orifice capillary tube, or other device having a restricted passage of fixed size for liquid refrigerant. It restricts flow of vapor of that same liquid to a lower rate of flow with the same pressure difference.

vapor mass fraction
vapor pickup tube

usually located at the bottom of large absorber tube bundles.

vapor pressure

the pressure exerted by a vapor. If a vapor is kept in confinement over its liquid so that the vapor can accumulate above the liquid with the temperature constant, the vapor pressure reaches a maximum called the saturated vapor pressure.

vapor quality

ratio of the mass of vapor to the total mass when a substance exists as part liquid and part vapor at the saturation temperature. Note: quality has meaning only when the substance is in a saturated state, i.e., at saturation pressure and temperature.

vapor refrigerating system

system using a condensable vapor as the refrigerant.

vapor retarder

material or construction that adequately impedes the transmission of water vapor under specified conditions. Compare to structural barrier.

vapor sample

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


change of phase from liquid to vapor.

vaporizing burner

burner designed to utilize the temperature of the heated combustion chamber to vaporize the liquid fuel fed into such chamber.

vapor-pressure thermometer

one based on pressure built up by the vapor evolved by liquid in a sealed container.

variable air volume (VAV)

HVAC system that controls the dry-bulb temperature within a space by varying the volumetric flow of heated or cooled supply air to the space.

variable flow

throttling control of water during a cooling or heating process.

variable output

for each element of a control system, a physical quantity whose changes are governed ultimately by the functioning of the element.

variable refrigerant flow (VRF) system

an engineered direct-expansion (DX) multisplit system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to multiple indoor fan-coil units, each capable of individual zone temperature control, through integral zone temperature control devices and common communications network. Variable refrigerant flow utilizes three or more steps of control on common, interconnecting piping.

variable volume, variable temperature (VVT)

combination of varying both airflow and temperature in response to space load, for the purpose of resetting temperature to maintain greater low-load airflow to the space than in a true variable volume system.

variable-capacity equipment

heating and cooling equipment that operates in stages of different capacity depending on building load, e.g., electric furnaces with several separate heater elements.

variable-frequency drive (VFD)

electronic device that varies its output frequency to vary the rotating speed of a motor, given a fixed input frequency. Used with fans or pumps to vary the flow in the system as a function of a maintained pressure.

variable-refrigerant-flow (VRF) system

an engineered direct expansion (DX) multisplit system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to multiple indoor fan-coil units, each capable of individual zone temperature control, through integral zone temperature control devices and common communications network. Variable refrigerant flow uses three or more steps of control on common, interconnecting piping.

variable-speed control of a fan

a method of controlling airflow produced by a fan by means of adjusting the speed or revolutions per minute (rpm) of the fan.

variable-speed drive
VAV box

variable-air volume terminal device


a measurement of the distance traveled per unit of time. This quantity is defined by its magnitude and direction at any point of the flow.

velocity coefficient

ratio of the actual velocity of gas emerging from a nozzle to the velocity calculated under ideal conditions, it is less than one because of friction losses.

velocity constant

ratio of the rate of change of the input command signal to the steady-state error, in a control system where these two quantities are proportional.

velocity head
velocity pressure

(1) in a moving fluid, the pressure that would induce an equivalent velocity if applied to move the same fluid through an orifice, so that all pressure energy is converted into kinetic energy. Velocity pressure is always a positive value, in. H2O (kPa). (2) the difference between the total pressure and static pressure (relative to the same datum).

velocity profile

graph that represents, in a plane section, the velocity distribution in a flowing fluid.

velocity reduction method duct sizing

method in which ducts are sized so that selected velocities occur in specific duct lengths.

vena contracta

smallest area of a fluid stream leaving an orifice.


a connection in any system or enclosure that may be open to atmosphere or to a lower pressure, space, or vessel and is intended for the transfer of any fluid. A vent can be used for either intake or relief purposes.

vent connector

the portion of the venting system that connects the gas appliance or its draft hood to the chimney or vent terminal.

vent damper

a device intended for installation in the venting system of an individual, automatically operated, fossil-fuel-fired appliance in the outlet or downstream of the appliance draft control device, which is designed to automatically open the venting system when the appliance is in operation and to automatically close off the venting system when the appliance is in a standby or shutdown condition.

vent gas

see flue gas.

vent limiter

a device that limits the flow of air from the atmospheric diaphragm chamber of a gas pressure regulator to the atmosphere. A vent limiter may be a limiting orifice or other limiting device.

vent or air intake terminal

a device that is located on the outside of a building and may be connected to a furnace or boiler by a system of conduits. It is composed of an air intake terminal through which the air for combustion is taken from the outside atmosphere and an exhaust terminal from which flue gases are discharged.

vent pipe

(1) discharge piping connected to a safety or relief valve. (2) passages and conduits in a direct vent or direct exhaust system through which gases pass from the combustion chamber to the outdoor air.

vent relief

(1) opening in a tank, duct, or other piece of equipment, sealed to prevent escape of material within normal pressures, but arranged to open automatically to relieve excessive pressure. (2) relief opening in a pressure regulator, normally open to the atmosphere.

vented crawlspace

a crawlspace with an open vent area 1/150th of its floor area with vents distributed over all exterior surfaces.


(1) the process of supplying air to or removing air from a space for the purpose of controlling air contaminant levels, humidity, or temperature within the space. (2) the process of supplying or removing air by natural or mechanical means to or from any space. Such air may or may not have been conditioned.

ventilation air

the minimum amount of outdoor air required for the purpose of controlling air contaminant levels in buildings.

ventilation effectiveness

value indicating how effectively specified contaminants in a defined volume are removed or diluted based on the relationship of the concentrations of those contaminants in the air supplied to, removed from, and remaining within the defined volume.

ventilation effectiveness concentration

measure of the relationship between the pollutant concentration in the exhaust air and the pollutant concentration in the specified zone.

ventilation effectiveness concentration

measure of the relationship between the pollutant concentration in the exhaust air and the pollutant concentration in the specified zone.

ventilation rate

rate of airflow to or from a space, expressed in terms of volume per unit of time, either through the ventilation system or infiltration through building envelope. These are commonly expressed as cubic feet per minute (CFM) in USCS units or liters per second (l/s) in SI units.

ventilation system motor nameplate horsepower (hp)

the sum of the motor nameplate horsepower of all fans that are required to operate as part of the system.


contraction in a pipeline or duct that increases the fluid velocity to lower its static pressure, followed by a gradual expansion to allow recovery of static pressure. Used for metering and other purposes that involve change in pressure.

Venturi tube

(1) specially shaped construction inserted in a pipe to measure the rate of flow from induced changes in pressure in the constricted section. (2) specially shaped construction that creates a vacuum in a steam/water refrigeration system.


the process by which specific documents, components, equipment, assemblies, systems, and interfaces among systems are confirmed to comply with the criteria described in the rating program.

verification and testing provider (V&T provider)

an entity who completes the activities needed to implement the building functional performance testing (FPT) activities or verify that elements of the building project meet stated requirements.

vertical axis of measurement (y-axis)

an axis of measurement perpendicular to the horizontal and axial axes of measurement of a piece of equipment but not necessarily in the horizontal plane.

very cold climate

climates that have more than 9000 annual heating degree-days base 65°F-day (5000 annual heating degree-days base 18°C-day).


container or structural envelope in which materials are processed, treated, or stored. Compare to tank.


an oscillation wherein the quantity is a parameter that defines the motion of a mechanical (physical) system. The term is usually used to mean an objectionable movement.

vibration isolation

in any rotating equipment where the isolation of its vibration is by devices external to the unit. Note: examples: flex duct connections, pipe flex connections, and spring isolators.

vibration severity

the magnitude of the vibration expressed in engineering units (usually velocity [mm/s]). Note: vibration severity values may represent either the overall value or a value obtained from a frequency analysis. It is extremely important to denote which choice was made. It is also necessary to denote whether the value is zero-to-peak, peak-to-peak, or root mean square (rms).


instrument for measuring viscosity.


(1) property of a fluid to resist flow or change of shape. (2) property of semifluids, fluids, and gases by which they resist an instantaneous change of shape or arrangements of parts. It causes fluid friction whenever adjacent layers of fluid move with relation to each other.

viscosity coefficient

force per unit area required to produce unit relative viscosity between two parallel areas of fluid unit distance apart.

viscosity index (VI)

(1) characteristic of lubricating oil that indicates the change in viscosity with temperature, i.e., a high VI oil has less change in viscosity with temperature change than a lower VI oil. (2) empirical number evaluated by comparing the viscosity of a substance with that of a standard substance.

viscosity manometer

pressure gage which uses the variations of viscosity of a gas with pressure (in vacuum techniques).

viscous filter

filter in which the medium is made from materials which have been impregnated with a viscous oil.

viscous flow

(1) laminar flow or streamline flow. (2) type of gas flow in which the average free path of gas molecules is much smaller than the smallest cross-sectional dimension of the pipe conveying the substance.

visible transmittance (VT)

the ratio of visible radiation entering the space through the fenestration product to the incident visible radiation, determined as the spectral transmittance of the total fenestration system, weighted by the photopic response of the eye and integrated into a single dimensionless value.

volatile liquid

liquid that evaporates readily at atmospheric pressure and room temperatures.

volatile organic compounds (VOC)

organic compounds in the vapor state present in an indoor atmosphere.

volatile refrigerant

a refrigerant that changes from the liquid to the vapor state in the process of absorbing heat.

volt ohmmeter (VOM)

a device for measuring voltage, or the resistance of an electrical circuit. Some models also measure current flow through components in electrical circuits.


electric potential or potential difference expressed in volts.

voltage drop

a decrease in voltage caused by losses in the lines connecting the power source to the load.

voltage relay

(1) device that is voltage sensitive with the ability to sense between two or more voltage levels to operate a set of contacts. (2) relay that responds to a predetermined voltage.

voltampere reactive (VAR)

unit of voltampere reactive power. For a two wire circuit, the product of the voltage times the current times the sine of the angular phase difference by which the voltage leads or lags the current. VARs and watts combine in a quadrature to form voltamperes.

volume control damper

device mounted in a duct or opening used to vary the volume of air flowing through.

volumetric air flow rate

volumetric flow of air over specified time, usually expressed in l/s or m3/h.

volumetric efficiency due to cylinder heating

ratio of the total to the apparent volumetric efficiency. Also called real or no-clearance volumetric efficiency.

volumetric heat capacity

the change in heat stored in unit volume of material for unit change of temperature.

volumetric moisture capacity

the change in stored moisture per unit volume of porous material and per unit moisture potential change.

volumetric total efficiency

ratio of the actual volume of gas moved by the compressor or pump to actual displacement of the compressor or pump.