ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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ring gasket of circular cross section, a torus.

O&M designer

designer specializing in the operations and maintenance aspects of the project.

object plane resolution

size in the object plane which corresponds to the products of the system’s instantaneous field of view in radians and the distance from the system to the object.

occupant sensor

a device that detects the presence or absence of people within an area and that causes lighting, equipment, or appliances to be regulated accordingly.

occupant-controlled naturally conditioned spaces

those spaces where the thermal conditions of the space are regulated primarily by the opening and closing of windows by the occupants.

occupiable space

(1) any enclosed space inside the pressure boundary (including, but not limited to, all habitable spaces, toilets, closets, halls, storage and utility areas, and laundry areas) and intended for human activities. (2) that portion of the premises accessible to or occupied by people, excluding machinery rooms.

occupied zone

the portion of the space that is normally occupied. The occupied zone is typically defined as encompassing all space from the floor level, excluding the space from the floor to 0.25 ft (0.076 m) above the floor, to 6 ft (1.83 m) above the floor and excluding the space from the wall to 2 ft (0.61 m) away from any wall.

octave band

a frequency band with an upper frequency limit twice that of its lower frequency limit. Octave and one-third octave bands are identified by their center frequencies, which are the geometric means of the upper and lower band limits. Three one-third octave bands make up one octave band. Octave 1- Center 63 HZ, Low 50 Hz, Upper 80 Hz, 2-100/125/160, 3- 200/250/315, 4-400/500/630, 5-800/1000/1250, 6-1600, 2000, 2500, 7-4000/5000/6300, 8-6300/8000/10000


(1) outside diameter. (2) outside dimension.


a quality of gases, liquids, or particles that stimulates the olfactory organ.

odor dispersion time

time taken to reduce odor to a defined level from a given concentration and resulting from a standard test.

odor reduction time

efficiency of the reduction of odors by a device.

off-cycle defrosting

(1) method of defrosting in which the temperature of the evaporator coils is allowed to rise naturally during an off-cycle, during which no refrigerant is supplied. (2) rapid heating of the evaporator coil during the off part of each cycle.

off-peak period

time of day other than the on-peak period of electrical demand.

off-peak system

refrigerating or cooling system with control that normally avoids use of power during peak-load periods and usually requires means for storage of energy.

office building

building or structure for office, professional, or service-type transactions (such as a medical office, bank, library, or business), including governmental offices.

Ohm’s law

direct current flowing in an electric circuit is directly proportional to the voltage applied to the circuit, Ohm's law is valid for metallic circuits and many circuits containing an electrolytic resistance.

oil charge

normal quantity of oil in a reciprocating compressor or engine.

oil cooler

a heat exchanger that can be cooled by air, brine, water, or refrigerant vaporization to cool oil in a lubrication system.

oil cooling

transfer of heat from oil in a pipe or tubing to a refrigerant or brine.

oil drain valve (oil purge valve)

valve for draining out the oil from all collection points in a system.

oil fired

support of combustion in a furnace, stove, etc. by the injection of fuel oil.

oil pressure cut-out

(also known as differential oil pressure switch or oil failure switch), safety device used to stop the compressor or burner when the oil pressure reaches a preset, abnormally low value.

oil pressure delivery

pressure of an oil pump in pressure-lubricated machines or oil burners.

oil pressure gage

gage fitted to the delivery side of an oil pump.

oil receiver

vessel for receiving and separating collected oil prior to returning to or discharging from the refrigerating system.

oil rectifier

(also known as oil still), apparatus for purifying oil by vaporizing the refrigerant contained.

oil removal

removal of oil or oil vapor from a refrigerant vapor.

oil return

migration of oil from the evaporator to the crankcase of the compressor. See compressor oil return.

oil separator

a device for separating oil and oil vapor from the refrigerant, usually installed in the compressor discharge line.

oil still

a device to separate oil from refrigerant by a distillation process.

oil temperature cut-out

safety device used to stop the compressor when the oil temperature reaches a preset, abnormally high value.

oil trap

device for separating and collecting oil at a given point in a refrigerating circuit.

oil-burner nozzle

nozzle for mechanically atomizing fuel oil in a burner.

oil-charge valve

valve enabling one to charge or top a refrigerating system with oil.

oil-charging pump

a hand or electrically operated pump for forcing oil into the compressor crankcase.

oil-circulation rate

the ratio of the mass of lubricant circulating through a refrigerant system to the total mass of refrigerant and lubricant flowing through the system at a specified set of operating conditions.

oil-free compressor

compressor in which no oil is used in the compression chamber. See dry piston compressor.


a hydrocarbon molecule containing at least one carbon-to-carbon double bond.


unit for quantifying the source strength of air pollution. One olf is the emission rate of air pollutants (bioeffluents) from a standard person.

on-demand heat
on-peak period

(1) time of day during which creating electrical demand incurs more cost. (2) time of day when use of power within a building is at a maximum, which may be the peak period of either the consumer or of the utility.

one-pipe hydronic system

piping system in which a single pipe loop provides the cooling or heating distribution to multiple devices or through secondary or tertiary loops.

one-pipe steam system

piping system in which the condensable vapor withdrawn from the supply main passes into a heating unit and returns as condensate to the same supply main.

ongoing commissioning process (OCx)

a continuation of the commissioning process well into occupancy/operations to continually improve the operation and performance of a facility.

opacity index

a number that expresses the relative dust accumulation on a dust spot sampling target, corrected for the nonlinearity of opacity increase at the constant dust accumulation rate. See dust spot opacity index.

open shell-and-tube condenser

condenser in which the water passes in a film over the inner surfaces of the tubes, which are open to the atmosphere.

open system

heating or refrigerating piping system in which the circulating water or brine return main is connected to an open-vented elevated tank that serves as a reservoir to accommodate expansion and contraction of the fluid and as an inspection point for the condition of the fluid. See also down-feed system, water system.

open-brine refrigerating system

system in which the circulating brine returns to an open tank which serves as a reservoir for the pump suction and as an inspection tank for the condition and flow of brine.

open-cell foamed plastic thermal insulation

cellular plastic in which interconnected cells predominate.

open-loop control

control system in which the effect of the control action is not felt by the sensed variable (e.g., outside air when it is used for reset). Compare to closed-loop control.

open-loop control system

control system that controls outputs by inputs only and where the actual system output is not considered.

open-spray recovery loop exchanger

extended surface cooling towers with interconnecting piping placed in supply and exhaust airstreams. A circulated heat and mass transfer fluid is alternately brought in direct contact with each airstream.

open-type compressor

refrigerant compressor with a shaft or other moving part extending through its casing to be driven by an outside source of power, thus requiring a shaft seal or equivalent rubbing contact between fixed and moving parts.

operating differential

difference between the cut-out and cut-in at the sensing element.

operating efficiency

ratio of output to input.

operating life

expected useful life of a device, usually expressed in number of operations or years/months/hours of typical operation.

operating opening

the position of the sash at which the fume hood user places the sash while working at the face of the fume hood. The operating opening should take into consideration all of the procedures to be conducted in the fume hood. There may be more than one operating opening.

operating pressure

the pressure occurring at a reference point in a system when the system is in operation.

operational carbon emissions

the total greenhouse gas emissions associated with the operation of an asset (i.e., building) during the use stage of the asset

operational carbon emissions

the total greenhouse gas emissions associated with the operation of an asset (i.e., building) during the use stage of the asset.

operational cleanroom

a cleanroom in normal operation with all services functioning and with production equipment and personnel present and performing normal work functions.

operative temperature

the uniform temperature of an enclosure in which an occupant would exchange the same amount of heat by radiation plus convection as in the actual nonuniform environment. Compare to mean radiant temperature.


(1) in the description of a process, that which indicates the action to be performed on operands. Compare to actuator. (2) person responsible for operating building automation equipment or who operates a machine.

operator station

point of operator interface to a building automation system using a graphic user interface and optional printers.

optical Doppler effect

change in the observed frequency of light or other electromagnetic radiation caused by relative motion of the source and the observer. Note: the frequency change does not depend on whether it is the source or the observer that is moving.


(1) collection of data in a control system in order to produce the best possible output, usually in accordance with what is most economical. (2) procedure used in the design of a system to maximize or minimize some performance index. May entail the selection of a component, a principle of operation, or a technique. (3) refinement of a system to reduce its energy requirements.

optimum operative temperature

operative temperature that satisfies the greatest possible number of people at a given clothing and activity level. See operative temperature.

optimum refrigerant charge

charge achieving maximum possible refrigerant effect within design limitations.

optimum start

a control system that is designed to automatically adjust the start time of an HVAC system each day with the intention of bringing the space to desired occupied temperature levels immediately before scheduled occupancy.

orifice meter

instrument that measures fluid flow by recording differential pressure across a restriction placed in the flow stream and the static or actual pressure acting on the system.

orifice plate

relatively sharp-edged orifice in a plate used for the calculation of fluid flow rates from measurements of the pressure difference between the two sides of the orifice.

Orsat apparatus

gas analyzer based on absorption of CO2, O2, etc. by separate chemicals that have a selective affinity for each of those gases.

orthographic drawing

plan and elevation detail drawing.

osmotic pressure

differential pressure that must be applied to the fluid on one side of a semipermeable membrane to create diffusion through the membrane.

Otto cycle

in a two- or four-stroke reciprocating engine, the compressed fuel air mixture is ignited by a spark at or near the start of the power stroke, as in conventional automobile engines.

outdoor air

(1) air outside a building or taken from the external atmosphere and, therefore, not previously circulated through the system. (2) ambient air that enters a building through a mechanical ventilation system, through intentional openings for natural ventilation, or by infiltration. (3) compare to outside air.

outdoor air change rate

the ratio of the volumetric rate at which outdoor air enters a building space to the building volume with identical volume units (normally expressed in units of air changes per hour).

outdoor air fraction

the outdoor air fraction is the ratio of the volumetric flow rate of outdoor air brought in by the air handler to the total supply airflow rate.

outdoor coil

the heat exchanger that rejects heat to, or absorbs heat from, a source external to the conditioned space.

outdoor dry-bulb temperature (ODB)
outdoor side

that part of the system that rejects heat to or absorbs heat from a source external to the indoor airstream.

outlet area

gross overall discharge area of a given component in an air-distribution system.

outlet spacing

the distance, measured from centerline to centerline, between adjacent air outlets.

outlet valve
outlet velocity

average velocity of fluid emerging from an outlet measured in the plane of the outlet.


(1) (general) current, voltage, power, or driving force delivered by a circuit or device, terminals or other places where current, voltage, power, or driving force may be delivered by a circuit of device. (2) capacity, duty, performance, net refrigeration produced by a system. (3) process of transferring data from an internal storage.

outside air

air external to a defined zone (e.g., corridors).

overall heat transfer coefficient

heat flow per area for a given construction and for an overall temperature difference of one degree.

overall system efficiency

ratio of the useful energy at the point of use to the thermal energy input over a designated time period.

overall thermal transfer value (OTTV)

quantity of heat transferred per unit of temperature difference into a building through its walls or roof, due to solar heat gain and outdoor/indoor temperature difference.


any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.

overcurrent motor protection

(1) device used to interrupt the supply voltage to a motor when the allowable full-load current is exceeded. (2) overload devices, either eutectic alloy or bimetal, that protect a motor against an overcurrent. Fuses and circuit breakers do not protect a motor, they only protect branch wiring.

overhead system

heating, air-conditioning, or radiant system in which the heating or cooling emission is overhead.


pressure in excess of the designed normal operating range.


(1) amount of the overtravel of an indicator beyond its final steady deflection when a new, constant value of the measured quantity is suddenly applied to the instrument. (2) condition in a control system where the controlled variable exceeds the desired setpoint as a result of approaching that setpoint too quickly. (3) excursion of the controlled variable beyond the differential or proportional band.


a person or legal entity that will own the delivered facility or an agent representing the owner, who defines the project requirements.

owner's program

the document that outlines the owner’s overall vision for the facility and expectations of how it will be used and operated.

owner's representative

person(s) authorized to function on behalf of the owner.

Owner’s Project Requirements

a written document that details the functional requirements of a project and the expectations of how it will be used and operated. These include project goals, measurable performance criteria, cost considerations, benchmarks, success criteria, and supporting information. (The term project intent is used by some owners for their commissioning process Owner’s Project Requirements.)


triatomic form of oxygen, O3. Sometimes used as an oxidant in air conditioning and as an odor eliminator in cold storages or exhaust stacks. It is toxic in concentrations of 1 ppm and higher.

ozone depletion potential (ODP)

a numerical quantity describing the extent of ozone depletion calculated to arise from the release to the atmosphere of one kilogram (2.2046 lb) of a compound relative to the ozone depletion calculated to arise from a similar release of the refrigerant R-11. The calculation is an integration of all known potential effects on ozone over the whole time that any portion of the compound could remain in the atmosphere.

ozone, air cleaner

process creates triatomic form of oxygen, O3, a type of reactive oxygen species (ROS). In occupied spaces, higher ozone concentrations can be an irritant or adversely affect health. In non-occupied spaces (exhaust stacks), it can be used as an odor eliminator.