ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

provided by a manufacturer or vendor in a substantially complete and operable condition.

packaged air conditioner

(also known as self-contained unit), complete air-conditioning unit, including refrigeration compressor, cooling coils, fans, filters, automatic controls, etc., assembled into one casing.

packaged boiler

a boiler that is shipped complete with heating equipment, mechanical draft equipment, and automatic controls, usually shipped in one or more sections. A packaged boiler includes factory-built boilers manufactured as a unit or system, disassembled for shipment, and reassembled at the site.

packaged direct-evaporative cooler

a self-contained unit including a fan and fan motor whose primary functions are (1) the conversion of the sensible heat of unsaturated air passing through the cabinet to latent heat by the process of evaporating recirculating or nonrecirculating water directly exposed to this air and (2) the movement of this air through the unit.

packaged indirect evaporative cooler

an indirect evaporative cooler with integrated or nonintegrated primary and secondary air passages and provided with both primary and secondary air moving devices. This device also includes the entire water distribution, collection, and recirculation system with pump and piping. This type may have provisions for installation of other heat and mass transfer devices, such as a direct evaporative cooler and auxiliary heating and cooling coils.

packaged systems

heat recovery devices that combine the exchanger with filters, fans, and controls.

packaged terminal air conditioner (PTAC)

a factory selected wall sleeve and separate unencased combination of heating and cooling components, assemblies, or sections. It may include heating capability by hot water, steam, or electricity and is intended for mounting through the wall to serve a single room or zone.

packaged terminal heat pump (PTHP)

a PTAC capable of using the refrigerating system in a reverse cycle or heat pump mode to provide heat.


(1) cooling tower packing. See stuffing box. (2) stuffing around a shaft or valve stem to prevent fluid leakage.

packing plant

production operation for slaughtering, dressing, and processing animals, also used in connection with processing vegetables and fish.

packless valve

valve that does not use packing to seal against leaks around the stem (e.g., bellows valve).

palladium cells

found in large, direct-fired and small, indirect-fired machines that continuously remove the small amount of hydrogen gas that is produced by corrosion.

panel cooler

refrigerated flat surface.

paraffinic oil

a petroleum lubricating oil fraction in which straight and/or branched-chain hydrocarbon structures predominate.


an inaccurate reading of a meter due to line of vision between the scale and the indicator being misaligned or angled.

parallel fan-powered VAV box

terminal device that combines a true VAV box in parallel with a fan and optional heating coils. Fan operation is intermittent.

parallel-flow heat exchanger

heat exchanger in which fluids flow approximately parallel to each other and in the same direction.

parallel-flow heat transfer

directional pattern of heat transfer fluids used in energy exchange equipment where the warmest fluid “A” indirectly contacts the coldest fluid “B” at the entering side of the exchanger.

part-load ratio

the ratio of the net refrigeration effect to the adjusted net total capacity for the cooling coil. (Also see net refrigerating effect and adjusted net total capacity.)

part-load value

single number figure of merit expressing part-load efficiency for equipment on the basis of weighted operation at various partial-load capacities for the equipment, expressed in kilowatts per ton of refrigeration.

partial pressure

(1) partial pressure of component i for any (ideal or nonideal) gas mixture is the product of the mole fraction of the component, xi, and the mixture pressure, p. [ pi = xi • p for any gas mixture]. (2) portion of total gas pressure of a mixture attributable to one component (Dalton’s law of gases).

partially halogenated chlorofluorocarbon

a chlorofluorocarbon in which hydrogen atoms exists in the compound with chlorine and fluorine atoms. Partially halogenated chlorofluorocarbons are not expected to have high ozone depleting potential (ODP). Average atmospheric lifetime is about 2 years for HCFC 21 and 13 to 25 years for HCFC 22. Compare to halogenated chlorofluorocarbon.

particle size

apparent maximum linear dimension of a particle in the plane of observation. Particulate state of matter in which solid or liquid substances exist in the form of aggregated molecules or particles. Airborne particulate matter is typically in the size range of 0.01 to 100 micrometers. See also atmospheric dust, respirable particles, and total suspended particulates.

partition fan

see fan types.

parts per million (ppm)

parts per million by mass.

party wall

a fire wall on an interior lot line used or adapted for joint service between two buildings.

Pascal’s law

confined fluid transmits externally applied pressure uniformly in all directions without change in magnitude.

passive chilled beam

uses a pipe surrounded by a coil in order to form a radiator system. Passive chilled beams have no method for maintaining the humidity of a room and must be paired with a ventilation system in order to maintain latent heat gains.

passive system

(1) a combined appliance that does not use a water pump but relies on thermosiphon flow through the desuperheater and water heater, generally the desuperheater is mounted on the side of the water heater or at an elevation lower than the water heater. (2) a system where natural forces are utilized for HVAC purposes in lieu of mechanical/electrical/chemical sources.


tendency of a metal to become inactive in a given environment.


any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa, or fungi. Microorganisms are not considered pathogenic until they have reached a population size large enough to cause disease.


a person receiving medical, surgical, or psychiatric care in an inpatient or outpatient facility.

peak demand

The highest measured demand encountered during a specified period of time (e.g., month, year, or finite set of time intervals).

peak discharge rate

the maximum rate at which heat is added to storage (cooling discharged from storage).

peak draw capacity

the maximum number of standard drinks drawn under standard test conditions without the beverage exceeding 40°F (4.4°C) at a draw rate defined by the manufacturer within a given time limit.

peak load

The maximum amount of power delivered (load) for a given time period. (NEMA glossary (

peak-to-peak (p-p) vibration value

the total distance traveled by the vibrating part, from one extreme limit of travel to the other extreme limit of travel, usually expressed in terms of linear displacement.

Peclet number

dimensionless number used in calculation of heat and mass transfer, consisting of the mass velocity times a characteristic length and the specific heat at constant pressure, divided by the thermal conductivity. Symbol Pe, Pe, or NPe.

Peltier effect

evolution or absorption of heat produced by an electric current passing across junctions of two suitable dissimilar metals, alloys, or semiconductors.


the ratio, fraction, or percentage of particles that pass through the air cleaner or filter, as measured downstream of the device.

penetration data acceptance criteria

criteria used to determine the adequacy of the penetration data.

percent outdoor air

volumetric outside airflow rate expressed as a percentage of total supply airflow rate.

perfect combustion

fuel-burning condition in which all combustibles are consumed with no excess air so that only the theoretical amount of oxygen is used.

perfect mixing

a theoretical airflow distribution pattern within a ventilated space where the supply air is instantaneously and uniformly mixed with the air in the space such that the concentration of all constituents in the air, and the age of air, are spatially uniform.


a halocarbon composed only of fluorine and carbon.

perforated ceiling

perforated ceiling panels used to distribute the air uniformly throughout the ceiling or a portion of the ceiling. Filter pads may be used to achieve a similar result.

perforated plate

manner in which an individual, a building, a system, or a component fulfills specified behavior.

performance additives

used in most lithium bromide equipment to achieve design performance. The typical additive is one of the octyl alcohols.

performance contracts

a binding agreement between two parties prescribing the range and magnitude of achievement required of equipment, subsystems, or systems, which is provided by one party for the benefit and use of the other.

performance factor

the ratio of capacity to power input at specified operating conditions. Using consistent units, the performance factor may be expressed in dimensionless form as a coefficient of performance (COP) or as the energy efficiency ratio (EER).

performance rating method

a calculation procedure that generates an index of merit for the performance of building designs that substantially exceed the energy efficiency levels required.

performance rating of a fume hood

a series of numbers and letters consisting of the letters AM, AI, or AU and a two or three digit number. AM identifies an \"as manufactured\" test, that is, the laboratory hood is built and assembled by the manufacturer and testing is performed at the factory. The typical equipment remains in the hood and other activities in the laboratory continue. AI identifies an \"as installed\" test, that is, the laboratory hood is installed at the location of the customer. The hood is tested empty, but with the ventilation system in the installation balanced and the hood in its final location. AU identifies an \"as used\" test, that is, the tests are conducted after the hood has been installed and used by the chemist. An example of the series of numbers and letters is AM yyy where AM identifies an \"as manufactured\" test and yyy is the control level of tracer gas established by the test in ppm. A test rating of AU 0.5, for example, would indicate that the hood controls leakage into the laboratory to 0.5 ppm at the mannequin's sensing point with a tracer gas release rate of 4.0 Lpm (70 mL/s).

perimeter zone

any space with at least one surface exposed to exterior loads.

periodic law

properties of the chemical elements and their compounds are a periodic function of their atomic weights.

perlite thermal insulation

insulation composed of natural perlite ore, a glassy volcanic rock expanded by heat to form a cellular structure.

permanent bleed rate

the capacity of the permanent bleed provision, expressed either as a percentage of the nominal capacity or in kW (Btu/h or tons) of refrigerating effect produced by the evaporation of that amount of refrigerant flow.

permanent-bleed-type valve

a valve that has a fixed flow passage incapable of being closed by action of the valve. Such a fixed orifice permits a flow through or in parallel with the main valve port.


time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. When permeability varies with psychrometric conditions, the spot or specific permeability defines the property at a specific condition. Note: permeability is a property of a material, but the permeability of a body that performs like a material may be used. Permeability is the arithmetic product of permeance and thickness.


a performance evaluation of water vapor transmission given a certain differential in partial pressures on either side of a material or membrane. An accepted unit of water vapor permeance is the perm, gr/(h·ft2·in. Hg) (kg/[m2·s·Pa]).

permissible exposure level (PEL)

the time weighted average concentration (set by OSHA, the U.S. Occupational Safety and Health Administration) for a normal 8 hour work day and a 40 hour work week to which nearly all workers can be repeatedly exposed without adverse effect. Chemical manufacturers publish similar recommendations [e.g., acceptable exposure level (AEL), industrial exposure limit (IEL) or occupational exposure limit (OEL), depending on the company], generally for substances for which PEL has not been established.

permissible range

range of a physical quantity which satisfies the different parameters for each of the categories of the specified environment.

person-to-person transmission

spread of infectious disease from an infected individual to a susceptible person


perfluorocarbon, a hydrocarbon composed only of fluorine and carbon.


logarithm of the reciprocal of the hydrogen ion concentration of a solution. pH values below 7 are increasingly acidic, those above 7 are increasingly alkaline.

phase-change material

substance that undergoes changes of state while absorbing or rejecting thermal energy at constant temperature.


the perceived loudness of a complex sound that is equal to the sound pressure level of a 1000 hertz reference tone as judged by the average listener.

photo catalytic oxidation, air cleaner

process that uses light energy and titanium oxide to catalyze the oxidation of contaminants in air or water. The light energy causes a chemical reaction that produces reactive oxygen species (ROS), which are highly reactive and break down bioaerosol into carbon dioxide and water.

photo catalytic oxidation, air cleaner

process uses light energy and titanium oxide to catalyze the oxidation of contaminants in air or water. The light energy causes a chemical reaction that produces reactive oxygen species (ROS), which are highly reactive and break down bioaerosol into carbon dioxide and water.


radiometer designed for measuring luminous intensity, luminous flux, or illumination.


a system capable of generating a voltage as a result of exposure to visible or other radiation.

photovoltaic cell

device that detects or measures electromagnetic radiation by generating a potential at a junction (barrier layer) between two types of material, on absorption of radiant energy. Also known as barrier layer cell, barrier layer photocell, boundary layer photocell, or photronic photocell.

pickup load

actual load for heating the system following setback.


instrument for measuring pressure or compressibility.

piggyback refrigerating system

two chilling plants, centrifugal and absorption, usually steam driven, using the same steam supply in sequence, with chilled water running counterflow to stream.

pilot duty

class of operation in which the ultimate load is controlled by auxiliary means.

pilot duty rating

volt ampere rating that is applied to electric devices intended to control an electromagnetic load other than an electric motor, such as a control coil or other solenoid.

pilot operated relief valve

a pressure relief valve in which the major relieving device is combined with and is controlled by a self-actuated auxiliary pressure relief valve.

pilot valve

small valve where the opening or closing directly influences a larger valve, as in a servo system.

pilot-operated solenoid valve

a valve in which the solenoid functions to directly open and close a relatively small pilot flow port. Flow through the pilot port parallels the flow path of the main port. Starting or stopping flow through the pilot port creates a pressure imbalance on the main valve member, causing the main valve port to open or close.

pilot-operated valve

expansion valve of a type used on large capacity systems (e.g., direct-expansion chillers) where the required capacity per valve is beyond the range of direct-acting valves, this type of valve is under the control of a direct acting valve.

pipe fittings

parts used to join, adapt, or adjust other parts of the piping (e.g., flange, joint, manifold).


system to carry fluids. Piping includes pipes, flanges, boltings, gaskets, valves, fittings, pipe-supporting fixtures, structural attachments, and the pressure-containing parts of other components, such as expansion joints, strainers, filters, and devices that serve such purposes as mixing, separating, muffling, snubbing, distributing, metering, or controlling flow.

Pitot tube

small bore tube inserted perpendicular to a flowing stream with its orifice facing the stream to measure total pressure. At present, the term is often used for a double-tube instrument from which the flow velocity can be calculated with one orifice facing the flowing stream to register total pressure and the other perpendicular to the stream to register static pressure.


formation of small depressions in a surface due to sand blasting, mechanical gouging, acid etching, or corrosion.

Pittsburgh lock joint

longitudinal folded joint used in rectangular sheet-metal duct construction. Pocket lock joint transverse joint used in rectangular sheet metal duct construction.

plan (drawing)

two dimensional graphic representation of the design, location, and dimensions of a project, or parts thereof, seen in a horizontal plane viewed from above.

plane radiant temperature

uniform temperature of an enclosure where the radiance on one side of a small plane element is the same as in the nonuniform actual environment.

planned maintenance

classification of maintenance department resources that are invested in prudently selected functions at specified intervals. All functions and resources attributed to this classification must be planned, budgeted, and scheduled. Planned maintenance embodies two concepts: preventive and corrective maintenance.

plate evaporator

evaporator consisting of two plates containing channels for the circulation of refrigerant or a set of tubes connected to and between the two plates.

plate fin

plate drilled or punched to receive a number of parallel tubes, with the purpose of increasing the heat transfer area.

plate freezer

contact freezer in which the refrigerated surface is a flat metal plate. Compare to double-contact freezer.

plate heat exchanger

fixed plates that separate and keep the hot and cold fluids separate. See also plate liquid cooler.

plate liquid cooler

heat exchanger made of thin plates so formed that liquid to be cooled flows through the passage between the plates and the cooling fluid flows through the alternate passages.

plate-type condenser

(1) air-cooled condenser consisting of plates between which are arranged channels for the circulation of refrigerant. (2) water-cooled condenser consisting of a coil through which the refrigerant circulates, arranged between two plates, with water circulating between coil and plates.


(also called plenum box and plenum chamber) (1) a compartment or chamber, to which one or more ducts are connected, that forms a part of the air-distribution system and that is not used for occupancy or storage. A plenum often is formed in part or in total by portions of the building. (2) an air compartment that is attached to, or is an integral part of, a forced-air furnace which is designed to either distribute the heated air after it leaves the heat exchanger in the case of a supply plenum or to collect air that enters the return inlet in the case of a return plenum. (3) component forming an interface between a ductwork and one or more air terminal devices, by virtue of its design or by the inclusion of accessories, it can also be used to equalize the pressure/velocity across the air terminal device.

plenum fan

fan assembly consisting of a single inlet impeller mounted perpendicular to airflow that pressurizes a plenum chamber in an air-distribution system. The impeller is typically an airfoil or backward-inclined design.

plug flow

(1) (also known as piston flow), flow of solids suspended in liquid, characterized by slippage of the cylindrical mass and equal velocity in cross section. (2) flow regime where the flow is predominantly in one direction and any contaminants are swept along with the flow. (3) form of two-phase flow in which plugs of gas and liquid flow alternately in the pipe.

plug load

A device that is powered by means of an electrical plug and matching socket or receptacle. This excludes devices that are accounted for as part of major building end uses such as HVAC, lighting systems, and water heating.

plug valve

valve in which the fluid passage is a hole in a rotatable, tapered plug in the valve body.

plumbing fixture

(1) a device that is part of a system to deliver and drain away water and waste but is also configured to enable a particular use. (2) end-use equipment such as sinks, lavatories, toilets, showers, drinking fountains, etc.

pneumatic accessories

equipment such as air compressors, air line dryers, compressed air filters, and indicating devices (pressure and temperature) to facilitate monitoring of control system activity.

pneumatic/electric (electronic) transducer

device that converts a pneumatic value to an equivalent electric (volt, milliamp) value.

pneumatic/electric control

(1) control device that interfaces pneumatic to electric systems. (2) control device with a pneumatic input and an electric output.

pneumatic/electric switch

control device that makes and/or breaks an electrical circuit based on the level of a pneumatic signal. The device has an adjustable setpoint and optional adjustable differential.

point of operation

the relative position on the cooling unit characteristic curve corresponding to a particular airflow rate. It is controlled during a test by adjusting the position of the throttling device, by changing flow nozzles or auxiliary fan characteristics, or by any combination of these.


presence of undesired elements that are deleterious to the comfort, health, and welfare of persons or the environment (pollution includes elements such as noise, vibration, odors, and gases).

polyalkylene glycol (PAG)

a synthetic ethylene oxide and/or propylene oxide polymer normally initiated with an alcohol and sometimes capped.

polyalphaolefin (PAO)

a synthetic, saturated acyclic hydrocarbon lubricant prepared from alphaolefins.


a characteristic of an aerosol for which the width of its number distribution shows a geometric standard deviation of dg > 1.5.

polyolester (POE)

a synthetic ester lubricant prepared from a polyol and monocarboxylic acids. For refrigeration applications, polyol esters are prepared from neopentyl polyols that do not contain beta hydrogens.


graphical line representing a polytropic process.

polytropic compression

compression of a fluid involving heat exchange with temperature change and temperature variation (not isothermal).

polytropic expansion

expansion of a fluid, involving heat exchange.

polytropic process

a process in which heat is being exchanged with the surroundings, represented by the equation pvn = constant (n is the polytropic exponent).

polyvinyl ether (PVE)

a synthetic lubricant prepared from vinyl ether monomers. The main chain of the molecule consists of carbon-to-carbon bonds, with ether-based side chains.


any structure, basin, or tank containing an artificial body of water for swimming, diving, or recreational bathing. The term includes, but is not limited to, swimming pools, whirlpools, spas, and hot tubs.

poppet valve

valve composed of a mushroom-shaped circular head and integral stem.

positional control level

the average tracer gas concentration at a position during a test.

positional sash movement effect

the maximum 45 second rolling average of the tracer gas concentration observed during a series of sash movement tests at one ejector and mannequin position.

post purge

the design that permits the continued operation of the combustion blower in a power vented unit, power burner unit, or forced draft unit for a period of time after the main burner is shut off for the purpose of venting of residual flue gas in the heat exchanger and the venting system.

post-retrofit period

the time following a retrofit, during which savings are to be determined.


suitable for human consumption.

potassium permanganate filter, gas

a salt and a strong oxidizing agent that causes substances to lose electrons.  Can convert ozone (O3) into oxygen (O2).  Does not generate toxic byproducts.

potassium permanganate filter, gas

a salt and a strong oxidizing agent that causes substances to lose electrons.  Can convert ozone (O3) into oxygen O(2).  Does not generate toxic byproducts.

potential energy

energy possessed by a system caused by the attractive forces existing between molecules or the elevation of the system.

pour point

the lowest temperature at which a lubricant will flow under prescribed conditions.

power adjustment factor

a modifying factor that adjusts the effective connected lighting power (CLP) of a space to account for the use of energy-conserving lighting-control devices.

power burner

a burner that supplies air for combustion at a pressure exceeding atmospheric pressure, a burner that depends on the draft induced by a fan incorporated in the furnace, a fan-powered burner that depends on the natural draft developed by a chimney for proper operation, a pulse-combustion burner.

power factor (PF)

a factor, equal to the cosine of the phase angle between current and voltage, by which the product of voltage and current is multiplied to convert volt amperes to power in watts.

power input

a term used to refer to the power input to the shaft of open compressors, a term used to refer to the power input at the motor terminals for hermetic compressors, semihermetic compressors, or motor compressors, a term used to refer to refer to the power absorbed by all ancillaries that are necessary to sustain the operation of the compressor or condensing unit (e.g., oil pump, fan motors, controls components, and circulating pumps).

power roof ventilator

(also referred to as an up blast fan). A fan designed for curb mounting on a roof or, within size and design constraints, for wall mounting. Air enters the fan in a typical axial arrangement but discharges radially from the centrifugal impeller and turns 90 degrees to exit through the top of the fan. It may be listed under UL Standard 705, Standard for Power Ventilators, in accordance with UL Subject 762, “Outline of Investigation for Power Roof Ventilators for Restaurant Exhaust Applications.”

power roof/wall ventilators (PRV)

a fan consisting of a centrifugal or axial impeller with an integral driver in a weather-resistant housing and with a base designed to fit, usually by means of a curb, over a wall or roof opening.

power ventilator

fan consisting of a centrifugal- or axial-type impeller with integral driver in a weather-resistant housing with base designed to fit (usually by means of a curb) over a wall or roof opening.

Prandtl number

(1) dimensionless number used in calculation of forced and free convection, equal to the dynamic viscosity times the specific heat at constant pressure, divided by the thermal conductivity. (2) dimensionless number used in calculation of diffusion in flowing systems, equal to the kinematic viscosity divided by the molecular diffusivity. Symbol is Pr or Pr. Also known as Schmidt number 1 (Sc, Sc, or NSc1).


device to remove fine ash, tars, dusts, or smoke particles from flue gases or other gaseous streams, the device may employ mechanical, electrostatic, or chemical means, or a combination of these. See air cleaner.


(1) closeness of agreement among repeated measurements of the same physical quantity by the same method under the same conditions and with the same instrument. (An instrument may be precise but not accurate.) (2) the statistical error of an instrument subjected to repeated measurements over a range of input values.


(1) a device for transferring heat from the incoming potable water to the spill. (2) cooler for removing sensible heat before shipping, storing, or processing. (3) device for cooling a fluid before it enters a system.

predicted mean vote (PMV)

index that predicts the mean value of thermal sensation votes of a large group of persons, expressed on a seven-point scale.

predicted percentage dissatisfied (PPD)

index that predicts the percentage of a large group of people who are likely to feel thermally dissatisfied for the body as a whole (i.e., feel either too warm or too cold).

predictive maintenance

function of corrective maintenance. Statistically supported objective judgment is implied. Nondestructive testing, chemical analysis, vibration and noise monitoring, as well as visual inspection and logging are all classified under this function, providing that the item tested or inspected is part of the planned maintenance program.

preheat coil

heating coil installed upstream of cooling coil or at the front of an air-handling system to preheat air.


(1) heating a food product prior to a subsequent treatment. (2) in air conditioning, to heat the air ahead of other processes. (3) to heat the water prior to heating by the auxiliary thermal source.

preliminary operating manual

an elaboration of the design intent that includes operating information developed during the construction documents design stage.

preliminary run

successive sets of readings compiled during continuous cooler operation with the objective of establishing a steady-state condition.

prepurge operation

operating a burner blower prior to flame ignition to blow out possible leaked gas. See purge.

prerotary vane assembly

in a turbocompressor, an assembly of adjustable guide vanes fitted to the inlet, the prerotary vane assembly's function is to orient the fluid so that the pressure/flow rate characteristics can be modified.

prescriptive design

for energy compliance alternatives , a prescriptive design includes specified assumptions concerning shape, orientation, HVAC, and other system design features. The prescriptive design is compared with the proposed design using the annual energy cost method.


(1) force exerted per unit area. Note: the terms head and pressure often are used interchangeably, however, head is the height of a fluid column supported by fluid flow, while pressure is the normal force per unit area. For liquids, it is convenient to measure the head in terms of the flowing fluid. With a gas or air, however, it is customary to measure pressure on a column of liquid. (2) thermodynamically, the normal force exerted by a homogeneous liquid or gas, per unit of area, on the wall of the container.

pressure control

device that responds directly or indirectly to deviation from a desired pressure by actuating a control or initiating a control sequence.

pressure dependent (PD)

the flow rate through a flow control device varies in response to changes in system pressure.

pressure difference

difference between pressures measured at two points or levels in fluids or gases.

pressure differential cut-out

cut-out control reactive to the difference between two pressures.

pressure differential valve

(also known as spring-loaded check valve), automatic valve that opens in response to a pressure difference across the valve port. Commonly used in conjunction with a head-pressure control valve (backup valve) between discharge line and receiver inlet.

pressure drop

(1) difference in pressure between two points in a flow system, usually caused by frictional resistance to fluid flow in a conduit, filter, or other flow system. Compare to pressure loss. (2) loss in pressure, as from one end of a refrigerant line to the other, from friction, static, heat, etc.

pressure equalizing

allowing high- and low-side pressures to equalize or nearly equalize during idle periods by an unloading valve, by a vapor-lock liquid control, or by nearly equalizing inlet and discharge pressures on compressors to reduce starting torque load.

pressure factor

ratio between the suction pressure effect and the pressure due to an air velocity passing over an inlet or outlet.

pressure head

hydrostatic height of fluid, equal to the fluid pressure divided by the density times the gravitational acceleration.

pressure imposing element

any device or portion of the equipment used to increase refrigerant pressure.

pressure independent

the flow rate through a flow control device is not affected by changes in system pressure.

pressure limit of watertightness

maximum pressure difference at which the rated watertightness is assured under test conditions.

pressure loss

decrease in total pressure due to friction and turbulence. It is a measure of the mechanical energy lost by the flow per unit volume of the fluid.

pressure loss coefficient

factor for mechanical energy loss as a result of flow.

pressure powered pump

receiver device having inlet and outlet nonreturn valves and a float-operated mechanism, enabling a secondary medium (steam, compressed air, or inert gas) to pressurize the receiver body through a fill and discharge cycle actuated from the float mechanism.

pressure reducing valve (PRV)

(1) valve used to reduce a high supply pressure to a usable level. (2) valve that maintains a uniform pressure on its outlet side irrespective of pressure variation on its inlet side.

pressure regulator

device to maintain controlled downstream fluid pressure with varying upstream pressure.

pressure relief device

(1) a pressure-actuated (not temperature-actuated) valve or rupture member designed to automatically relieve excessive pressure. (2) means for relieving internal pressure and preventing explosive shattering of the housing. (3) valve or rupture member designed to relieve excessive pressure automatically.

pressure relief valve
pressure switch

regulating or safety device actuated by change in pressure.

pressure temperature relief device

automatic pressure and temperature sensitive device actuated by a predetermined internal pressure, temperature, or both and used to prevent or relieve excessive pressure or temperature within a tank.

pressure vessel

(1) any refrigerant-containing receptacle in a refrigerating system. This does not include evaporators where each separate evaporator section does not exceed 0.5 ft3 (0.014 m3) of refrigerant-containing volume regardless of the maximum inside dimension. This also does not include evaporator coils, compressors, condenser coils, controls, headers, pumps, and piping. (2) container for fluids at a pressure different from atmospheric pressure (vacuum to high), capable of withstanding associated stresses.

pressure volume temperature (PVT)

relationship of pressure, volume, and temperature as fundamental, thermodynamic properties of a fluid in a single-phase region.

pressure-compensating control system
pressure-dependent control system

control system in which the airflow through the air terminal varies with system pressure.

pressure-independent control system

control system in which the airflow through the air terminal is independent of system pressure. Also known as “pressure-compensated.”

pressure-limiting device

a pressure-responsive electronic or mechanical control designed to automatically stop the operation of the pressure-imposing element at a predetermined pressure.

pressure-sustaining valve

valve providing maintenance of designated pressure level within a system.

pressure-type air cooler

an air cooler designed for use with external elements that impose air resistance.

pressurized stair shafts

a type of smoke control system in which stair shafts are mechanically pressurized with outdoor air to keep smoke from contaminating them during a fire incident.

prevention of vacuum system

a refrigerant pressure-control system that prevents refrigerant loss and infiltration into idle low-pressure chillers and is also used to pressurize for leak testing without the use of noncondensables.

prevention-of-vacuum system

a refrigerant pressure control system that prevents refrigerant loss and infiltration into idle low-pressure chillers and is also used to pressurize for leak testing without the use of noncondensables.

preventive maintenance

classification of resources allotted to ensure proper operation of a system or equipment under the maintenance program by providing for systematic inspection, detection, and correction of incipient failures either before they occur or before they develop into major defects. Durability, reliability, efficiency, and safety are the principal objectives.

primary air

(1) any air that is mixed with fuel at or in a burner prior to burning. (2) in a cleanroom, air that recirculates through the work space. (3) treated supply air that enters the space through any supply air device, such as air outlet or through any air supply terminal, such as a VAV unit or fan terminal unit. The air is not mixed with space air before entering the space.

primary air temperature

air temperature of the primary airflow (e.g., supply airflow from an air-handling unit).

primary alternate test method

test procedure using an alternate method of installing a test unit, or an alternate test apparatus, that produces reproducibility and accuracy equivalent to the primary test method.

primary refrigerant

working fluid of a refrigerating cycle (as opposed to secondary refrigerant).

primary surface

(1) (direct surface), portion that is exposed to radiation from fire and transfers heat directly to the air being heated. (2) surface that is in direct contact with both the heat-absorbing and heat-emitting media.

primary test

the test from which ratings are calculated.

primary test method

test procedure incorporating the preferred method of installing a test unit and the preferred test apparatus that produces the desired reproducibility and accuracy.

primary transfer fluid

the fluid that flows through the solar collector.

prime contractor

contractor on a project having a contract directly with the owner.

prime mover

engine, turbine, water wheel, or similar machine that drives an electric generator.

prime professional

person or entity having a contract directly with the owner for professional services.

prime surface

surface having the heating medium on one side and fluid (or extended surface) on the other.


the approach to and execution of a sequence of work operations to yield a repeatable and defined result.


(1) a sequence of events. (2) change in thermodynamic state that can be defined as any change in the properties of a system. Note: a process is described by specifying the initial and final equilibrium states, the path (if identifiable), and the interactions that take place across system boundaries during the process.

process application

a manufacturing, industrial, or commercial procedure or activity where the primary purpose is other than conditioning spaces and maintaining comfort and amenities for the occupants of a building.

process control

(1) closed-loop control as applied to processes or operations or both. Note: HVAC systems may be considered as processes. (2) control for automatic regulation of operations or processes or both.

process energy

energy consumed in support of a manufacturing, industrial, or commercial processes other than conditioning spaces and maintaining comfort and amenities for the occupants of a building.

process load

the load on a building resulting from the consumption or release of process energy.

product-integrated control (PIC)

direct digital control (DDC) panel, factory mounted and connected, able to monitor, control, and diagnose the significant functions of the equipment of which it is a part. Compare field-installed device.

product-integrated control (PIC)

direct digital control (DDC) panel, factory mounted and connected, able to monitor, control, and diagnose the significant functions of the equipment of which it is a part. Compare to field-installed device.

program maintenance

a systematic approach to maintenance in terms of time and resource allocation. It documents the objectives, establishes the criteria for evaluation, and commits the maintenance department to basic areas of performance such as prompt response to mechanical failure, maintenance, and attention to planned functions that protect the capital investment and minimize downtime or failure response.

programmable read-only memory (PROM)

not field modifiable, nonvolatile (requires no batteries).

programmable thermostat

a thermostat with the ability to preset different temperature/time settings for heating and cooling equipment.

programming control

control system that provides for performing various operations in predetermined sequences related to time or other variables.

project team

select members of all the teams involved in a building project. The project team provides a venue for coordinating actions and information flows between all staff who are involved in the project and commissioning (Cx) Activities.

projected baseline

the baseline energy use or demand applied to the post-retrofit period and conditions.

projection factor

the ratio of the horizontal depth of the external shading projection divided by the sum of the height of the fenestration and the distance from the top of the fenestration to the bottom of the farthest point of the external shading projection in consistent units.

propeller fan

(1) fan in which the air enters and leaves the impeller in a direction substantially parallel to its axis. (2) propeller- or disc-type wheel within a mounting ring or plate and including driving mechanism supports for either belt drive or direct connection.

proportional control mode

mode that requires offset from the desired setpoint to move the controlled device through its proportional band.

proposed building performance

the annual energy cost calculated for a proposed design.

proposed design

a computer representation of the actual proposed building design, or portion thereof, used as the basis for calculating the design energy cost.

propylene glycol

clear, colorless liquid used to depress the freezing point of water for use as a secondary coolant in HVAC&R systems. Note: inhibitors are required to control corrosion caused by glycol solutions.

PSC motor

a permanent split-capacitor motor.


particle size removal efficiency


instrument for measuring relative humidities with wet- and dry-bulb thermometers. See hygrometer.

psychrometer wet-bulb temperature

temperature indicated by a wet-bulb sensing element when the sensor 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 an equilibrium temperature of water evaporating into air when the heat of vaporization is supplied by the sensible heat of the air. Psychrometer wet-bulb temperature is not a thermodynamic property.


relating to the measurement or determination of atmospheric conditions, particularly regarding moisture mixed with air.

psychrometric chart

graphical representation of the properties of moist air, usually including wet- and dry-bulb temperatures, specific and relative humidities, enthalpy, and density.


branch of physics concerned with the measurement or determination of atmospheric conditions, particularly the moisture in air.

public areas

designated spaces freely accessible to the public. These include parking areas, secured entrances and areas, entrance lobbies, reception and waiting areas, public toilets, snack bars, cafeterias, vending areas, gift shops and other retail locations, health education libraries and meeting rooms, chapels, and gardens.

public way

a street, alley, transit right of way, or other parcel of land open to the outdoors and leading to a street or transit right of way that has been deeded, dedicated, or otherwise permanently appropriated for public use and that has a clear width and height of not less than 10 ft (3 m).

published ratings

statement of the assigned values of those performance characteristics, under standard rating conditions, by which a unit may be chosen to fit its application.

pull-down load

the unmet cooling load that accumulates during a period when cooling is not provided to the load and that must be met upon system start-up. Maximum pull-down load generally occurs on a Monday morning.

pull-down test

first operational check on a refrigerating installation to measure the time taken to pull the temperature down to the desired conditions while also measuring the temperatures, pressures, and associated data.

pulsating flow

fluid flow phenomenon characterized by periodic (or pseudoperiodic) variation in velocity.

pulse width modulation (PWM)

a method of converting a fixed source of voltage (most commonly direct current [DC]) into a controlled voltage by breaking the voltage source into a series of rectangular pulses. PWM is commonly used in the output of a variable-frequency drive (VFD) to control the output voltage supplied to the motor.


machine for imparting energy to a fluid, causing it to do work, drawing a fluid into itself through an entrance port, and forcing the fluid out through an exhaust port. Main types are air lift, centrifugal, diaphragm, positive displacement, reciprocating, and rotary. Pumps can be configured as individual (one motor, one impeller, one fluid stream) or combined (one motor, multiple impellers, multiple fluid streams).

pump bowl assembly

impeller housings and impellers are always placed sufficiently below the expected pumping level to prevent cavitation at the peak production rate. For example, this pump should be placed at least 115 ft (30 m) below the casing top [pump setting depth = 115 ft (30 m)] to allow for adequate submergence at peak flow. The specific net positive suction head (NPSH) pressure required for a pump varies with each application and should be carefully considered in selecting the setting depth.

pump down

of refrigerant, withdrawal of all refrigerant from the low side of a system by pumping it to either the condenser or the liquid receiver.

pump laws

(1) (also known as affinity laws for pumps), power varies directly with specific gravity or density changes of the fluid. (2) with constant impeller diameter and variable speed, flow of a constant gravity pump varies directly with the speed change, head varies as the square of the speed change, and power changes as the cube of the speed change. (3) with variable-impeller diameter and constant speed, flow varies directly with the diameter change, head varies as the square of the diameter change, and power varies as the cube of the diameter change.

pump system power

the sum of the nominal power demand (nameplate horsepower) of motors of all pumps that are required to operate at design conditions to supply fluid from the heating or cooling source to all heat transfer devices (e.g., coils, heat exchanger) and return it to the source.

pump-down charge

the quantity of refrigerant stored at some point in the refrigeration system for operational, service, or standby purposes.

pumping water level (PWL)

level that exists under specific pumping conditions. Generally, this level is different for different pumping rates (higher pumping rates mean lower pumping levels).

purchased-energy rates

costs for units of energy or power purchased at the building site. These costs may include energy costs as well as costs for power demand as determined by the adopting authority.

pure ice

ice made by using water with less than 5 ppm of solids concentration.

pure tone

a sound wave that is at a single frequency.


(1) removal of air from a hydronic system. (2) removal of noncondensable gases from a refrigeration system. (3) removal of unburned gases from a combustion chamber. (4) to rid of whatever is impure or undesirable.

purge recovery system

(also known as purger), system used to recover refrigerant from purged mixture of gases and water.

purge system

required on lithium bromide absorption equipment to remove noncondensables (air), which leak into the machine, or hydrogen (a product of corrosion), which is produced during equipment operation. Even in small amounts, noncondensable gases can reduce chilling capacity and even lead to solution crystallization. Purge systems for larger sizes above 100 tons (359 kW) of refrigeration typically consist of these components: vapor pickup tube(s), noncondensable separation and storage tank(s), and vacuum pump or valving system.

purge valve

a device to allow noncondensable gases to flow out of a system.


removing noncondensable gases from the system.

purging device

an automatic, semiautomatic, or hand-operated device that collects gases from the condenser or receiver, condenses the condensable refrigerant therefrom, and releases the remainder to the atmosphere.


radiometer used to measure solar radiation received from the whole hemisphere (two steradians). Note: the instrument is suitable for measurement of global or diffuse solar radiation.


a radiometer used for measuring the incoming atmospheric infrared radiation spectrums from approximately 4 µm to 100 µm on a black surface at ambient air temperature. The solar shortwave radiation is excluded from the energy measured.


radiometer used to measure the direct or beam solar irradiance incident on a surface normal to the sun’s rays.


instrument for measuring temperature, usually above the range of mercury thermometers.


a radiometer used to measure the total solar radiation incident upon a surface per unit time per unit area. This energy includes the direct radiation, the diffuse sky radiation, and the solar radiation reflected from the foreground.