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

(1) measure of the acoustical absorption properties of a room, the average absorption of all the surfaces in a room times the total surface divided by the average reflection. (2) (also known as thermal resistance), quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow rate through unit area (R = ?T/q). R-value is the reciprocal of thermal conductance.

radial-bladed impeller (paddle-bladed impeller)

one in which the blades are flat and disposed radially from the fan hub to the outer edge.

radial-energy distribution system

system of radial feeders extending outward from a centrally located district energy plant (heating or cooling). Each feeder normally is composed of one supply pipe and one return pipe.


total radiant flux emitted from a surface through unit projected area by unit solid angle. It includes the self-emitted radiation plus reflections from sources other than the object evaluated as interpreted from the direction of measurement. The term intensity of radiation is sometimes used as a synonym for radiance. See also radiosity.

radiance at a point on a surface

the quotient of the flux incident on an element of a surface containing the point, by the area of that element, measured in watts per square meter, W/m2 (Btu/ft2).


radiating rays of light, emitting or reflecting beams of light, vividly shining, glowing, brilliant.

radiant asymmetry temperature

difference between the plane radiant temperature of the two opposite sides of a plane element. See plane radiant temperature.

radiant barrier

a surface of low emissivity (less than 0.1) placed inside an attic or roof space above (but not touching) the distribution system to reduce radiant heat transfer.

radiant energy

energy passing through space in the form of electromagnetic radiation (such as light or ultraviolet or infrared radiation) or as a stream of particles (e.g., electrons or protons).

radiant flux

the time rate of flow of radiant energy (watts).

radiant flux density

measure of radiant power per unit area flowing across or onto a surface. (Also called irradiance).

radiant heat

heat transferred by radiation.

radiant heating system

a heating system that transfers heat to objects and surfaces within the heated space primarily (greater than 50%) by infrared radiation.

radiant panel

a heating or cooling surface that delivers 50% or more of its heat transfer by radiation, which may be either an integral part of the building (e.g., floor or ceiling heating) or detached from the building elements (e.g., suspended ceiling panel).

radiant reflectance (luminous reflectance)

ratio of the reflected radiant (or luminous) flux to the incident radiant (or luminous) flux.

radiant transmittance (luminous transmittance)

ratio of the transmitted radiant (or luminous) flux to the incident radiant flux.

radiant-cooling system

a sensible cooling system that provides more than 50% of the total heat flux by thermal radiation.

radiant-heating system

a sensible heating system that provides more than 50% of the total heat flux by thermal radiation.

radiated sound power level

sound power that radiates from terminal casings and induction ports for induction terminal units.


act or process of radiating, specifically the process by which energy is emitted from molecules and atoms, owing primarily to internal temperature change.

radiation angle factor

(also known as shape factor), when the space above an element of surface is partially occupied by a body exchanging radiant energy with the surface, the angle factor represents the fraction of the angular field of view where energy exchange is taking place.

radiation pasteurization

irradiation, using moderate doses, to enhance keeping quality by killing most of the spoilage microorganisms present, viruses are not affected.

radiation shield

(1) device designed to intercept radiated heat. (2) in a furnace, a sheet of metal or other material supported between the heat exchanger and the casing to protect the casing from heat, not to be considered as a heat transfer surface.

radiation sterilization

irradiation using high doses to kill the microorganisms present.

radiative forcing

represents the net amount of infrared radiation absorbed by gases in the atmosphere. The radiative forcing of a gas depends on the efficiency with which it traps infrared radiation and its concentration in the atmosphere. Atmospheric concentration depends on emission rates and the atmospheric lifetime of the gas.

radiator valve

manual or automatic valve that controls the flow of the fluid to a radiator.


materials that emit ionizing radiation.


instrument for measuring irradiance in energy or power units.


total radiant flux that leaves unit area of a surface. The sum of radiant flux emitted and reflected by the surface, plus any radiant flux transmitted through that surface. Compare to exitance.

rain louver

see louver.

ramp rate

The rate at which a generator changes its output or a demand resource changes its demand.

random error

an error that causes readings to take random values on either side of some mean value. Measurements may be precise or imprecise depending on how well an instrument can reproduce subsequent readings of an unchanged input.

random-access memory (RAM)

memory providing access time that is independent of the address and is addressable for both reading from and writing into memory.


(1) difference between the highest and the lowest operational values, such as pressure, temperature, rate of flow, or computer values. (2) region between limits within which a quantity is measured, transmitted, or received, expressed by stating the lower and upper range values.

Rankine cycle

theoretical thermodynamic cycle used in steam engines, comprising four principal stages: (1) vaporization of water under high pressure, (2) expansion of steam, (3) condensation of steam, and (4) pumping of the water back to initial pressure.

Rankine temperature

absolute temperature scale conventionally defined by the temperature of the triple point of water equal to 491.68°R, with 180 divisions between the melting point of ice and the boiling point of water under standard atmospheric pressure (1°R = 1°F). See triple point.

Ranque-Hilsch effect

spot-cooling effect produced in a tube into which gas is introduced tangentially, producing vortex flow.

Ranque-Hilsch tube

(also known as pulse tube), device in which the Ranque-Hilsch effect is produced.

Raoult’s law

molar weights of nonvolatile nonelectrolytes, when dissolved in a definite mass of a given solvent under the same conditions, lower the solvent’s freezing point, elevate its boiling point, and reduce its vapor pressure equally for all such solutes.


such as frequency, input motor power, conditions, and voltage indicates the condition as specified by the manufacturer or the conditions of design.

rated airflow

the airflow rate in m3/s (cfm) at which the device is tested.

rated final resistance

the operating pressure loss at the airflow rate at which a disposable device (e.g., filter) should be replaced or renewed, as recommended by the manufacturer, expressed in Pa (in. of water).

rated operating pressure

tested maximum positive pressure at which a device or component is rated.

rated R-value of insulation

the thermal resistance of the insulation alone as specified by the manufacturer in units of h·ft2·°F/Btu (m2·K/W) at a mean temperature of 75°F (24°C). Rated R-value refers to the thermal resistance of the added insulation in framing cavities or insulated sheathing only and does not include the thermal resistance of other building materials or air films.


the assigned values of those performance characteristics, under stated rating conditions, by which a unit may be chosen to fit its application. These values apply to all units of like nominal size and type (identification) produced by the same manufacturer.

raw water

(1) untreated water. (2) water used for ice making, except distilled water.

reactive power

portion of apparent power that does no work. It is measured commercially in kilovars. Reactive power must be supplied to most types of magnetic equipment, such as motors. It is supplied by generators or by electrostatic equipment such as capacitors.

read-only memory (ROM)

memory with stored data that can be read but not changed or written into.

real power

energy or work producing part of apparent power. The rate of supply of energy, measured commercially in kilowatts. The product of real power and length of time is energy, measured by watthour meters, and expressed in kilowatt hours (kWh).


a vessel in the refrigerating system designed to ensure the availability of adequate liquid refrigerant for proper functioning of the system and to store the liquid refrigerant when the system is pumped down.

reciprocating compressor

a positive displacement compressor that changes internal volume of the compression chamber(s) by the reciprocating motion of one or more pistons.

recirculated air

air taken from a space and returned to that space, usually after being passed through a conditioning system. The part of the return air that is reused. Air removed from a space and reused as supply air.

recirculating system

a domestic or service hot-water distribution system that includes a closed-circulation circuit designed to maintain usage temperatures in hot-water pipes near terminal devices (e.g., lavatory faucets, shower heads) in order to reduce the time required to obtain hot water when the terminal device valve is opened. The motive force for circulation is either natural (due to water density variations with temperature) or mechanical (recirculation pump).

recirculating unit

a remote unit with cooling that is provided at the dispensing valve and accomplished by circulating cold water or cold carbonated water through one of the lines and returning the same to a refrigeration unit.

recirculation of discharge air

(1) a condition pertaining to air-cooled condensers in which a portion of the discharge air enters along with the fresh air, the amount of recirculation is determined by equipment design, placement in regard to adjoining objects, and atmospheric conditions. The effect is generally evaluated on the basis of the decrease in unit capacity. (2) condition pertaining to cooling towers and evaporative condensers in which a portion of the discharge air enters along with the fresh air, the amount of recirculation is determined by equipment design, placement in regard to adjoining objects, and atmospheric conditions. The effect is generally evaluated on the basis of the increase in entering wet-bulb temperature compared to the ambient.


to process used refrigerant to new product specifications.

reclaimed refrigerants

refrigerants reprocessed to the same specifications as new refrigerants by any means, including distillation. Such refrigerants have been chemically analyzed to verify that those specifications have been met.

recommissioning (Cx)

an application of the commissioning process requirements to a project that has been delivered using the commissioning process. This may be a scheduled recommissioning developed as part of an ongoing commissioning process or it may be triggered by use change, operations problems, or other needs.


to lower the temperature of air that has been previously heated by a mechanical heating system.


(1) cooling of air that has been previously heated. (2) lowering the temperature of air that has been previously heated by a mechanical heating system.

record documents

drawings and other documents that record the conditions of the project as constructed. These include any refinements of the construction or bid documents.

record drawings

drawings that record the conditions of the project as constructed. These include any refinements of the construction or bid documents.

recording thermometer (temperature recorder, thermograph)

a thermometer that indicates temperature by traces on a moving chart, recording may be done on a cylinder, a circular chart, or a strip chart.


to remove refrigerant in any condition from a system and store it in an external container.

recoverable heat

portion of thermal input to a prime mover and that is not converted to mechanical power and can be reclaimed.

recovered energy

energy reclaimed for useful purposes and that would otherwise be wasted.

recovered or reclaimed heat

comes from internal heat sources. It is used for space heating, domestic or service water heating, air reheat in air conditioning, process heating in industrial applications, or other similar purposes. Recovered heat may be stored for later use.

recovered refrigerants

refrigerants removed from a system in any condition without necessarily testing or processing them.

recovery unit (refrigerant recovery unit)

a machine that removes refrigerant from a refrigerating system, preventing refrigerant discharge to the environment.


(1) externally cooled heat exchanger in the high side of a refrigerating absorption system for condensing absorbent and separating it from the refrigerant before passing it to the condenser. (2) in electricity, a device for converting AC to DC current.


to reduce contaminants in used refrigerants by separating oil, removing noncondensables, and using devices such as filter driers to reduce moisture, acidity, and particulate matter.

recycled refrigerants

refrigerants for which contaminants have been reduced by oil separation, removal of noncondensable gases, and single or multiple passes through filter driers or other devices that reduce moisture, acidity, and particulate matter.

reduced input rate with manually adjustable modulating control

the input rate resulting from burner operation at the manufacturer's recommended adjustment setting or the midpoint setting.

reduced pressure detector check (RPDC)

back-pressure and back-siphonage-type device designed to serve as a detector check on fire protection systems where contaminants are involved. Note: RPDC includes a line-size approved, reduced pressure zone backflow preventer with a metered bypass into which has been incorporated a three-quarter inch, approved, reduced pressure zone backflow preventer.

reduced pressure zone backflow preventer

back-pressure and back-siphonage-type device designed to operate under continuous pressure where contaminants are involved.

reduced-absorptivity exterior coating

an exterior finish applied to roof systems in order to reduce the absorption of solar radiation. The solar absorptivity must be 0.4 or less.

reduced-heat input rate

for furnaces or boilers equipped with automatic modulating control, the input rate resulting from the lowest factory-adjusted setting of the control.


deliberate duplication of components, equipment, controls, or systems and their interconnections to enable continued operation at needed functional capacities during and after the loss of the primary components, equipment, controls, or systems due to failure, maintenance, servicing, or other modification activities.

reed valve

(1) a type of check valve that restricts flow of fluids to a single direction and consists of thin strips of flexible metal, fiberglass, or other materials fixed on one end. They open and close with changing pressures across opposite sides of the valve. (2) compressor valve consisting of a plate having a number of slots and flexible steel strips fitted over the slots.

reference filters

dry-media-type filters that are carefully measured for resistance and initial efficiency immediately after a test system is qualified. These filters serve as references to ensure that the test system continues to operate as it did when it was qualified.

reference temperature

the temperature at an appropriate, fixed location within the test zone, for example, at 42 in. (1.1 m) above the floor for an office space served by a mixing air system.


(1) portion of the incident radiation on a surface that is reflected from the surface. Note: for an opaque surface, the sum of the reflectance and the absorptance is unity at equilibrium. Absorptances and reflectances are of various types, as are emittances. (2) the ratio of the light reflected by a surface to the light incident upon it.

reflective thermal insulation

insulation that reduces radiant heat transfer across spaces by use of one or more surfaces of high reflectance and low emittance, for example, aluminum foil.


portion of the radiation-striking unit area of a surface that is not absorbed or transmitted by the surface. See reflectance.


the fluid used for heat transfer in a refrigerating system, which absorbs heat at a low temperature and a low pressure of the fluid and rejects heat at a higher temperature and a higher pressure of the fluid, usually involving changes of the state of the fluid.

refrigerant blend

a refrigerant composed of two or more different chemical compounds often used individually as refrigerants for other applications.

refrigerant charge

(1) actual amount of refrigerant in a closed system. (2) weight of refrigerant required for proper functioning of a closed system.

refrigerant circuit

an assembly of refrigerant-containing parts connected to allow the flow of refrigerant in the refrigerating cycle. The refrigerant-containing parts are considered part of the circuit even if isolated by a valve. A system or equipment may be considered to have multiple circuits only if there is no intended path for the refrigerant to cross over from circuit to circuit.

refrigerant distributor

device used in conjunction with a thermostatic expansion valve to ensure equable refrigerant distribution from the valve to individual parallel sections of an evaporator.

refrigerant heat rejection

total useful capacity of a refrigerant condenser for removing heat from the refrigerant circulated through it.

refrigerant hydrolysis

splitting of refrigerant compounds by reaction with water (e.g., reaction of dichlorodifluoromethane or methyl chloride with water, forming acid products).

refrigerant mass flow rate

the mass flow rate of the volatile refrigerant, which is potentially mixed with lubricant.

refrigerant metering device

device that controls the flow of liquid refrigerant to an evaporator.

refrigerant pressure-imposing device

device or portion of equipment used for the purpose of increasing refrigerant pressure.

refrigerant pump

recirculates liquid refrigerant from the refrigerant sump at the bottom of the evaporator to the evaporator tube bundle in order to effectively wet the outside surface and enhance heat transfer (when used).

refrigerant pump-out system

dedicated apparatus for transfer of refrigerant from a chiller or other refrigerating system to a separate and distinct storage vessel.

refrigerant receiver

vessel in a refrigerating system designed to ensure the availability of adequate liquid refrigerant for proper functioning of the system and to store the liquid refrigerant when the system is pumped down.

refrigerant recovery unit

a machine that removes refrigerant from a refrigerating system, preventing refrigerant discharge to the environment.

refrigerant release

any movement of refrigerant out of its containment and into the atmosphere, including, but not limited to, movement by a leak, by an action of filling or testing, or by failure.

refrigerant subcooler

heat exchanger, after the condenser, for subcooling the condensed refrigerant.

refrigerant subcooling

process of cooling refrigerant below condensing temperature for a given pressure, also, cooling a liquid below its freezing point where it can exist only in a state of unstable equilibrium. See supercooling.

refrigerated plate freezing

heat removal by direct contact of the packaged product with refrigerated plates.

refrigerated truck end bunker

a refrigerated vehicle where the space given to the ice or cooling element is in the end of the truck or rail car.

refrigerating cycle

thermodynamic cycle of a system that transfers heat from a low-temperature reservoir to a high-temperature reservoir.

refrigerating effect

in a refrigeration system, the rate of heat removal.

refrigerating engineering

technique of design, manufacture, application, and operation of refrigerating machinery and its primary equipment.

refrigerating heating system

interconnected parts forming a closed circuit in which refrigerant is circulated and having the condenser located to transfer heat to the zone to be heated.

refrigerating load

amount of heat per unit time imposed on a refrigerating system for the required rate of heat removal.

refrigerating service load

quantity of heat to be removed from a refrigerated space to compensate for opening doors, presence of personnel, heat from electric lights, etc.

refrigerating system

a system that, in operation between a heat source (evaporator) and a heat sink (condenser) at two different temperatures, absorbs heat from the heat source at the lower temperature and rejects heat to the heat sink at the higher temperature. See also refrigerating unit.

refrigerating system classification

refrigerating systems are classified according to the degree of probability, low or high, that leaked refrigerant from a failed connection, seal, or component could enter an occupied area. The distinction is based on the basic design or location of the components.

refrigerating system contaminants

contaminants include water (the most important and universal contaminant), dirt, particles, organic materials such as waxes, acids, and sludges, or other products of chemical reactions taking place while the system is operating.

refrigerating unit

unit assembly composed of a compressor, evaporator, condenser, and expansion device, used for refrigerating and for extracting heat.

refrigerating-system high side

parts subjected to approximately the condenser pressure.

refrigerating-system low side

parts subjected to approximately the evaporator pressure.

refrigerating-system machinery

refrigerating equipment forming a part of the refrigerating system, including any or all of the following: compressor, condenser, generator, absorber (adsorber), liquid receiver, connecting piping, and evaporator.

refrigerating-system performance factor

ratio of the useful refrigerating effect of the system to the power input.

refrigerating-system restrictor

device capable of providing the necessary difference in pressure between the high- and low-pressure sides of the refrigerating system.


(1) Cooling of a space, substance or system to lower and/or maintain its temperature below the ambient one (removed heat is rejected at a higher temperature) (2) Artificial cooling

refrigeration circuit

assembly of refrigerant-containing parts used in a refrigerating cycle.

refrigeration lubricant

a stable fluid that is compatible with system components, will form a friction-reducing film between rubbing surfaces and seal critical clearances, and has low-temperature transport properties suitable for the application in which it is used.

refrigeration oil

a stable fluid that is compatible with system components, will form a friction-reducing film between rubbing surfaces, will seal critical clearances, and has low-temperature properties suitable for the application.

refrigerator baffle

plate, wall, or partition that is designed to perform one or more of the following functions: (1) prevent contact of food with refrigerated surfaces, (2) prevent dripping of condensate on food, (3) regulate and/or direct circulation of refrigerated air.

refrigerator cabinet breaker strip

separate insulating element or integral insulating extension of the cabinet interior surfaces around the periphery of the cabinet door or drawer opening(s), which functions as a thermal barrier to minimize heat flow to the cabinet interior (i.e., it breaks a thermal leakage path).

refuse-derived fuel (RDF)

refuse used as a combustion fuel source to provide heat in a district-heating plant.

regain of moisture

mass of moisture reabsorbed by a dried material when exposed to humid air.

regeneration air

the airstream used as a carrier for the desorbed moisture and/or a mechanism to transfer heat for the regeneration of the desiccant in a dry desiccant system.

regeneration heater

device used to heat the solid desiccant regeneration air or the liquid desiccant.

regeneration specific heat input

energy per unit moisture removed expressed in kJ/kg (Btu/lb).

regenerative air-cycle system

air-cycle refrigerating system fitted with heat regenerators.

regenerative cooling

process of using heat that must be rejected or absorbed in one part of the cycle to perform a useful function in another part of the cycle.

regenerative cycle gas turbine

simple cycle gas turbine to which has been added a heat exchanger to warm incoming combustion air with heat from exhaust gases in order to increase thermal efficiency.

regenerative heating

heat rejected in one part of a system and used to perform a useful function in another part.

regenerative scrubber

scrubber in which the active reagent can be regenerated.


the structure or section containing the desiccant to be regenerated.


combination grille and damper assembly over an air opening. See air outlet.

registered engineer

appropriately qualified and licensed professional engineer. See also design professional.

regulating valve

valve capable of regulating the flow of water through a condenser according to changes in condensing pressure or water temperature.


device to establish or adjust the time, amount, degree, or rate of a unit or process.

regulatory bodies

agencies whose rules are mandated by law or by owner’s specification. Examples include the U.S. Coast Guard, American Bureau of Shipping, and U.S. Public Health Service.


application of sensible heat to supply air that has been previously cooled below the temperature desired for maintaining the temperature of the conditioned space.

reheat coil

heating coil installed downstream of cooling coil.

reheat terminal

terminal that heats a single source of supply air.

reheat VAV box

true VAV box with a reheat coil mounted on the discharge of the unit.


raising the temperature of air that has been previously cooled either by mechanical refrigeration or an economizer system.

relative humidity (rh, RH)

(1) ratio of the mole fraction of water vapor to the mole fraction of water vapor saturated at the same temperature and barometric pressure. (2) ratio of the partial pressure or density of water vapor to the saturation pressure or density, respectively, at the same dry-bulb temperature and barometric pressure of the ambient air.

relative light transmission

the light transmission of a dust spot sampling target relative to that of a translucent standard.


(1) electrical mechanism that uses the current in a control circuit to open or close electric contacts. (2) fluid (liquid or pneumatic) device that uses variations in fluid pressure to actuate final control devices.

releasable water

in freeze drying, that part of the water contained in the product and that may be extracted without spoiling the quality of the product.


any movement of refrigerant out of its containment, including, but not limited to, by a leak, by an action of filling or testing, or by failure.

release rate

the rate of release, in actual liters per minute (Lpm), of tracer gas during a hood test.


(1) mathematical probability that a device will perform its objective adequately for the period of time intended under the operating conditions specified. (2) probability that a device will function without failure over a specified time period or amount of usage. See accuracy, precision, repeatability. (3) probability that an instrument’s repeatability and accuracy will continue to fall within specified limits.

relief air

(1) all return air that is discharged directly to the outside or exhausted by separate exhaust fans. (2) building return air discharged by the air-handling unit (AHU) equipment to control building pressure when an HVAC system is operating in the economizer cycle.

remote-bulb thermostat

thermostat in which the sensing bulb can be located at a distance from the instrument proper while remaining flexibly connected to it.

renewable energy

energy obtained from sunlight, wind, earth, geothermal sources, or bodies of water to provide heating, cooling, lighting, or water-heating services to buildings. See nondepletable energy.


empirical unit of resistance to water vapor flow through a material or construction. One rep = 1 hour – square foot – in. Hg pressure difference between the two surfaces per grain (avoirdupois) water vapor (h·ft2·in. Hg/gr). (The resistance may be stated in other units consistent with this value.) The rep is the reciprocal of the perm (1 rep = 1/perm). It is not an SI unit.


(1) the reconstruction or renewal of any part of an existing building for the purpose of its maintenance. (2) to restore to good or sound condition within the following constraints: operation must be fully restored without embellishment and failure must have occurred.


(1) closeness of agreement among consecutive measurements of the output for the same value of input approaching from the same direction. Compare to accuracy, precision, reliability. (2) closeness of agreement among repeated measurements of the same variable under the same conditions. (3) the ability to obtain the same observed value in repeated experiments. Lack of repeatability is commonly associated with precision error, the random component of the total error.

replacement air

outdoor air that is used to replace air removed from a building through an exhaust system. Replacement air may be derived from one or more of the following: makeup air, supply air, transfer air, and infiltration. However, the ultimate source of all replacement air is outdoor air. When replacement air exceeds exhaust, the result is exfiltration.

replacement cost

estimate of the current or future cost to replace existing facilities either as currently structured or as redesigned to embrace new technology with facilities that will perform the same functions.


repetitions of measurements at the same conditions that are taken to estimate the uncertainty in the results.


studious inquiry, usually critical and exhaustive investigation or experimentation with the aim of revision of accepted conclusions in light of newly discovered facts.


automatic adjustment of the controller setpoint to a higher or lower value.

reset control

control method using a remote or external signal to modify the setpoint of a controller.

residence time

period that the contaminant molecule is within the boundaries of the media bed.


the occupant or occupants of an individual unit in a multiple-occupancy residential building who have entered into an agreement with the owner.

resident engineer

engineer employed by the owner to represent the owner’s interests at the project site during the construction phase.


spaces in buildings used primarily for living and sleeping. Residential spaces include, but are not limited to, dwelling units, hotel/motel guest rooms, dormitories, nursing homes, patient rooms in hospitals, lodging houses, fraternity/sorority houses, hostels, prisons, and fire stations.

residential air conditioning

air conditioning as applied to private houses or apartments.

residential application

providing comfort heating for single-family living quarters.

residential health care facility

a facility, building, or portion of a building that provides housing and services for a resident or group of residents.

residual moisture

in a freeze-dried product, the ratio of the mass of residual water to the original mass of product.

residual pressure

in freeze drying, the pressure of the gas (usually air) at the trap.


(1) property of an electric circuit, or of any object used as part of an electric circuit, that determines for a given current the rate at which electric energy is converted into heat or radiant energy and that has a value such that the product of the resistance and the square of the current gives the rate of conversion of energy. (2) property opposing movement of material or flow of energy and involving loss of potential (voltage, temperature, pressure level). (3) thermal resistance. See electrical resistance (O), thermal resistance.

resistance coefficient

(1) dimensionless group equal to four times the Fanning friction factor. Also known as Darcy-Weisbach coefficient and Darcy number 1. (2) dimensionless number used in the study of flow resistance, equal to the resistance force in flow divided by one-half the product of fluid density, the square of fluid velocity, and the square of a characteristic length.

resistance heating element

the electrical conducting medium that is heated by an electric current and that also dissipates this heat into the air or a fluid.

resistance temperature device (RTD)

temperature sensors that utilize the predictable change in electrical resistance of some materials with changing temperature. They are commonly made of platinum. There are two broad categories, film and wire-wound types. Film temperature sensors have a layer of some resistive material, such as platinum, on a substrate: the layer may be extremely thin, perhaps 1 micrometer. Wire-wound temperature sensors can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift.

resistance welding

welding with resistance heating and pressure, the work being part of the electrical circuit.

resistive load

(1) electric load with all energy input converted to heat and light. (2) electric load without capacitance or inductance or one in which inductive portions cancel capacitive portions at the operating frequency.


(1) for digital techniques, the number of discrete values that can be indicated by the digital word from zero to full scale (e.g., a ten bit binary word has a resolution of one point in 1024). (2) in analog to digital conversion, resolution refers to the smallest input increment that can be measured and indicated by an output change. This can also be called sensitivity.


condition of high-vibration response. The exact resonance of a system in forced vibration exists when any change, however small, in the frequency of excitation causes a decrease in the response amplitude (i.e., when the driving frequency equals the natural frequency).

resonance frequency

the frequency at which operation of the equipment leads to a peak in the response spectrum. Note: for lightly damped structures, the resonance frequency can be taken to be the natural frequency.

resorption-type refrigerating system

system in which the refrigerant vapor is not condensed to a pure liquid but is absorbed in a weak solution from which it is subsequently evaporated at a lower temperature to produce refrigeration.

resource energy impact

product of an energy resource [includes application of resource utilization factor (RUF)] anticipated to be used in providing fuel or energy to a building site, multiplied by an RIF for that particular form of energy resource. Total resource energy impact is the sum of all resource energy impacts for a building project.

resource impact factor (RIF)

multipliers applied to fuel and energy resources required by a building project to permit a quantitative evaluation on the economy of those resources resulting from the selection of on-site fuel and energy forms. Availability and social, economic, environmental, and national interest issues are considered.

resource utilization factor (RUF)

multiplier, applied to the quantity of fuel or energy delivered to a building site, that provides a quantitative estimate of the energy resources consumed in providing that fuel or energy. Variant multipliers account for the burden of processing, transporting, converting, and delivering fuel or energy from the point of extraction to the building site.

respirable particles

particles small enough to be inhaled into a nonciliated portion of the lung. Peak deposition of respirable particles occurs within the size range of 0.2 to 5.0 micrometers. Particles greater than 10 micrometers in aerodynamic diameter are not respirable.


(1) breathing process of animals. (2) production of carbon dioxide and heat by ripening of perishables in storage.

respiratory heat

in plants, heat created during the respiratory process (absorption of oxygen and evolution of CO2).

response time

(1) output, expressed as a function of time, resulting from the application of specified input. (2) time (preferably in seconds, may also be in cycles of supply frequency) required for the output quantity to change by some agreed-on percentage of the differential output quantity in response to a step change input. Note: in measurement, the initial and final output quantities shall correspond to the test-output quantities. The response time shall be the maximum obtained including differences arising from increasing or decreasing output quantity or time phase of signal application. (3) time for a measuring sensor to reach 90% of the final value after a step change. For a measuring system that includes only one exponential time constant function, the 90% response time equals 2.3 times the time constant.


of the adsorbent, the amount remaining after a saturated bed reaches equilibrium in clean air.


the commissioning process applied to an existing facility that was not previously commissioned. The same process for retrocommissioning needs to be followed from predesign through occupancy and operations to optimize the benefits of implementing the commissioning process philosophy and practice.


modification of existing equipment, systems, or buildings to incorporate improved performance, updated operation, improved energy performance, or all three. Derived from retroactive refit.

retrofit isolation

the savings measurement approach that determines energy or demand savings through the use of meters to isolate the energy flows for the system(s).


pipe or duct to carry fluid back to the source.

return air

air removed from a space to be recirculated or exhausted. Air extracted from a space and totally or partially returned to an air conditioner, furnace, or other heating, cooling, or ventilating system.

return flow nozzle

nozzle for a mechanical atomizing oil burner in which part of the oil supplied to the atomizer is withdrawn and returned to storage or to the oil line to the atomizer.

return fluid temperature

temperature of the heat transfer fluid at the exit of a test panel.

return-flow compressor

compressor in which the suction valve is located in the cylinder head of the compressor.


persistence of sound to bounce around in an enclosed space after the sound source has stopped.

reverberation time

time in seconds for sound energy to decay 60 decibels. A common reference is often the time for the sound in the 500 Hz octave band to decay 60 dB.

reverse acting

device or control where the action of the device or control decreases (or increases) as the variable increases (or decreases). Compare to direct acting.

reverse cycle

cycle obtained when a fluid is made to follow the different thermodynamic stages of a cycle in the reverse order.

reverse cycle defrosting

defrosting an evaporator by reversing its function with that of the condenser.

reverse flow

airflow within the hood when smoke released in the hood moves forward, toward the front of the hood. This term does not apply to the forward motion of the roll inside the hood that occurs in the upper cavity of the hood above the hood opening or to the cyclonic motion that occurs behind a closed horizontal sash.

reverse pressure

(1) output pressure of a reversing relay that changes in opposition to the input signal (i.e., as the input pressure increases, the output pressure decreases). (2) pressure differential existing in a heat pump reversing valve to affect a reversal of flow through the valve.

reverse return piping system

two-pipe system in which the heat transfer medium supplied to the first load is the last returned to the heat transfer equipment. A system in which the water return piping from terminal units is sized to provide equal lengths for balanced flow rates. Compare to direct-return piping system.

reversible cycle

theoretical thermodynamic cycle composed of a series of reversible processes.

reversible work

maximum useful work obtained for a given change of state. It includes heat supplied from other systems but excludes work done on the surroundings.

reversing relay

relay designed to reverse the direction of current flow or of function, on actuation.

Reynolds number

a dimensionless number, designated Re, that indicates whether the fluid flow is laminar or turbulent. For flow in a pipe, transition generally occurs between Reynolds’ numbers of 2300 and 4000.


density of refrigerant, kg/m3 [lbm/ft3]

Richardson number

dimensionless number used in studying the stratified flow of multilayer systems, equal to the acceleration of gravity g times the density gradient of a fluid divided by the product of the fluid’s density and the square of its velocity gradient at a wall (symbol NR).

rigid duct

ducts constructed of rigid materials such as metal and fiberglass duct board.

rigid duct flow area

calculated by using the average inside duct dimensions determined by measurement of a minimum of three representative sections of the duct envelope.

rime ice

granular deposit of opaque ice on a surface, formed by quick freezing of supercooled water droplets.


alternating-current component from a direct-current power supply arising within a power supply.

rise of an air jet in mixing air diffusion

vertical distance (hv) between the highest horizontal plane tangent to a specified isovel and the center of the core of an air jet.

rise time

(1) measure of the time required for a circuit to change its output from a low voltage level to a high voltage level, normally from 10 to 90 percentage points. (2) time required for the output of a system (other than first order) to make the change from a small specified percentage (often 5% to 10%) of the steady-state increment to a large, specified percentage (often 90% to 95%) either before overshoot or in the absence of overshoot.

rock bed regenerative cooling system

system of air conditioning in which packed beds of crushed stone or gravel are used for both evaporative cooling and heat energy storage.


the rotation of air in the upper cavity of the hood. The roll is induced by the momentum of the air entering the hood through the hood opening.

roll bond evaporator

evaporator consisting of two metal plates that are weld bonded together (with the exception of the printed circuit forming the refrigerant passage, which is obtained by inflation under pressure).

roll filter

filter in which the filter medium is a continuous belt on movable rolls to bring clean filter media into the airstream, either automatically or manually.

roller bearing

bearing consisting of a number of rollers in a cage between the inner and outer races.

rolling piston compressor

special type of small rotary compressor having a rotor aligned eccentrically within the stator, used in domestic refrigerators.


the upper portion of the building envelope, including opaque areas and fenestration, that is horizontal or tilted at an angle of less than 60° from horizontal.

roof assembly

all components of the roof/ceiling envelope through which heat flows, thus creating a building transmission heat loss or gain, where such assembly is exposed to outdoor air and encloses a heated and/or mechanically cooled space.

roof spray cooling

system that reduces heat gain through a roof by cooling the outside surface with a water spray, suited for only temporary treatment because high humidity may be introduced by air intakes on the roof.

roof with insulation entirely above deck

a roof with all insulation: a. installed above (outside of) the roof structure and b. continuous (i.e., uninterrupted by framing members).

rooftop air conditioner

packaged air conditioner mounted on a roof, the conditioned air being discharged directly into the rooms below or through a duct system.

room calorimeter

a test facility consisting of a room-side compartment and an outdoor-side compartment, each of which is equipped with instrumented reconditioning equipment. The output of this equipment is measured and controlled to counterbalance the room-side net total cooling effect of the air conditioner under test.

room cavity ratio (RCD)

a factor that characterizes room configuration as a ratio between the walls and ceiling and is based upon room dimensions.

room criterion curve (RC curve)

a series of curves of octave band sound spectra in a system for rating the noisiness of an occupied indoor space, an actual octave band spectrum is compared with this set of curves to determine the RC level of the space. Rates sound from 16 to 16,000 Hz octave bands.

root mean square (RMS)

(1) for a sinusoidal motion, the RMS value is 0.707 times peak. (2) for a velocity measurement, root mean square is often used when vibrations are random or consist of a number of sinusoidal vibrations of different frequencies. The RMS value is a measure of the effective energy used to produce the vibration of the machine. (3) root mean square is the square root of the time average of the sound (vibration) wave(s).

rotary atomizing burner

burner in which atomization is accomplished by feeding oil to the inside of a rapidly rotating cup.

rotary compressor

a positive displacement compressor in which the change in internal volume of the compression chamber is accomplished by the rotary motion of a positive displacement.

rotating blackouts

process of deliberately interrupting preselected loads from an electric power system, as a nonroutine remedy of energy management implemented through transmission supervision and substation automation, on a substation bus or distribution feeder basis in a sequentially timed pattern for the purpose of matching demand to temporarily limited supply.

rotating vane anemometer

device consisting of rotating-propeller-type vanes, the air velocity is indicated from the rotational speed of the vanes.

roughness factor

ratio of size of projections from the surface of a pipe or duct to the diameter of the pipe or duct.

runaround heat exchanger

finned-tube coils (closed system) or spray chambers (open systems) in which a liquid is circulated by gravity or pump action through a heat source exchanger and then through a heat sink exchanger. Antifreeze may be used in the coil loop and a desiccant in the spray system.

runaround system

regenerative-type closed secondary system in which a continuously circulated fluid absorbs heat from the primary system fluid at one place and sends it to the primary system fluid at another place.

running cycle

(1) the period of time between the start and the stop of a cycle. (2) the period of time between the start of refrigeration after a defrost termination and the beginning of the next successive defrost.


(1) accumulated hours of equipment operation. (2) period of operation of a system on test.

rupture disc

(also known as pressure relief device or pressure limiting device), valve or rupture member designed to relieve pressure at a predetermined setting by mechanical failure of the disc. A rupture disc is a single-use device and must be replaced after the incident (as opposed to a relief valve, which may be manually or automatically reset).