ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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TAB technician

the person who measures and adjusts the system.


receptacle, open or closed, for holding, transporting, or storing fluids.

tankless heater

a heat exchanger for indirect heating of water, typically for domestic use, which is designed to be used without a water storage tank. See instantaneous heater.


manually operated device on the end of a pipe in a fluid supply system to enable drawing off quantities of the fluid.

task/ambient conditioning (TAC) system

any space-conditioning system that allows occupants to individually control the thermal environment in the localized zone of their workspace while still maintaining acceptable environmental conditions in the surrounding ambient spaces.


measurement of warmth or coldness with respect to an arbitrary zero or to the absolute zero. Temperatures are indicated on defined scales, such as Kelvin and Rankine for absolute temperatures, and Celsius and Fahrenheit for ordinary temperatures.

temperature controller

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

temperature difference

difference between the temperatures of two substances, surfaces, or environments involving transfer of heat.

temperature difference method

procedure used in design and system analysis to calculate flow (of air, water, or steam) from load or to determine load when flow and temperature differential are known.

temperature differential (?t)

the difference between the average test zone temperature (tac) and the average supply air temperature (tdc).

temperature differential sensor

a sensor system composed of two temperature sensors that is capable of providing a signal that is related to the temperature differential of the two sensors.

temperature differential within the occupied zone

largest value of the difference between the measured air temperatures within the occupied zone.

temperature glide

see glide.

temperature gradient

temperature variation per unit distance or time along the heat flow path.

temperature gradient risk

percentage of people predicted to be dissatisfied due to a difference in air temperature between ankle and head.

temperature index coefficient of thermal performance

ratio of the difference in temperature between the inside surface and the exterior ambient to the difference in temperature between the inside ambient and the exterior ambient across a component of the building envelope. The coefficient can be used to estimate the apparent thermal resistance of the component.

temperature of flowing fluids

the mixed mean-stream temperature at a station perpendicular to the flow direction.

temperature profile

graph representing the distribution of temperatures in a plane section of a body or a space, or over a period of time.

temperature sensor

a sensor located in the fluid that is capable of producing a signal (output) that is related to the temperature.

temperature-controlled surfaces

interior surfaces whose temperature is controlled or monitored for heating and cooling purposes.

temperature-sensing element

the part of the expansion valve that senses the temperature at the superheat control point, normally located at the outlet of the evaporator. This element may be remote or integral to the expansion valve body.

temperature, dry bulb
temperature, wet bulb
tenable environment

an environment in which the characteristics, quantity, and location of smoke are limited or otherwise restricted to allow for ready evacuation through the space. Maintenance of a tenable environment in the smoke zone is not within the capability of zoned smoke control.


a device by which energy from a system is finally delivered, e.g., registers, diffusers, lighting fixtures, faucets, etc.

terminal casing leakage

amount of air in ft²/min (L/s at standard conditions) escaping from the terminal at a given inlet pressure with only the outlet(s) blocked and with the damper/valve fully opened.

terminal damper leakage

amount of air in ft²/min (L/s at standard conditions) passing through a fully closed damper/valve at a given inlet pressure.

terminal unit

a device that regulates the volumetric flow rate and/or the temperature of the controlled medium.

terminal-unit casing leakage

air in cubic feet per minute (liters per second) leaking from a terminal unit at a given inlet pressure with the outlets and inlets blocked and with the damper/valve fully opened.

terminal-unit damper leakage

air in cubic feet per minute (liters per second) leaking through a fully closed damper/valve of a supply/exhaust terminal unit at a given inlet/discharge pressure.


system of terms belonging to, or peculiar to, a science, art, or specialized subject.


(1) the recorded group of readings of required test data taken while equilibrium is maintained and used in the computation of results, those observed or recorded during a sufficient period to indicate that equilibrium was attained prior to the actual test. (2) the recorded group of readings of required test data taken while equilibrium is maintained and used in the computation of results, those recorded during the period of the test. (3) a series of determinations for various points of operation.

test air

the air that flows through the device being tested. During the test, test air should be at the temperature, humidity, pressure, and atmospheric dust concentration prevailing at the time of the test. Test air for arrestance and dust-holding capacity measurement may be indoor ambient air.

test authority

the designated person, company, or agent who specifies the test requirements.

test condition tolerance

the maximum permissible variation between the average of a measured quantity and the desired test condition specified in the standard.

test method

definitive procedure that produces a test result. Note: appropriate functions of a test method are identification, measurement, or evaluation of one or more qualities, characteristics, or properties of a material, product, system, or service.

test operating tolerance

the maximum amount that a designated measured quantity shall vary (i.e., maximum–minimum) during the entire or a specified interval of a test.

test panel

any sensible heating or cooling panel that is used in testing for performance and/or rating purposes.

test period

the time over which quasi-steady-state conditions are maintained for each measured point.

test pressure

pressure, usually higher than the design working pressure, to which a piece of equipment is subjected for testing according to specified procedures.

test pressure loss

differential in total pressure between the inlet and the outlet sections of a test duct or across a test fitting. For test fittings, the fitting is assumed to have zero length. For multiflow fittings, the total pressure loss shall be determined for each stream separately.

test rooms

the two environmental chambers where all components of the combined appliance are installed and tested, one chamber is used to maintain specified indoor ambient conditions, while the second chamber is used to maintain specified outdoor ambient conditions.


the use of specialized and calibrated instruments to measure conditions such as temperatures, pressures, rotational speeds, electrical characteristics, velocities, fluid flows, etc., used in HVAC&R.

testing standard

standard that sets forth methods of measuring capacity, or other aspects of operation, of a specific unit or system of a given class of equipment, together with a specification of instrumentation, procedure, and calculations. See MOT.

testing, adjusting, and balancing

a systematic process or service applied to heating, ventilating, and air-conditioning (HVAC) systems and other environmental systems to achieve and document air and hydronic flow rates. The adjustment of fluid flow rates through distribution systems by manually adjusting the position of dampers, valves, etc., or by using automatic control devices to control the position of dampers, actuators, valves, etc.


changing the solid phase of water, or frozen moisture within a substance, to the liquid phase by the application of heat.

theoretical storage capacity

the sum of the products of masses and heat capacities of all components (including the transfer fluid) contained within the insulating envelope of the thermal storage device.


quantity of heat equivalent to 100,000 Btu.

thermal absorptance

ratio of the radiant flux absorbed by a physical surface to that incident on it.

thermal anemometer

device that relies on the cooling effect of the airflow to change the temperature of a heated body in proportion to the air speed. Types include hot-wire anemometer, heated-bulb thermometer, heated-thermocouple anemometer, and heated-thermistor anemometer.

thermal anomalies

heat loss characteristics of a physical condition or structure that are not in accordance with intended design or calculated characteristics.

thermal boundary resistance

(also known as thermal contact resistance), ratio of temperature difference to heat flux across the boundary between two distinct media (solid/solid or solid/fluid).

thermal break

nonconducting physical structure, such as a frame around a door or window acting to retard heat flow.

thermal bridge

low thermal resistance path connecting two surfaces.

thermal comfort

the condition of mind which expresses satisfaction with the surrounding thermal environment and is assessed by subjective evaluation. Thermal comfort is affected by heat conduction, convection, radiation, evaporative heat loss, and relative air motion.

thermal conductance (C-factor)

time rate of steady-state heat flow through unit area of a material or construction, induced by a unit temperature difference between the body surfaces. Units are Btu/h·ft2·°F (W/[m2·K]). Note that the C-factor does not include soil or air films.

thermal conductivity

(k-factor), time rate of steady-state heat flow through unit thickness of unit area of a homogeneous material, induced by a unit temperature gradient in a direction perpendicular to the isothermal planes of that unit. Units of k are in Btu·in/(h·ft2·°F), Btu·ft/(h·ft2·°F), or W/(m·K). Thermal conductivity must be evaluated for a specific mean temperature, thickness, age, and moisture content. See also thermal conductance.

thermal conductor

material that transmits heat by conduction.

thermal convection

transfer of heat by a fluid moving by natural variations in density (in the absence of conduction and radiation).

thermal delay

time period between the energization of a heat-producing device and the measurable effect of the heat produced until equilibrium conditions are reached.

thermal diffusion

phenomenon in which a temperature gradient in a mixture of fluids gives rise to a flow of one constituent relative to the whole mixture.

thermal diffusivity

physical quantity that determines the rate of heat propagation in transient state processes. Thermal conductivity divided by the product of density and specific heat. Units are ft2/s or m2/s.

thermal efficiency

energy output as a percentage of the energy input of a machine or process.

thermal emissivity

radiation property of a material, evaluated with its surface optically smooth and clean, and of sufficient thickness to be opaque.

thermal emittance

a surface property of a material governing the emission of thermal radiation relative to that emitted by a perfect emitter, or black body, at the same surface temperature.

thermal energy

energy possessed by a system caused by the motion of the molecules and/or intermolecular forces, i.e., heat.

thermal energy meter
thermal energy storage

(1) thermal energy storage may refer to a number of technologies that stores energy in a thermal reservoir for later reuse. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) than that of the ambient environment. The principal application today is the production of ice, chilled water, or eutectic solution at night, which is then used to cool environments during the day. (2) thermal energy storage technologies store heat, usually from active solar collectors in an insulated repository for later use in space heating, domestic or process hot water, or to generate electricity. Most practical active solar heating systems have storage for a few hours to a day's worth of heat collected. There are also a small but growing number of seasonal thermal stores used to store summer heat for space heating during winter.

thermal envelope

elements of a structure that enclose conditioned spaces and that control transmission of heat, air, and water vapor between the conditioned spaces and the exterior. See also building thermal envelope.

thermal environment

the surrounding atmosphere characterized by parameters such as air temperature, wet-bulb temperature, dew-point temperature, water vapor pressure, total atmospheric pressure, relative humidity, and specific humidity.

thermal equilibrium

equilibrium obtained in a system when the temperatures are nonvarying.

thermal expansion

increase in one or more of the dimensions of a solid body or a liquid volume, caused by a temperature rise.

thermal frequency response

response of a thermal system to a periodic thermal excitation expressed as a function of frequency. Note: thermal frequency response usually is displayed by polar plot of amplitude attenuation and time lag versus frequency.

thermal inertia

ability of a material, depending on its density and specific heat, to store heat and to resist temperature change.

thermal input

heating or cooling effect delivered to a product or space.

thermal insulation

material or assembly of materials used to provide resistance to heat flow. Also see blanket thermal insulation.

thermal insulation fill

insulation in granular, nodular, fibrous, powdery, or similar form designed for installation by pouring, blowing, or hand placement. Examples are mineral or glass fiber, cellulosic fiber, diatomaceous silica, perlite, silica aerogel, and vermiculite.

thermal lag

delay in action of the sensing element of a control device due to the time required for the sensing element to reach equilibrium with the property being controlled or measured.

thermal load

thermal requirement of a system under specified conditions.

thermal neutrality

the indoor thermal index value corresponding with a mean vote of neutral on the thermal sensation scale.

thermal output

heating or cooling effect put out by a source or removed from a storage device.

thermal plume

airflow created by a convective heat source that rises due to natural thermal buoyancy. Plume formation and growth are dependent on the intensity of the heat source and on the degree of stratification of the ambient air.

thermal radiance

rate of radiant emission through unit solid angle over unit projected area of a source in a stated angular direction from the surface (usually the normal). Units are watts per square metre.

thermal radiant flux density

rate of radiant energy emitted from unit area of a surface in all radial directions of the overspreading hemisphere. See also radiant flux density.

thermal radiation

transmission of energy by means of electromagnetic waves emitted due to temperature. Radiant energy of any wavelength when absorbed may become thermal energy that increases the temperature of the absorbing body. See also heat transfer radiation coefficient.

thermal reflectance

fraction of the incident radiation on a surface that is reflected from that surface. Note: for an opaque surface, the sum of reflectance, transmittance, and absorptance is unity at any wavelength of radiation.

thermal regain

the fraction of distribution system losses (gains for cooling) that are returned to the conditioned space.

thermal relay

(1) relay activated by change in temperature. (2) relay in which the displacement of the moving contact member is produced by heating of a part of the relay under the action of electric currents.

thermal resistance

(R-value), the reciprocal of the time rate of heat flow through a unit area induced by a unit temperature difference between two defined surfaces of material or construction under steady-state conditions. Units of thermal resistance are h·ft2·°F/Btu (m2·°K/W). Thermal resistance is the reciprocal of the thermal conductance.

thermal sensation

a conscious feeling commonly graded into the categories cold, cool, slightly cool, neutral, slightly warm, warm, and hot, it requires subjective evaluation.

thermal storage

(1) accumulation of energy in a body or system in the form of sensible heat (temperature rise) or latent heat (change of phase). (2) full storage: thermal storage system having capacity to meet all on-peak cooling or heating requirements by being charged off peak, and without energy added on peak. (3) fully charged condition: the state of a thermal storage device at which, according to the design, no more heat is to be removed from the thermal storage device. This state is generally reached when the control system stops the charge cycle as part of its normal control sequence. (4) fully discharged condition: the state of a thermal storage device at which no more usable cooling energy can be recovered from the storage device. (5) normally interchangeable term with cool storage or ice storage when addressing air-conditioning thermal storage systems. (6) technology or systems of accumulating cooling or heating capacity for subsequent use. (7) temporary storage of high or low-temperature energy for later use.

thermal storage charge

to supply cooling or heating to storage.

thermal storage efficiency

(also known as cycle figure of merit), ratio of the integrated discharge capacity to the hypothetical maximum available capacity for a single cycle of operation.

thermal storage load leveling

charging a thermal storage system at a constant rate during a complete cycle.

thermal storage medium

substance in which cooling or heating energy is stored.

thermal superinsulation

insulation of very high thermal resistance. Usually refers to that used in cryoengineering.

thermal time lag

(1) phase difference in hours between the exterior and interior surface temperatures when the exterior surface is subjected to a sine wave temperature change having a 24 h period. (2) time interval by which the peak thermal response falls behind (lags) the peak thermal excitation in a thermal system.

thermal transmittance

(also known as U-factor), heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. Note: this heat transmission rate is also called the overall coefficient of heat transfer. U, in Btu/h·ft2·°F (W/[m2·K]). Thermal transmittance is sometimes called the overall coefficient of heat transfer or U-factor. Thermal transmittance includes surface film conductance.

thermal unit

quantity or rate of heat energy or equivalent in work or electrical energy.


thermoelectrical element in which the electrical resistance falls appreciably with a rise in temperature, often used as a temperature sensor.


junction of two wires of dissimilar materials, not necessarily metal, with the property of generating an emf related to the temperature of their junction. Compare to thermopile.

thermodynamic equilibrium

equilibrium in a system when the physical variables have uniform values that do not change in time. Furthermore, if the system is not an isolated one, these variables should have the same values for both the system and its surroundings.

thermodynamic properties

those data needed to calculate the equilibrium relations among pressure, volume, and temperature along with the enthalpy and entropy of the fluid in the liquid and vapor states.

thermodynamic shock

implosive impact in liquid, caused by sudden condensation of vapor into its subcooled liquid. This phenomenon can occur when the liquid is about 60°F to 85°F (33°C to 47°C) cooler than the saturation temperature of the contact vapor. It creates a loud sound and can cause severe local pressure stresses in the container or piping system.

thermodynamic system

in thermodynamics, a region in space or a quantity of matter bounded by a closed surface in which thermal actions occur. The surroundings include everything external to the system, and the system is separated from the surroundings by the system boundaries. These boundaries can be either movable or fixed, either real or imaginary.

thermodynamic trap

steam (disk) trap constructed with a cap containing a steel disc, which fits against a flat seat. Condensate, discharging at close to saturation temperature, increases in velocity and draws the disc down toward the seat, due to the lower pressure caused by the increased velocity (Bernoulli effect). Condensate discharging from high to low pressure flashes off and creates the closing pressure above the disc within the cap. As this flash steam condenses, pressure is dissipated, and the cycle repeats. The trap has limited air venting capabilities.

thermodynamic work

mechanism that transfers energy from one system to another without accompanying transfer of entropy. Units of thermodynamic work are Btu (W·h).


science of the relation of heat to other forms of energy.

Thermodynamics First law

law of conservation of energy, which can be expressed as follows: heat and work are mutually convertible, or because energy can neither be created nor destroyed, the total energy associated with an energy conversion remains constant.

thermoelectric refrigeration

method for cooling by the Peltier effect.


photograph or two dimensional record of an image that maps the apparent temperature of a scene as sensed by an infrared imaging system.


process of generating a thermogram by using an infrared imaging system, usually with some means of temperature calibration.


instrument for measuring temperature.

thermometer electric resistance

a temperature measuring and display instrument in which an electric resistance varies as a function of temperature.


part of applied physics relating to the measurement of temperature.

thermophysical properties

those data needed to calculate heat transfer and fluid flow characteristics of the fluid. Thermophysical properties include both thermodynamic (equilibrium) and transport properties.


study of physical phenomena related to heat.


a number of thermocouples wired consistently in series or parallel to measure small or average temperature differences.


circulation by the forces induced by the differences in densities of cooler and warmer fluids.

thermosiphon exchanger

a tube, or coils with interconnecting piping, placed in supply and exhaust airstreams and filled with a refrigerant heat transfer fluid.


an automatic control device used to maintain temperature at a fixed or adjustable setpoint.

thermostat anticipator

device that adds heat within a thermostat in anticipation of overshoot.

thermostat compensator

in a gas charged thermostat, a device that compensates for fluctuations caused by temperatures, in certain parts of the power system, different from the temperature of the bulb.

thermostat offset

difference between the cut in point and the scale setting.

thermostatic balanced pressure steam trap

trap installed on the discharge side of a heating unit and designed to pass air freely on start-up and condensate at a subcooled temperature, but to prevent steam vapor passing into the return. It can have a bellows or encapsulated metallic diaphragm containing a small quantity of volatile liquid. At the bottom of the diaphragm or bellows is attached a hardened, self centering valve head operating on the pressure side of the valve seat. At ordinary temperatures and atmospheric pressure, the valve is fully open to permit free passage of air and cold condensate. The trap discharges at a fixed temperature below that of steam saturation temperature and closely follows the steam pressure/temperature curve.

thermostatic bimetallic steam trap

trap installed where low-temperature discharge is required. It incorporates a bimetallic element that, when heated, deflects and causes a downstream valve head to be drawn up, closing the orifice. It discharges air and cold condensate freely on start-up.

thermostatic control

an automatic control device or system used to maintain temperature at a fixed or adjustable setpoint.

thermostatic expansion valve

a device for controlling superheat by regulating the mass flow of refrigerant to a refrigeration load, actuated by changes in equalizer pressure and temperature sensing element temperature.

thermostatic regulator

evaporator pressure regulator that is sensitive to temperature.

thermostatic switch

device within an electric controller for completing or interrupting an electrical circuit in response to a temperature change.

third octave (1/3) band sound pressure level

the sound pressure level at all frequencies contained within a 1/3 octave band filter.

three piece split system

any air conditioner or heat pump that has three major assemblies separated from each other. The compressor would be in a first assembly, with one refrigerant heat exchanger in a second assembly, and the second refrigerant heat exchanger in a third assembly.

three-phase electrical service

(1) electrical service of a three-phase power form. (2) electrical service supplied to the user by the utility company.

three-pipe air-conditioning system

multipiping arrangement in which each unit is fitted with two supply pipes (hot and chilled water) and a single return pipe common to the central heater and refrigerating system.

three-way valve

valve having either a single inlet and two outlets (diverting) or two inlets and a single outlet (mixing), in which either one or the other is open. Can also be a service valve for dual-mounted safety relief valves. See also diverting valve, mixing valve.

threshold limit value®—time weighted average (TLV®—TWA)

(1) the time-weighted average concentration for a normal 8 hworkday and a 40 h workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. (2) the refrigerant concentration in air for a normal 8 h workday and a 40 h workweek to which repeated exposure, day after day, will not cause an adverse effect in most persons.

threshold limit values®

refers to airborne concentrations of substances and represents conditions under which it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects. Because of the wide variation in individual susceptibility, however, a small percentage of workers may experience discomfort from some substances at concentrations at or below the threshold limit, a smaller percentage may be affected more seriously by aggravation of a preexisting condition or by development of an occupational illness. Smoking of tobacco is harmful for several reasons. Smoking may act to enhance the biological effects of chemicals encountered in the workplace and may reduce the body’s defense mechanisms against toxic substances. Individuals may also be hypersusceptible or otherwise unusually responsive to some industrial chemicals because of genetic factors, age, personal habits (smoking, use of alcohol or other drugs), medication, or previous exposure. Such workers may not be adequately protected from adverse health effects from certain chemicals at concentrations at or below the threshold limits. An occupational physician should evaluate the extent to which such workers require additional protection. Threshold limit values® (TLVs®) are based on the best available information from industrial experience, from experimental human and animal studies, and, when possible, from a combination of the three. The basis on which the values are established may differ from substance to substance, protection against impairment of health may be a guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance, or other forms of stress may form the basis for others. (This definition reprinted by permission of the American Conference of Governmental Industrial Hygienists [ACGIH].)

throttling expansion

expansion (via flow restriction) across any orifice during which no mechanical work is transferred by the fluid to the surroundings.

throttling range

change in controlled variable required to move the actuator or valve from one of its extreme limits of travel to the other.

throttling valve

valve used to restrict (throttle) the flow of fluid.

through the wall air conditioner
throw (T)

the distance, in ft (m), from the center of the air device outlet to a point in the mixed airstream where the highest sustained velocity of the mixed airstream has been reduced to a specified level.

tilt angle

angle between the horizontal plane and the plane of the collector aperture.

time defrosting

defrosting process automatically and intermittently operated for a predetermined period.

time delay

(1) time interval between the manifestation of a signal at one point and the manifestation or detection of the same signal at another point. Note: generally, the term “time delay” is used to describe a process whereby an output signal has the same form as the input signal causing it, but is delayed in time, that is, the amplification of all frequency components of the output are related by a single constant to those of corresponding input frequency components, but each output component lags behind the corresponding input component by a phase angle proportional to the frequency of the component. (2) time interval between when a command is given and when it is executed.

time of use pricing

A rate structure characterized by different prices for electricity use in a 24-hour time frame. Time of use pricing is generally used to encourage electricity use during periods of lower demand and discourage electricity use during periods of high demand.


(as applied to fenestration) bronze, green, blue, or gray coloring that is integral with the glazing material. Tinting does not include surface applied films such as reflective coatings, applied either in the field or during the manufacturing process.

titanium tetrachloride

the chemical TiCl4 that generates white visible fumes used in preliminary testing in laboratory fume hoods. (Caution: Titanium tetrachloride is corrosive and irritating, skin contact or inhalation should be avoided.)

toggle action

action of a switch to change to an alternate contact position as in a flip flop.


difference between upper and lower limits of size for a given nominal dimension or value.

ton day of refrigeration

heat removed by a ton of refrigeration operating for a day, 288,000 Btu (approximately 84.3 kWh).

ton of refrigeration

time rate of cooling equal to 12,000 Btu/h (approximately 3517 W). It is a quantity approximately equal to the latent heat of fusion or melting of 1 ton (2000 lb) of ice, from and at 32°F (0°C).


metric ton of 1000 kg. Equivalent to 1.10225 short ton (2000 lb).

torsional excitation

as applied to a fan, a type of excitation in which the external force is applied through the hub in the form of torque pulsations.

total acid number (TAN)

a measure of the acidity of a lubricant.

total air

combination of primary air and secondary air at a specific point.

total cooling effect

(1) amount of sensible and latent heat removed from the conditioned space. (2) difference between the total enthalpy of the dry air and the water-vapor mixture entering and leaving the cooler.

total head

sum of static head and velocity head.

total heat

see enthalpy.

total heat rejection
total irradiance

the quantity of radiant energy incident upon a surface over all wavelengths.

total organic acid (TOA)

total concentration of organic acids present in the lubricant

total pressure

the pressure which exists by virtue of the degree of compression and the rate of motion. It is the algebraic sum of the velocity pressure and the static pressure at a point. Thus, if the fluid is at rest, the total pressure will equal the static pressure.

total pump head pressure

total pump head pressure is composed of four primary components: lift, column friction, surface requirements, and injection head.

total refrigerant heat rejection effect

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

total refrigerating effect

(water or brine cooler), product of the mass rate of refrigerant flow and the difference in enthalpy of the entering and leaving refrigerant fluid, expressed in heat units per unit of time.

total refrigeration capacity

the product of the mass flow rate of refrigerant and the difference in enthalpy between the leaving and entering refrigerant, expressed in energy units per unit of time.

total static pressure loss
total suspended particulates

mass of particulates suspended in a unit volume of air as collected by an air sampler.

total thermal emittance

emittance that is an integrated average for all wavelengths of radiant energy emitted.

total ton

total heat load expressed in tons of cooling, the sum of the sensible tons (dry tons) and the latent tons (wet tons).

total-to-static efficiency

in a turbocompressor, the ratio of the variation of enthalpy of an isentropically compressed vapor to the work to be effectively supplied to the compressor (the enthalpy relating to the total pressure of the fluid at inlet and the static pressure at outlet).

total-to-total efficiency

(also known as stagnation efficiency), in a turbocompressor, the ratio of the variation of enthalpy of the vapor isentropically compressed from the total pressure at inlet to the total pressure at outlet to the work effectively supplied to the compressor.

tower scrubber

vertical vessel filled with plates or suitable packing, through which scrubbing fluid flows upward through the liquid, separating entrained liquids or solids from the gas.


the ability of a substance to be harmful or lethal due to acute or chronic exposure by contact, inhalation, or ingestion. The effects of concern include, but are not limited to, those of carcinogens, poisons, reproductive toxins, irritants, corrosives, sensitizers, hepatoxins, nephrotoxins, neurotoxins, agents that act on the hematopoietic system, and agents that damage the lungs, skin, eyes, or mucous membranes.

toxicity of a refrigerant

the ability of a refrigerant to be harmful or lethal due to acute or chronic exposure by contact, inhalation, or ingestion. The effects of concern include, but are not limited to, those of carcinogens, poisons, reproductive toxins, irritants, corrosives, sensitizers, hepatoxins, nephrotoxins, neurotoxins, agents that act on the hematopoietic system, and agents that damage the lungs, skin, eyes, or mucous membranes.

training plan

a written document that details the expectations, schedule, budget, and deliverables of commissioning process activities related to training of project operating and maintenance personnel, users, and occupants.


(1) a device designed to receive energy from one system and supply energy, of either the same or of a different kind, to another system in such a manner that the desired characteristics of the input energy appear at the output. (2) a device that changes one form of physical quantity into another. In the measurement field, transducers are generally used to sense a variety of measurands, such as line voltage, current, power, pressure, and temperature, and to convert these to a common output signal for use with a controlling or recording instrument.

transfer air

air transferred from one room to another through openings in the room envelope, whether it is transferred intentionally or not. The driving force for transfer air is generally a small pressure differential between the rooms, although one or more fans may be used.

transfer fluid

the fluid that carries energy between two heat transfer devices.

transformation fitting

a piece of electrical equipment used to convert electric power from one voltage to another voltage.


pulse or other temporary phenomenon occurring in a system that is not in a steady-state condition.

transient state

the state in which the system undergoes a normal change in operation, such as thermostat cycling or actuation of a defrost control.

transition point

at a stated pressure, the temperature (or at a stated temperature, the pressure) at which two phases exist in equilibrium, that is, an allotropic transformation temperature (or pressure).


transport of substances, energy, or indicated values from one place to another with or without impedances.

transmission loss

reduction in magnitude of some characteristic of a power or a signal between two stated points in a system.

transmittance radiation

portion of thermal radiation incident on a physical surface that is transmitted through that surface. Compare to thermal transmittance. Note: thermal transmittance usually is used for heat flow through walls, but transmittance (or transmissivity) is more often referred to as a radiation property.

transpiration cooling

cooling produced by evaporation of fluid lost by a body or material.

transport property data

properties that describe the capability of a fluid to transfer heat and momentum, typically thermal conductivity and dynamic or kinematic viscosity.

transverse joint

transverse (girth) joint used to assemble duct and fittings.


device for preventing passage of one type of fluid, often while allowing other fluids to proceed. Compare to steam trap.


method of measuring air and fluid volumetric flow in ductwork and piping systems.

trend log (trend record)

record of events taken on a regular schedule or equal time intervals or by change of state or value.

triple point

particular temperature and pressure at which three different phases of one substance can coexist in equilibrium. Water is an example of a substance that has a well known triple point.


(also known as luminaire), electric lighting fixture that may be equipped with a means to provide for air supply, air return, and/or heat extraction.

trombe wall

wall that is sun facing and built from material that can act as a thermal mass (such as stone, metal, concrete, adobe, or water tanks). A highmass wall that stores heat from solar gain during the day and slowly radiates the heat.

true solar time

local standard time adjusted by the equation of time (determined from an astronomical almanac) and the longitude correction (four times the difference between the standard longitude of the observer’s time zone and the observer’s actual longitude). A time reference used to compute the apparent position of the sun.


tubular conduit for transport of fluids or finely divided solids, also, a hollow structural member, a hollow product of round or other cross section. A tube may be helical welded, lap welded, spiral welded, butt welded, or seamless. A tube is designated by its exact outside diameter and its exact wall thickness, which may be described in gage numbers or other units. As an example, copper tube is commonly used in the piping and plumbing industry and the normal wall thickness is 0.125 in. (3.2 cm). When describing the outside diameter, it is referred to as copper tube. When describing the nominal pipe size, it is referring to the inside diameter. Tube or tubing identifies the outside diameter. Pipe or piping identifies the inside diameter.

tube-in-sheet evaporator

(plate evaporator), evaporator constructed from a pair of plates assembled to form a shallow compartment containing a coil through which refrigerant flows, with the fluid to be cooled circulating in the compartment.

tube-in-tube condenser

condenser consisting of a tube inserted in a second tube in a helical coil, serpentine coil, or parallel tubes.

tube-on-sheet evaporator

(plate coil), type of extended surface evaporator consisting of one or several metal sheets with a coil through which refrigerant flows, brazed to one face.

tubeaxial fan

(1) axial fan whose blades revolve in a cylindrical casing. The term “ducted fan” is used when the casing is of substantial length. (2) propeller or disc-type wheel within a cylinder, and including driving-mechanism supports for either belt drive or direct connection.


formation over a surface of scattered, knob-like mounds of localized corrosion products.

tubular centrifugal fan

(also referred to as an in-line fan) a centrifugal impeller located within a cylindrical or rectangular housing, discharging in an axial direction. Its performance is similar to that of a centrifugal blower except its capacity and pressure are lower due to the less efficient fan arrangement.

tunnel cooler

chilled, elongated space for cooling foodstuffs on a movable transport system by rapid circulation of cold air.

tunnel freezer

elongated enclosure provided with rapid cold air circulation for the freezing of foodstuffs. Also called a freezing tunnel or blast freeze tunnel.


fluid-energized acceleration machine for generating rotary mechanical power from the energy in a fluid stream.


turbine-driven compressor, usually a centrifugal compressor.

turbocompressor stall

phenomenon of instability that may occur in centrifugal or axial flow compressors, characterized by aerodynamic blockage or the breakaway of the flow from certain sections of the passage between the blades.

turboexpander (expansion turbine)

in cold air or gas refrigeration cycles, a turbine in which the compressed gas expands and produces mechanical energy.


flow-enhancing device to increase coil heat transfer efficiency.

turbulence intensity

the ratio of the standard deviation of the air velocity (SDv) to the mean air velocity (v). Turbulence intensity may also be expressed in percent (i.e., Tu = [SDv/va] · 100).

turbulent flow

fluid flow in which the velocity varies in magnitude and direction in an irregular manner throughout the mass. Turbulent flow exists when the Reynolds Number exceeds a value of 2000 to 4000.

turning vane

a series of single thickness or airfoil radius sheet metal guides placed within a rectangular duct elbow to reduce turbulence and associated pressure drop within the elbow and to direct air around the bend.

two piece split system, indoor/indoor

any air conditioner or heat pump that has two major assemblies separated from each other with both assemblies being indoor. The compressor and one refrigerant heat exchanger would be in one assembly, with the other refrigerant heat exchanger in a second assembly.

two piece split system, indoor/outdoor

any air conditioner or heat pump that has two major assemblies separated from each other between indoor and outdoor. The compressor and one refrigerant heat exchanger would be in one assembly, with the other refrigerant heat exchanger in a second assembly.

two-phase flow

simultaneous flow of two phases of a fluid, usually gas liquid flows.

two-pipe system

piping system in which the fluid withdrawn from the supply passes through a heating or cooling unit to a separate return main.

two-stage control

a modulating control that both cycles a controlling device between two preset conditions, could be between OPEN and CLOSED, or between ON and OFF, or between two stages or levels of capacity control.

two-stage thermostat

(1) single temperature controller designed to control temperature at two distinct setpoints. (2) thermostat that handles two separate circuits in sequence.

two-way valve

valve having a single inlet and single outlet. Uses of two way valves could be for throttling, isolation, or shutoff.

Type I hood

a hood designed to capture smoke and/or grease-laden vapor produced by a cooking process, incorporating listed grease removal devices and fire suppression equipment. Type I hoods fall into two categories: listed and nonlisted. Listed hoods have been tested in accordance with UL Standard 710.1. Conventional, or nonlisted hoods are hoods that meet the design, construction, and performance criteria of the applicable national and local codes.

Type II hood

a hood designed to capture heat, odors, products of combustion, and/or moisture where smoke or grease laden vapor is not present. A Type II hood may or may not have filters or baffles and does not have a fire suppression system.