ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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daily range

difference between high and low temperatures for a typical day. Used in HVAC load calculations.

Dalton’s law

each constituent of a mixture of gases behaves thermodynamically as if it alone occupied the space. The sum of the individual pressures of the constituents equals the total pressure of the mixture.

damp building

a building that has a persistent and excessive accumulation of moisture which will, if allowed to persist, shorten the intended useful life of the building’s contents, materials, structural fasteners, or systems.


element inserted into an air-distribution system or element of an air-distribution system permitting modification of the air resistance of the system and consequently changing the airflow rate or shutting off the airflow.

damper actuator

device that provides the necessary force to position a damper.


(1) progressive diminution with time of certain quantities characterizing a phenomenon. (2) reducing the amplitude of vibrations by dissipating the corresponding mechanical energy to some suitable absorbing sink.

darcy unit

measure of permeability. At one darcy, a material will pass a fluid of one centipoise viscosity through a section of one square centimeter at a rate of one cubic centimeter per second, with a drop in pressure of one standard atmosphere.


(1) general term used to denote any or all facts, numbers, letters, and symbols that refer to or describe an object, idea, condition, situation, or other factors. (2) information obtained by experimental means, assumed to be in numerical form, recorded values of the variables, readings.

data bank

any electronic depository of data.

data base

ordered and named collection of data, particularly for use in computerized information systems.

data center

a room or building, or portions thereof, including computer rooms being served by the data center systems, serving a total ITE load greater than 10 kW and 20 W/ft2 (215 W/m2) of conditioned floor area.

data center ITE design power

the combined power in kilowatts of all the ITE loads for which the ITE system was designed. The data center ITE power shall not include any additional loads, such as cabinet fans or other devices, that are not inherent parts of the ITE, even if the loads are part of the UPS operational design load.

data center systems

HVAC systems, electrical systems, equipment, or portions thereof, used to condition ITE or electrical systems. Data center systems may also be shared, serving other data center additions or non-data-center loads.

data display module

device that stores computer output and translates this output into signals that are distributed to a program-determined group of lights, annunciators, and numerical indicators in operator consoles and remote stations.

data logging

(1) (also known as real time processing) processing of data in synchronism with a physical process so that the results of the data processing are useful to the physical operation. (2) device used for collection of characters or analog signals. (3) recording of data about events that occur in time sequence.

data reduction

process of transforming masses of raw test- or experimentally obtained data (usually gathered by automatic recording equipment) into useful, condensed, or simplified intelligence.

data table

collection of data with each item uniquely identified either by some label or by its relative position.

daylight area

the floor area substantially illuminated by daylight.

dead air pocket

(also known as dead zone) stagnant area in a space unaffected by air circulation.

dead band

the range of values within which a sensed variable can vary without initiating a change in the controlled process.

dead-end trap

piping arrangement for collecting oil or liquid refrigerant from suction gas prior to entry to a compressor.


(1) to isolate and remove a mistake or malfunction. (2) to operate equipment prior to use to detect and replace parts that are defective or expected to fail and to correct error in fabrication or assembly.


the process of removing or reducing greenhouse gases

decay rate

any physical attribute that decreases with time in a regular fashion. An example is the rate of decay of the concentration of a tracer gas, as used to measure the air infiltration rate of a building. See also sound decay rate.

decision maker

designated individual responsible for risk management of a given facility.

declination of sun

angle of the sun above or below the equatorial plane. The value is plus if north of the plane and minus if south.


process of chemical change, breaking up of structures, spoilage.

dedicated outdoor air

a ventilation system that delivers 100% outdoor air to each individual space in a building.

dedicated outdoor air system (DOAS)

1. A dedicated outdoor air system (DOAS) uses separate equipment to condition all of the outdoor air brought into a building for ventilation and delivers it to each occupied space, either directly or in conjunction with local or central HVAC units serving those same spaces. The local or central HVAC units are used to maintain space temperature. 2. Primary Air System

dedicated outdoor air system unit (DOASu)

Outdoor air unit that provides ventilation directly to a space or sensible heating/cooling units. If equipped with an exhaust fan it can control building pressure. Can have an air-to-air recovery device to save energy . Other options include custom air cleaning, enhanced dehumidification devices, compressors, condensors. 2. Make up air unit.

deep ground temperature

ground temperature at or below a soil depth of two meters.

deep vacuum

(also known as high vacuum), a vacuum of 1000 µm Hg (130 Pa) or less of absolute pressure.

definite composition law

a given chemical compound always contains the same elements in the same fixed proportions by weight. Also known as the definite proportions law.

deflecting vane anemometer

analog device containing a pivoted vane that rotates within its casing from the force of air moving through the casing so that the indicated tension in a coil spring, with damping, is related to the velocity of the air at that instant.

defrost control

act of controlling the refrigerating cycle or heating cycle to periodically melt the unacceptable accumulation of ice on evaporator tubes, windows, etc.


the process of removing unwanted ice or frost from a surface.

defrosting cycle

duration of the off cycle of a refrigerating system sufficient to permit defrosting of a cooling coil.

defrosting heat ratio

ratio between the energy transferred into the supply air and the maximum recoverable energy in exhaust air, excluding the energy input for defrosting.

defrosting system

equipment and controls designed to remove frost (ice) from cooling coils of a refrigerating system.

degradation coefficient

a measure of efficiency loss due to cycling of equipment.

degradation coefficient (CD)

factor of efficiency loss due to the cycling of the unit. It is the effect of reduction in performance under cyclic operation.

degree day

the difference in temperature between the outdoor mean temperature over a 24-hour period and a given base temperature. For the purposes of determining building envelope requirements, the classifications are defined as follows: (a) cooling degree day base 50°F, CDD50 (10°C, CDD10): for any one day, when the mean temperature is more than 50°F (10°C), there are as many degree days as degrees Fahrenheit or Celsius temperature difference between the mean temperature for the day and 50°F (10°C). Annual cooling degree days (CDDs) are the sum of the degree days over a calendar year. (b) heating degree day base 65°F, HDD65 (18°C, HDD18): for any one day, when the mean temperature is less than 65°F (18°C), there are as many degree days as degrees Fahrenheit or Celsius temperature difference between the mean temperature for the day and 65°F (18°C). Annual heating degree days (HDDs) are the sum of the degree days over a calendar year.

degree day (Kelvin-day)

the difference in temperature between the outdoor mean temperature over a 24-hour period and a given base temperature, used in estimating heating and cooling energy use. For any one day, there are as many degree days (Kelvin-days) as there are degrees Fahrenheit (degrees Celsius) departure of the mean temperature for the day from the base temperature


imposed transformation from a green color to a yellow or orange color (especially with citrus fruits) due to destruction of chlorophyll.

degrees of freedom (DOF)

the set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation of the body or system. This is a fundamental concept relating to systems of moving bodies in engineering. A particle that moves in three-dimensional space has three translational displacement components as DOFs, while a rigid body would have at most six DOFs, including three rotations. Translation is the ability to move without rotating, while rotation is angular motion about some axis.


removal of water vapor from air.


an air cooler or an absorption or adsorption device used for lowering moisture content.

dehumidifying effect

mass of water condensed during cooling or the equivalent refrigerating capacity expressed in terms of the latent heat of the water condensed per unit of time.

dehydration (drying)

removal of water from any substance.


a device for removing moisture from refrigerant or other substances.


intentional melting of an ice layer.

delivery effectiveness

the ratio of the thermal energy transferred to or from the conditioned space to the thermal energy transferred at the equipment distribution system heat exchanger. Energy delivered to or from the conditioned space includes distribution system losses to the conditioned space.


(1) the time rate of energy flow. In the United States, demand usually refers to electric power and is measured in kW (equals kWh/h) but can also refer to natural gas, usually as therms or ccf per day. In many other countries, demand is commonly used with other energy sources, especially district heat. (2) the highest amount of power (average Btu/h over an interval) recorded for a building or facility in a selected time frame.

demand charge

part of an electric bill based on kilowatt demand and the demand interval, expressed in dollars per kilowatt. Note: demand charges offset construction and maintenance of a utility’s need for large generating capacity.

demand control

an application that utilizes input signals that measure the past and current demand and provide control of future use to maintain or reduce the target level.

demand defrost

automatic defrosting system in which the defrost cycle is initiated by a drop in performance of the refrigerating system.

demand factor

ratio of the maximum electric demand to the connected load, usually monthly or annually. See also electric power load factor.

demand forecast

Predictions regarding future electrical demand.

demand interval

The period of time over which an energy supplier calculates demand. Typical values range from five minutes to 60 minutes.

demand limit tripping

removing interruptible or deferrable load(s) at the user level when electric power or current flow to that user, or to a portion of the load, exceeds a specified level for more than a specified time, as determined by agreement. The automatic function can be enabled or disabled remotely by the serving utility via centralized communication.

demand limiter

a device that monitors user electric power demand and causes that demand to be limited in a manner not to exceed a selected or programmed maximum value.

demand load

actual load on a circuit at any time. Sum of all loads which are on. Equal to the connected load minus the loads that are off.

demand period

electric power demand interval.

demand resource

A load, aggregation of loads, behind-the-meter generator, electrical storage system, or thermal storage system capable of providing measurable and verifiable demand response.

demand response

A temporary change in electricity usage by a demand resource in response to market or reliability conditions.

demand response event

A period of time defined by the program administrator, including notifications, deadlines, and transitions, during which demand resources provide demand response. All notifications, deadlines, and transitions may not be applicable to all demand response products or services.

demand savings

the reduction in the demand from the pre-retrofit baseline to the post-retrofit demand once independent variables (such as weather or occupancy) have been adjusted for. This term is usually applied to billing demand, to calculate cost savings, or to peak.

demand target

Target in demand management, peak demand is a key factor in contract charges for electricity use.

demand-limited storage

thermal storage system controlled to limit the electric power demand.

demonstrated accuracy

the accuracy of an instrument testing against a primary or calibrated instrument.

dense-air system

cold-air system maintained under pressure greater than atmospheric in which air is compressed, heat of compression dissipated, and the air, chilled by expansion and performance of work, can create useful refrigeration.


mass per unit of volume.

depletable (nonrenewable) energy

energy that comes out of the ground in the form of liquids, gases, or solids and is considered depletable or nonrenewable because it is energy that cannot be replenished in a short period of time.


direct formation of the solid phase by cooling a vapor below the triple point.

derivative control

control action in which the output is based on the rate of change of the input.

derivative control mode

mode that contributes to the output of the controller an amount equal to the derivative of the error signal, multiplied by the derivative gain.


(1) a solid that will collect and hold water from a liquid or gas. It must be insoluble in the refrigerating medium to be in order to be used in refrigerant driers. (2) absorbent or adsorbent liquid or solid that removes water or water vapor from a material.


process for evaporating water or removing water from a material.

desiccation ratio

in freeze drying, ratio of the mass of the substance resulting from a freeze drying process to the mass of the original product. Dryness ratio in freeze drying is the ratio of the mass of dry matter to the mass of frozen substance. See freeze drying (lyophilization).

design air temperature

air temperature which an HVAC system or apparatus is designed to maintain (indoor design air temperature) or to operate against (outdoor design air temperature).

design airflow

required airflow when the system is operating under assumed maximum conditions of design, including diversity.

design capacity

output capacity of a system or piece of equipment at design conditions.

design conditions

specified environmental conditions, such as temperature and light intensity, required to be produced and maintained by a system and under which the system must operate.

design electrical loss component (design ELC)

the design electrical loss component for the data center or data center addition shall be the combined losses (or the losses calculated from efficiencies) of three segments of the electrical chain: incoming electrical service segment, UPS segment, and ITE distribution segment. The design ELC shall be calculated using the worst-case parts of each segment of the power chain in order to demonstrate a minimum level of electrically efficient design. The design ELC does not, and is not intended to, integrate all electrical losses in the facility.

design energy cost

the annual energy cost calculated for a proposed design.

design intent

an initial version of the operating manual developed in the conceptual design stage. It contains a description of the building functions for which the design is intended, lists the design parameters of systems to perform these functions, and provides a brief description of the operating routines that are to be followed to comply with the functional requirements of the building.

design intent document

a detailed, written document evolving throughout the commissioning process, clearly defining items and criteria of the design intent.

design load

peak instantaneous load that a system is expected to meet.

design pressure

the maximum allowable working pressure that a system, a part of a system, or an apparatus is designed to and/or can withstand.

design professional

individual responsible for the design and preparation of architectural or engineering contract documents. An architect or engineer licensed to practice in accordance with applicable state licensing laws.

design temperature

(1) the temperature at which a system or zone is meant to maintain. (2) the temperature, or range of temperatures, at which a piece of equipment is selected to perform.

design voltage

specific voltage for which a line or piece of equipment is designed. A reference level of voltage for identification and not necessarily the precise level at which it operates.

design working pressure

in the United States, the maximum working pressure for which an apparatus has been designed. See design pressure. In some countries, the design pressure is greater than the maximum working pressure. Compare to operating pressure.


liberation of a gas held in a substance by sorption.


A heat exchanger in which superheated gas exiting a refrigeration (or air-conditioning/heat pump) system releases heat whenever the system is operating, thus enabling energy recovery to be performed (e.g., for domestic or service water heating).

desuperheating coil

heat exchanger, preceding the condenser or incorporated in it, for removing all or part of the superheat.

desuperheating heat rejection

sensible heat rejection from gaseous refrigerant in a condenser. Occurs prior to the gas-to-liquid phase change.


a complete set of measurements for a particular point of operation of a fan.


(1) difference between the setpoint and the value of the controlled variable at any point in time. (2) the difference between a single result and the mean of many results.


piece of equipment or a mechanism designed to serve a special purpose or to perform a special function.


deposit of water droplets on cold surfaces. Formed by the condensation of water vapor.

dew point

temperature at which water vapor has reached the saturation point (100% relative humidity). Temperature of the air at which it must be cooled at constant barometric pressure for water vapor to condense.

dew-point depression

difference between dry-bulb temperature and dew-point temperature. See wet-bulb temperature.

dew-point hygrometer

device that measures the temperature at which water droplets appear on a cooled, polished surface.

dew-point rise

increase in moisture content (specific humidity) of air expressed in terms of a rise in dew-point temperature.

dew-point temperature

temperature of moist air saturated at pressure p, with the same humidity ratio W as that of the given sample of moist air. It is defined as the solution td(p, W) of the equation: Ws(p, td) = W

Dewar flask

silvered vacuum flask with double walls with the space between the walls highly evacuated.

dial thermometer

a device that indicates temperature by a pointer moving over a circular scale.


(1) bellows whose elastic deflection can be increased by the use of corrugations. (2) flexible membrane separating two cavities. (3) in pneumatics or hydraulics, the membrane separating the fluid pressure system from the mechanical side.

diaphragm compressor

type of oil-free compressor in which flexion of a diaphragm creates compression.

diaphragm valve

membrane valve that closes with the admission of fluid pressure to a diaphragm and opens when pressure is reduced. See disc valve

diaphragm valve (membrane valve)

packless valve in which the seal between the adjustment spindle and the valve body is a diaphragm.


pertaining to the nature of a substance or a space that allows the passage of heat (more particularly, radiant heat).

dielectric constant

for an isotropic medium, the ratio of the capacitance of a capacitor filled with a given dielectric to that of the same capacitor having only a vacuum as a dielectric.

dielectric strength

maximum electric field that an insulator can withstand without breakdown.

dielectric thawing

process using dielectric heating with high voltage and high frequency or ultrahigh frequency electric fields.

diesel cycle

in a two- or four-stroke reciprocating engine, air without fuel is compressed, usually to more than 500 psig (3500 kPa), to raise its temperature above the ignition temperature of the fuel that is injected into the hot compressed air at or near the start of the power stroke. In modified diesel engines, a glow plug is used to aid ignition when the engine is started.

diesel engine

reciprocating engine that changes thermal energy to mechanical energy using the heat of compression to ignite the injected fuel.


an ester lubricant prepared from a dicarboxylic acid and monohydric alcohols.


(1) of a control, the difference between cut-in and cut-out temperatures or pressures. (2) range which the controlled variable must pass in order to actuate various control functions.

differential controller

(1) controller reactive to the difference between values of two variables. (2) device used to maintain a given difference in pressure or temperature between two elements or two points.

differential pressure

the difference in pressure between any two points in the system.

differential pressure control
differential temperature

(1) difference in temperature that exists between any two points or states when measured on the same temperature scale. (2) in control terminology, the difference in temperature between the high event and the low event.


indicates that flux propagates in many directions as opposed to a single direction, as in a direct beam, which refers to collimated flux . When referring to reflectance, it is the directional hemispherical reflectance less the specular reflectance. Note: diffuse has been used in the past to refer to hemispherical collection of transmitted or reflected radiation (including the specular component). This use is deprecated in favor of the more precise term hemispherical.

diffuse reflectance

directional hemispherical reflectance less the specular reflectance.

diffuse sky irradiance

solar radiation that has been scattered in passing through the earth’s atmosphere. It is equal to the global irradiance less the direct-normal and ground-reflected components.

diffuse solar radiation

the solar radiation received from the sun after its direction has been changed by scattering by the atmosphere or other objects, such as the ground.


(1) circular, square, rectangular, or linear air-distribution outlet, generally located in the ceiling, and composed of deflecting members discharging supply air in various directions and planes and arranged to promote mixing of primary air with secondary. (2) duct of increasing area following the outlet of a rotary blower so that the reduced velocity of the fluid will convert some of the kinetic energy into pressure energy.

diffuser radius of diffusion

see throw (T).

diffuser terminal

diffuser with an integral air terminal.


(1) displacement of the molecules of a fluid within another fluid. (2) distribution of air within a space by an outlet discharging supply air in various directions and planes.

diffusion absorption system

refrigerating absorption system that, in addition to refrigerant and absorbent, also has an inert medium (such as hydrogen) to balance pressure in the various parts of the refrigerating circuit.

diffusion area

effective area covered by a jet of air on leaving an outlet air device.

diffusion coefficient
digital to analog converter

(1) device that transforms digital data into analog data. (2) in data processing, a device that converts an input number sequence into a continuous variable. (3) in power system communications, a circuit or device whose input is information in digital form and whose output is the same information in an analog form.


neutral fluid added to another fluid to reduce the concentration of the second fluid in a mixture.

dilution flue

flue designed to effect the dilution of flue gases with air before discharge from an appliance.

dimensionless number

ratio of various physical properties (such as density or heat capacity) and conditions (such as flow rate or mass) of such nature that the resulting number has no defining units of mass, rate, etc. Also called a nondimensional parameter.

dip tank

liquid-holding process vessel in which components to be treated can be immersed.

direct acting

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

direct current (DC)

electric current in an electrical circuit that does not reverse polarity. Note: direct current is said to flow from positive to negative, but electrons travel from negative to positive.

direct digital control (DDC)

a type of control where controlled and monitored analog or binary data (e.g., temperature, contact closures) are converted to digital format for manipulation and calculations by a digital computer or micro processor, then converted back to analog or binary form to control physical devices.

direct drive

driver and driven equipment with positive connections for rotation at the same speed.

direct emissions

greenhouse gas emissions from sources owned or controlled by the reporting entity

direct exhaust system

mechanical venting system supplied or recommended by the manufacturer through which the products of combustion pass directly from the furnace or boiler to the outside and which does not employ a means of draft relief. Includes units that have small air passages in the flue that have an opening area that is not in excess of 10% of the cross-sectional area of the stack.

direct expansion (DX)
direct ice contact

ice storage system using a method of heat exchange in which ice is formed by direct refrigeration and melted by immersion in circulating water or secondary coolant. Also called static direct contact storage.

direct load control

A demand response activity by which the program administrator remotely shuts down or cycles a customer's electrical equipment (e.g., air conditioner, water heater). Direct load control programs are primarily offered to residential or small commercial retail customers.

direct normal irradiance

(beam) irradiance received from the sun without significant change of direction from the apparent position of the sun. See also pyrheliometer.

direct sound

sound that reaches a given location in a direct line of sight from the source, without any reflections.

direct vent system

system that provides outdoor air directly to a unit for combustion and for draft relief and which provides for discharge of all flue gases to the outside atmosphere.

direct-expansion (DX) refrigeration systems

(1) system in which the cooling effect is obtained directly from the expansion of the liquid refrigerant into a vapor. (2) common term applied to an air-conditioning or refrigeration system that utilizes the vapor-compression refrigeration cycle. In a vapor-compression refrigeration cycle, the refrigerant removes heat in the evaporator by directly expanding the entering liquid refrigerant into vapor as it leaves the evaporator. The vapor is then compressed and piped to a condenser where the heat removed by the evaporator and the heat of compression are rejected to another medium so that the gaseous refrigerant is condensed to a liquid. The liquid is then piped to a pressure reducing device/metering device to be supplied to the evaporator.

direct-fired heating

system where the combustion heat and the products of combustion are transferred directly to the heated medium.

direct-injection humidifier

device that injects steam directly into the airflow.

direct-load management

subfunction of power system distribution automation that controls interruptible user loads or otherwise selected user appliances from one or more centralized, utility-operated locations in chosen groups, load classes, and/or timed patterns. Characterized by remote control by the serving utility of user interruptible loads. Also called active load management. Compare with load management.

direct-return piping system

two-pipe system in which the heat transfer medium supplied to the first load is the first returned to the heat transfer equipment. Compare to reverse return piping system.

directional thermal emittance

ratio of the radiance from a surface in a particular direction to the radiance in that direction from a blackbody at the same temperature under the same conditions.

disc valve

compressor valve consisting of a metal disc that is lifted from an orifice in order to control fluid flow. See diaphragm valve.

discharge capacity

the amount of thermal energy that can be removed from the storage device during a period of time and for a specific set of values for the initial temperature of the storage device, the temperature of the entering fluid, and the mass flow rate of fluid through the storage system. Compare to storage capacity.

discharge coefficient

in the flow of fluids through nozzles and orifices, the product of the velocity coefficient and the contraction coefficient. See also air discharge coefficient.

discharge line

(also known as hot gas line), line through which refrigerant vapor flows from a compressor to a condenser.

discharge line valve

(also known as discharge stop valve), shutoff valve fitted to a compressor (or in the discharge line adjacent to it) which can be used to isolate the compressor from the discharge line.

discharge or intake velocity (Vk)

discharge or intake velocity (Vk) of an outlet or inlet, in fpm (m/s), is the velocity of the airstream measured at specified locations relative to the face of an outlet or inlet.

discharge pressure

(1) the pressure generated on the leaving side of a fan or pump. (2) the pressure generated on the output side of a gas compressor in a refrigeration or air-conditioning system. The discharge pressure is affected by several factors: size and speed of the condenser fan, condition and cleanliness of the condenser coil, and the size of the discharge line. An extremely high discharge pressure coupled with an extremely low suction pressure is an indicator of a refrigerant restriction. It is also called high-side pressure or head pressure. (3) operating pressure in a system measured in the discharge line at the compressor or fan outlet.

discharge sound power level

sound power that is transmitted from a device emanating sound and proceeding out of a device such as a terminal outlet.

discharge temperature

(also known as delivery temperature), temperature of fluid as discharged, such as from a fan or compressor.

discharge test time

the duration of a single transient test in which energy is removed from the storage device.

discharge valve

(also known as delivery valve or outlet valve), in a compressor, the valve that allows compressed fluid to flow from the cylinder and prevents return flow.

discharge-loss coefficient

actual discharge airflow rate divided by the theoretical discharge airflow rate at a given pressure difference.


withdrawing cooling or heating potential from thermal storage.


a device, group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.

dispatchable generator

Generators whose power supply may be increased or decreased for the purpose of decreasing or increasing the total electrical demand on the grid.

displacement flow

movement of air within a space in a piston- or plug-type motion. Minimal mixing of the room air occurs in ideal displacement flow, which is desirable for removing pollutants generated within a space. The laminar flow air-distribution system that sweeps air across a space may produce displacement flow.

displacement ventilation system

a type of air-distribution system, used only for cooling purposes, in which air at a temperature below room temperature is supplied to the floor level at a low discharge velocity [<100 fpm (0.5m/s)] and is returned near ceiling level. Thermal plumes, which develop over heat sources in the room, drive the overall floor to ceiling air motion, producing a stratified environment with cooler and fresher air near the floor and warmer and less fresh air near the ceiling.

disposable filters

filters that are designed to operate through a specified performance range and then be discarded and replaced. An example is the cartridge filter.

distributed energy costs

those fees, charges, or assessments for energy use that are apportioned to individual units on any prorata basis, rather than on the basis of measurements.

distributed energy resource

A small, modular, energy generation or storage device located within the electrical distribution system at or near the end user. Distributed energy resources may be connected to the local electrical power grid (e.g., for voltage support) or isolated from the grid in standalone applications, such as part of a micro grid.

distributed isolation material (DIM)

pads cut from sheets of material (usually cross ribbed, oil-resistant neoprene) placed between the deck and mounting surfaces of a heavy machine to isolate vibration.

distribution law

if a substance is dissolved in two immiscible liquids, the ratio of its concentration in each is constant.

distribution system

(1) conveying means, such as ducts, pipes, and wires, to bring substances or energy from a source to the point of use. The distribution system includes auxiliary equipment such as fans, pumps, and transformers. (2) parts of a cooling tower, beginning with the inlet connection, that distribute the hot, circulating water within the tower to the points where it contacts the air.

distribution system efficiency

ratio between the energy consumption by the equipment if the distribution system had no losses (gains for cooling) to the outdoors or effect on the equipment or building loads and the energy consumed by the same equipment connected to the distribution system under test.


device for dividing flow of fluids into parallel paths.

district cooling

concept of providing and distributing, from a central plant, cooling energy to a surrounding area (district) of tenants or clients (residences, commercial businesses, or institutional sites). Compare to district heating.

district energy system

centralized facility for generation and distribution of the heating and cooling and/or power needs of a community, rather than individual heat or cold generators (i.e., furnace or air conditioner) at each residential, commercial, or institutional site.

district heating

concept of providing and distributing, from a central plant, heating energy to a surrounding area (district) of tenants or clients (residences, commercial businesses, or institutional sites). Compare to district cooling.

district-cooling system cooling density

measure of cooling demand per unit area. Customary units are kW/hectare or tons/acre.

district-heating system heating density

measure of heating demand per unit area. Customary units are kW/hectare or therms/acre.

diversion pipe fitting

Venturi jet or orifice in a tee that induces flow through branch lines by reducing the pressure in the main line following the branch.

diversity factor

ratio, or percentage, obtained when the total output capacity of a system is divided by the total output capacity of all the terminal devices connected to the systems. Example: to express the ratio of VAV supply air fan capacity to the total capacity of the VAV terminal devices as a percentage.

diverting element

element to divert the flow of air from one duct to another.

diverting valve

three-way valve piped to supply a single source of fluid to either of two outlets. Compare to mixing valve.

domestic hot water

(1) heating water for domestic or commercial purposes other than space heating and process requirements. (2) potable hot water as distinguished from hot water used for heating.

domestic water coil

coil of tubing within a boiler to heat water for potable use, usually a hot-water tank is used for hot-water storage.

dominant peak

the point in the response spectrum (normally plotted as velocity versus frequency) that has the highest value.

door area

?total area of the door measured using the rough opening and including the door slab and the frame.

double break

breaker arrangement whereby dual circuit breakers are used to reduce likelihood of extended power outages of any circuit due to circuit breaker trouble, permits breaker maintenance without a feeder outage.

double island canopy hood

a double island canopy hood is placed over back-to-back appliances or lines of appliances. It is open on all sides and overhangs the front and the sides of the appliance(s). It may have a wall panel between the backs of the appliances. Mounting height varies.

double-acting compressor

compressor which has two compression strokes per revolution of crankshaft per cylinder (i.e., both faces of the piston are working faces).

double-bundle condenser

condenser that contains two separate tube bundles, allowing the option of either rejecting the heat to the cooling tower or to another building system requiring heat input.

double-check valve backflow preventer

a backflow prevention device designed to protect water supplies from contamination. It consists of two check valves assembled in series, usually with a ball valve or gate valve installed at each end for isolation and testing. Often, test cocks (very small ball valves) are in place to attach test equipment for evaluating whether the double-check assembly is still functional. The double-check valve assembly is suitable for prevention of back pressure and back siphonage but is not suitable for high-hazard applications. It is commonly used on lawn irrigation, fire sprinkler, and combi-boiler systems. May also be referred to as double-check assembly (DCA).

double-contact freezer

contact freezer in which the product to be frozen is placed between two refrigerated surfaces and makes contact with both.

double-detector check (DDC) backflow preventer

device designed to serve also as a detector check on fire-protection systems where pollutants are involved. Note: DDC includes a line size, approved double-check valve backflow preventer with a metered bypass, into which has been incorporated a three-quarter inch, approved, double-check valve backflow preventer.

double-pipe condenser (tube-in-tube condenser)

condenser constructed of concentric tubes in which the refrigerant circulates through the annular space and the cooling medium through the inner tube.

double-pipe heat exchanger

two pipes arranged concentrically, one within the other, and in which one fluid flows through the inner pipe and the other through the annulus between them.

double-pole switch

(1) contact arrangement of two separate contacts (i.e., two single-pole contact assemblies). (2) two isolated contacts operated in unison.

double-riser suction line

arrangement of two vertical suction lines in order to ensure oil entrainment or carryover at minimum load. Often the minimum load line is smaller than the full-load suction riser.

double-seated valve

valve having two seats and two discs arranged so that the upstream pressure is acting on one side of one disc and the opposite side on the other disc. This acts to cancel system forces and allows the use of a smaller actuator.

double-suction compressor

split suction valving arrangement on compressor for carrying two suction pressures.

double-suction riser
double-throw switch

three-contact switch that can change the circuit connections from one to the other of its two operating positions.

double-wall heat exchanger

two fluids separated by two walls with the space between open to the atmosphere so that a fracture on one wall will not transfer one fluid into the other. Used in water systems with a potable fluid and a potentially hazardous fluid.

down-discharge makeup air

makeup air delivered directly to the interior plenum of an exhaust hood such that it is introduced vertically downward, typically from the front edge of the hood. Sometimes this kind of makeup air is referred to as an air curtain.

down-feed system

piping arrangement for heating, air-conditioning, or refrigerating systems in which heating and cooling fluid is circulated through supply mains which are above the levels of the heating or cooling units they serve.

down-flow-type central furnace

one designed with airflow essentially in a vertical path, discharging air at or near the bottom of the furnace.


(1) current of air, when referring to pressure difference that causes a current of air or gases to flow through a flue, chimney, heater, or space. (2) current of air, when referring to localized effect (generally, the unwanted local cooling of the body caused by air movement) caused by one or more factors of high air velocity, low ambient temperature, or direction of airflow whereby more heat is withdrawn from a person’s skin than is normally dissipated.

draft hood

a device that is not an integral part of the furnace or boiler and is connected to the furnace or boiler by a short length of flue pipe. It is designed to (a) provide for the exhaust of the products of combustion in the event of no draft, back draft, or stoppage beyond the draft hood, (b) prevent a back draft from entering the furnace, and (c) neutralize the effect of stack action of the chimney or gas vent upon the operation of the furnace.

draft rate

percentage of people predicted to be dissatisfied due to draft.

draft regulator

device installed in the breeching between a fuel-fired appliance and the chimney to control chimney draft. Draft regulators can be barometric or power operated. Barometric draft regulators operate by gravity and are commonly provided with an adjustable counter weight to set the minimum draft requirements. Power-operated draft regulators include controls that are capable of maintaining a constant pressure in a furnace under all normal operating conditions. In addition, it includes a low draft cutoff that shuts off the burner when the draft falls below the preselected minimum value. See regulator.

draft relief

provision for drafts to escape the venting system of a boiler or furnace.

draft-hood relief opening

opening provided in draft hood to permit the ready escape to the atmosphere of the fuel gases in the event of no draft or stoppage beyond the draft hood and to permit inspiration of air into the draft hood to neutralize strong chimney or vent updraft.

draft-rating index

an index that establishes a quantitative prediction of the percentage of occupants dissatisfied due to draft.

drag coefficient

coefficient expressing the resistance encountered by a body when it is moving in a fluid.

drain back

refers to systems in which the fluid in the solar collectors is allowed to drain back to storage whenever solar energy is not being collected (i.e., the fluid-circulating pump is not operating).

drain cock
drain down

refers to systems in which the fluid in the solar collectors is drained from the system under prescribed circumstances.

drain pan

vessel or tray placed under an evaporator coil or cooling coil to receive condensed moisture, melted frost, or ice. Also called a drip tray or a defrost pan.

drain plug (drain cock)

removable plug or key-operated draw-off valve intended to permit the removal of fluids or condensates.

drain valve

valve fitted to the lowest point of an apparatus or refrigeration system for the purpose of tapping or draining the system.


see draft.


difference between the static water level and the active-pumping water level.


graphic and pictorial document showing the design, location, and dimensions of the elements of a project.

drier (dehydrator)

(1) desiccant or refrigeration device placed in the main air line of a pneumatic control system to reduce moisture. Compare to desiccant. See also dryer. (2) a device containing desiccant(s). It is used in the liquid line of a refrigerant system for the primary purpose of collecting and holding water that may have entered the system.

drier coil

additional length of pipe or coil added to a direct-expansion evaporator in order to provide superheat at the thermostatic expansion valve sensing bulb without using evaporator tubing.


(1) change in mechanical or electrical characteristics with the passage of time, change in temperature, or both. (2) change in output/input relationship over a period of time with the change unrelated to input, environment, or load. (3) in a cooling tower, water lost as liquid droplets entrained in the exhaust air. It is independent of water lost by evaporation. (4) movement of current carriers in a semiconductor under the influence of an applied voltage.

drift eliminator

(also known as carryover eliminator), component that removes entrained moisture from the ducted air.

drinking-water cooler

unit in which drinking water is cooled by refrigeration and usually dispensed by a manual valve.


(1) leak in a liquid system. (2) liquid which appears on thawing frozen food, water melting from evaporator, or water droppings from a cooling surface. (3) pipe or a steam trap and a pipe considered as a unit that conducts condensation from the steam side of a piping system to the water or return side of the system.

drip tray (defrost pan)

vessel or tray placed under the cooling coil to receive the melt from frost or ice. Also called drain pan.


(1) deviation from the no-load control point that results from a change in the heating or cooling load. (2) linear term referring to the percentage decrease in output voltage for a square wave or rectangular wave as a function of time. Also can apply to voltage error caused by leakage in a sample and hold circuit. (3) (of an air jet in mixing air diffusion), vertical distance (hv) between the lowest horizontal plane tangent to a specified isovel and the center of the core of an air jet.

drop-in refrigerant

replacement refrigerant that has thermodynamic properties similar to one being replaced and does not require air conditioning or refrigerating equipment to be replaced but may require modifications.

droplet condensation

phenomenon encountered when the condensate does not wet the cold surface but settles in the form of separate droplets.

drum cooler

rotating refrigerated cylinder whose surface is in contact with the liquid to be cooled.

dry air

(1) air unmixed with or containing no water. (2) air without entrained water vapor.

dry compression

compression of initially dry saturated or superheated vapor.

dry expansion

process of heat removal by a refrigerant in an evaporator fed by a flow control device, responsive to temperature, pressure, or both at some point in the evaporator or to the difference between high- and low-side pressures and not to the liquid level in the evaporator. All entering refrigerant is evaporated before being recirculated. See direct-expansion (DX) refrigeration systems.

dry expansion evaporator

DX refrigerant evaporator with organized distribution from which the refrigerant exits at a vapor quality of one, usually with superheat. Compare to flooded evaporator.

dry ice

solid carbon dioxide, CO2. A proprietary term.

dry ice bunker

in a refrigerated vehicle, a compartment containing dry ice to keep the load cooled.

dry layer filter

filter having a dry filtering medium (as opposed to a viscous filter).

dry piston compressor

type of oil-free compressor in which no lubricating oil is used in the cylinder, the piston is usually equipped with low-friction labyrinth rings.

dry saturated steam

steam at the saturation temperature corresponding to the pressure and containing no liquid water in suspension. Compare to wet saturated steam.

dry saturated vapor

vapor at the saturation temperature corresponding to the existing pressure and without any liquid phase.

dry ton

sensible heat load expressed in tons of refrigeration.

dry-air cooler

cooler that removes sensible heat from dehydrated air.

dry-bulb temperature

temperature of air indicated by an ordinary thermometer shielded from solar and long wave radiation

dry-bulb temperature (DBT)

(1) temperature of air indicated by an ordinary thermometer shielded from solar and long wave radiation. (2) in general, any thermometer that indicates the temperature of air (or other fluids), distinguished from a wet-bulb thermometer.

dry-return heating system

steam heating in which a return pipe carries both condensate and air. The return pipe is always above the waterline in the boiler in a gravity system.

dry-type air cooler

forced-circulation air cooler wherein heat transfer is not implemented by a liquid spray while in operation.

dry-type equipment

mechanical, refrigerated equipment using metal or plastic as a direct heat transfer medium and for reserve cooling capacity.

dry-type evaporator

continuous tube evaporator in which refrigerant from a pressure-reducing device is fed into one end and the suction line connects to the outlet end.

dry-type transformer

a transformer in which the core and coils are in a gaseous or dry compound.


appliance that removes moisture. Compare to drier.

dual compression

compression of refrigerant entering a single cylinder from two sources at different suction pressures.

dual thermostat

(1) pneumatic thermostat designed to maintain one temperature during the day and a lower temperature during the night. (2) changes thermostat output from direct acting (DA) to reverse acting (RA) for seasonal changeover. (3) two temperature controls in one enclosure.

dual-duct air-conditioning system

system of a central plant that produces conditioned air at two temperatures and humidity levels to supply air through two independent duct systems to the points of usage where mixing may be carried out.

dual-duct terminal

terminal that mixes, for individual zonal needs, varying portions of two independent sources of primary air.

dual-effect compressor

compressor in which the cylinders have an additional suction inlet partway along the compression stroke which enables refrigerant to be drawn in at two different suction pressures.

dual-effect control

device responsive to temperatures of two zones or to two variable conditions.

dual-fuel burner

burner designed to burn either gas or oil (but not both simultaneously).

dual-heating system

heating system utilizing two fuel or energy sources, such as gas, oil, coal, or electric power, either as alternate sources or with one as a booster to the other.

dual-pressure control

(1) combined pressure-regulating device, one part connected to the low pressure side of the system and one part to the high-pressure side of the system, with a common switch mechanism. (2) two pressure controls in one enclosure. (3) use of a single device to accomplish some form of pressure control at two distinct setpoints, such as a dual-pressure switch.

dual-pressure regulator

upstream pressure regulator equipped with two controls, used in refrigerant suction lines to provide freeze-up protection or for safety pressure relief.

dual-pressure relief device

two pressure relief devices mounted on a three-way valve that allows one device to remain active while the other is isolated.

dual-temperature refrigerator

two compartment refrigerated cabinet with one used for chilling foods and the other for either freezing foods or storing frozen products.


a tube or conduit used to convey or encase, an air duct is a tube or conduit used to convey air (air passages in self-contained systems are not air ducts), a pipe duct is a tube or conduit used to encase pipe or tubing.

duct board

rigid board composed of insulation material with one or both sides faced with a finishing material. The outer facing is normally a vapor barrier and can also be used as an air barrier.

duct breakout noise

transmission or radiation of noise from some part of a duct system to an occupied space in the building. Also called flanking path noise or duct radiation.

duct connection component

means intended to facilitate the joining of two components of ductwork.

duct distribution

distribution of air into a room or a building by means of ductwork.

duct fitting

transition between sizes or shapes of duct.

duct flow area

inside dimensions of the duct or duct liner if used in duct.

duct liner

insulation, usually fiberglass, applied to the inside of a steel ducts. It is used for both thermal retention and sound attenuation reasons.

duct sealing

means taken either to ensure the airtight sealing of the air-distribution system or to minimize leakage therefrom.

duct sizing

calculation of dimensions of ducting for a given air-distribution system with consideration for fan power, noise, etc.

duct support

means used to suspend or support ductwork within a building structure.

duct system

series of ducts, elbows, and connectors to convey air or other gases from one location to another.

duct transformation
duct transition section

section of duct, breeching, or stack used to connect those elements with structures of different cross-sectional dimensions.

duct-support spacing

distance between or frequency of supports along the length of a duct route.

ductless hood

a listed, packaged system incorporating a hood, a fan, and air-treatment devices designed to remove most grease and particulate matter from the airstream before reintroducing the treated air into the occupied space.


system of ducts for distribution and extraction of air. See system.

Dulong-Petit law

the product of the specific heat per gram and the atomic weight of many solid elements at room temperature have almost the same value, about 6.3 calories per degree Celsius (264 J/K).

dump water

the water drainage from an ice maker to control the clarity of ice or to prevent scaling.

dump-trap liquid-return heating system

method of separation and automatic return of liquid refrigerant from an accumulator in the low side to the high side of the plant.

duplex circuit

(1) circuit that has two separate sources of supply. (2) pair of circuits where either one usable when failure occurs in the other.

duplex transmission

(1) simultaneous two-way independent transmissions in both directions. (2) (also known as double transmission) transmission of each data word twice and comparison bit by bit for accuracy.


(1) capability of a building, assembly, component, product, or construction to maintain serviceability for which it was designed over at least a specified time, based on assumed levels of use and maintenance. Compare to serviceability. (2) the average expected service life of a system or facility. Individual manufacturers quantify durability as design life, which is the average number of hours of operation before failure, extrapolated from accelerated life tests and from stressing critical components to economic destruction.


air suspension (aerosol) of solid particles, usually with particle size less than 100 micrometers. See air contaminant.

dust eliminator

device, usually applied at a dust-generating source, that traps dust particles suspended in the air or gases passing through it. Compare to filter.

dust extracting plant

system that traps dust particles in the gas or airstream passing through it.

dust increment

the amount of dust fed during a definite part of the loading procedure.

dust spot opacity

the percent decrease in the relative light transmission of a dust-spot sampling target resulting from dust buildup on the target.

dust spot opacity index

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

dust-holding capacity

for disposable and manually renewable devices air filters, it is the average arrestance multiplied by the amount of ASHRAE dust fed to the device measured to the nearest gram. See also MERV rating.

dust-holding capacity per cycle

for devices whose renewal mechanism is designed to restore the performance characteristics of the device to starting conditions, dust -holding capacity per cycle is determined by feeding dust until a steady-state condition is achieved. The dust-holding capacity per cycle is averaged over at least four cycles. The result is expressed in g/cycle to the nearest gram.

dust-holding capacity per unit area

for disposable and manually renewable devices, this is dust-holding capacity divided by net effective filtering area. For self-renewable devices, dust feed is continued until the steady-state condition is achieved in several cycles. Dust-holding capacity per unit area is then the average arrestance multiplied by the dust fed during the steady-state period divided by the area of media consumed during the same period. Dust-holding capacity per unit area is expressed in g/m2 (g/ft2).

duty cycling

process of turning off electric consuming equipment for predetermined periods of time during operating hours to reduce consumption and demand.

dynamic characteristics of a fan

the resonance frequencies and mode shapes of a fan.

dynamic glazing

any glazing system/glazing infill that has the fully reversible ability to change its performance properties, including U-factor, solar heat gain coefficient, or visible transmittance. This includes, but is not limited to, shading systems between the glazing layers and chromogenic glazing.

dynamic head loss

the reduction in the velocity head during flow, consisting of friction or energy losses per length of pipe and losses associated with bends, fittings, valves, etc. The most common equation used to calculate major head losses is the Darcy–Weisbach equation. Older, more empirical approaches are the Hazen-Williams equation and the Prony equation.

dynamic ice

ice formed on a cooling surface, then removed to be stored in an insulated container (tank). Compare to ice harvester.

dynamic pressure
dynamic suction head

positive static suction head minus friction head and minus velocity head.

dynamic suction lift

sum of suction lift and velocity head at the pump suction when the source is below the pump centerline.

dynamic viscosity

force per unit area required to produce unit-relative velocity between two parallel areas of fluid unit distance apart. Also called coefficient of viscosity.


device for measuring power output of a running engine or motor.