ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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habitable space

building space intended for continual human occupancy, such space generally includes areas used for living, sleeping, dining, and cooking but does not generally include bathrooms, toilets, hallways, storage areas, closets, or utility rooms.

halide torch

device to detect gas leaks using the color changes of a flame in the presence of a halogenated hydrocarbon.


a hydrocarbon derivative containing one or more of the halogens bromine, chlorine, or fluorine, hydrogen also may be present.


one of the electronegative elements of Group VIIA of the periodic table of the elements (the group also includes fluorine, chlorine, bromine, iodine, and astatine); listed in the order of their chemical activity, fluorine being the most active of all chemical elements.

halogenated chlorofluorocarbon

fully halogenated chlorofluorocarbon is one in which all of the hydrogen atoms are replaced by chlorine and fluorine atoms. Atmospheric lifetimes of fully halogenated chlorofluorocarbons are long (75 years for CFC 11 and 111 years for CFC 12).

hand valve

valve that is operated manually by a handwheel attached to the spindle.

hand-washing station

an area that provides a sink with hot- and cold-water supply and a faucet that facilitates easy on/off/mixing capabilities. The station also provides cleansing agents and means for drying hands.


a sinusoid whose frequency is an integral multiple of the fundamental frequency.

harmonic order

integer that defines the numerical value of the multiple of the fundamental frequency.

Hartford loop

condensate return arrangement for low-pressure steam heating systems, featuring a constant waterline in the boiler.


(1) energy per unit mass of fluid divided by gravitational acceleration. (2) in fluid statics and dynamics, a vertical linear measure. Note: the terms head and pressure are often mistakenly used interchangeably. See also suction head. (3) operating pressure measured in the discharge line at a pump, fan, or compressor outlet (i.e., at the head).

head pressure control valve

automatic valve, located between the air-cooled condenser and the receiver, that will back up liquid in the condenser to reduce the effective area of the condenser, thus keeping the discharge pressure to a predetermined minimum value during low ambient temperatures. Used in conjunction with a pressure differential valve between the discharge line and the receiver.


a pipe or tube (extruded, cast, or fabricated) to which other pipes or tubes are connected.


(1) energy that is transferred in the direction of lower temperature. (2) form of energy that is exchanged between a system and its environment or between parts of the system induced by temperature difference existing between them.

heat anticipation

ability of a thermostat or control system to terminate the heat or cooling input at a temperature other than its setting and in advance of the time that the temperature at the thermostat or control system sensor normally would cause a control change.

heat balance

statement that shows the changes in a system from heat and work input to output losses.

heat bridge

(also known as heat channel or heat leak), part of the boundary construction of an insulated enclosure through which heat can flow readily by conduction.

heat capacity

(1) the amount of heat necessary to raise the temperature of a given mass one degree, numerically, the mass multiplied by the specific heat. (2) the capacity of a body to store heat.

heat equivalent of work

heat energy corresponding to a unit of mechanical energy.

heat exchange

(1) heat exchange surface area of any heat exchanger available for transmitting heat. (2) process of heat transfer in which heat flows from one substance to another. (3) (also known as heat interchanger), device to transfer heat between two physically separated fluids. See also superheater.

heat flow

passage of heat from one point to another or one space to another by one or more of the three modes: conduction, convection, and radiation. See also thermal transmittance.

heat flux

the flow of energy per unit of area per unit of time. It is sometimes called thermal flux and also referred to as heat flux density or heat flow rate intensity. It has both a direction and a magnitude, so it is a vectorial quantity.

heat gain (heat uptake)

quantity of heat absorbed by an enclosed space or system.

heat index

an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature (how hot it feels, also termed the felt air temperature). When the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Compare wind chill.

heat lag (thermal lag)

time elapsing between the initiation of a thermal phenomenon and the appearance of its effect.

heat loss

(1) (also known as infiltration losses) energy required to warm outdoor air leaking in through cracks and crevices around doors and windows, through open doors and windows, and through porous building materials. (2) (also known as transmission losses) heat transferred through confining walls, glass, ceilings, floors, or other surfaces. (3) See also heat gain (heat uptake).

heat loss coefficient

rate at which heat is lost from the storage device per degree temperature difference between the average temperature of the storage medium and the average temperature of the surrounding air or fluid.

heat loss rate

the rate at which heat is lost from the storage device per degree temperature difference between the average storage medium temperature and the ambient temperature (or ground temperature, if the storage device is buried).

heat meter
heat of combustion (HOC)

the heat released when a substance is combusted, determined as the difference in the enthalpy between the reactants (refrigerant[s] and air) and their products after combustion as defined in Section The heat or enthalpy of combustion is often expressed as energy per mass (e.g., kJ/kg or Btu/lb).

heat of fusion (fusion heat)

(1) heat energy required to cause a change of state from solid to liquid at constant temperature. For ice to water, 143.5 Btu/lb (333.8 kJ/kg). (2) latent heat involved in changing between the solid and the liquid states.

heat of reaction

heat per unit mass (or per mole) of reagents and substances in a chemical reaction, exothermal if heat is given off, endothermal if absorbed.

heat of subcooling

quantity of heat removed from a liquid to reduce it from its saturation temperature at saturation pressure to some lower temperature at the same pressure.

heat operated

describes any equipment whose energy source is thermal energy. Heat may be provided by combustion, steam, or other means. The heat may manifest itself as thermal energy (elevated temperatures) or other available energy (such as elevated pressure) and may activate a heat cycle (e.g., absorption process) or a work cycle (e.g., vapor compression cycle) by means of heat transfer surfaces and/or heat engines (prime movers).

heat pump

thermodynamic heating/refrigerating system to transfer heat. The condenser and evaporator may change roles to transfer heat in either direction. By receiving the flow of air or other fluid, a heat pump is used to cool or heat. Heat pumps may be the air source with heat transfer between the indoor air stream to outdoor air or water source with heat transfer between the indoor air stream and a hydronic source (ground loop, evaporative cooler, cooling tower, or domestic water).

heat pump balance point

temperature at which the heat pump capacity and the building heat requirement are equal. Heat pump heating effect. See compressor heating effect.

heat pump balance point temperature

temperature at which the installed heat pump capacity is equal to the heat requirement of the building. For a geoexchange system, the temperature at which supplemental heating or cooling is required. For an air-to-air system, the temperature at which supplemental heating is required. For a water loop system, the temperature at which heating and cooling requirements are equal.

heat pump compressor heating effect

(1) rate of heat delivery by the refrigerant assigned to the compressor in a heat pump. (2) rate of heat delivery by the refrigerant assigned to the compressor in a heat pump system equal to the product of the mass rate of refrigerant flow produced by the compressor and the difference in specific enthalpies of the refrigerant vapor at thermodynamic state leaving the compressor and saturated liquid refrigerant at the pressure of the vapor leaving the compressor.

heat rate

see heat flux.

heat recovery

use of heat that would otherwise be wasted from a system or process (e.g., heat-recovery chiller, a machine using hot waste gases as a heat source).

heat rejection

mass flow rate times the difference between the entering and leaving enthalpies.

heat reservoir

system that can absorb or reject heat from storage.

heat sink (cold source)

substance or environment into which unwanted heat is directed.

heat source

substance or environment from which heat is taken.

heat storage

technology or systems used to store heating capacity.

heat trace

a heating system where the externally applied heat source follows (traces) the object to be heated (e.g., water piping).

heat transfer

(also called heat transmission, heat transport, and thermal transmission), transfer of energy, in Btu/h (W), induced by a temperature difference. May occur by conduction, convection, radiation, mass transfer, or any combination of these.

heat transfer fluid

fluid used in a heat transfer process.

heat transfer radiation coefficient

an imaginary coefficient expressing the proportionality of radiant heat exchange between two bodies to their temperature difference.

heat transfer surface

outside area of a heat exchanger through which heat flows. See fin.

heat trap

a device or arrangement of the piping entering and leaving a water heater constructed to counteract the convective forces of the heated water (thermosiphoning) during standby periods.

heat treatment

heating and cooling a metal or alloy to obtain desired properties or conditions.

heat value

(also known as heat of combustion), amount of heat released in the oxidation of one mole of a substance at constant pressure or constant volume.

heated slab

concrete slab-on-grade floor containing wires, cables, pipes, or ducts that transfers heat to the conditioned space.


apparatus or appliance to supply heat to a space or a fluid. See also water heater.

heat-exchanger face area

in a heat exchanger with a bundle of tubes, the section that is effectively traversed by the external fluid just before it passes through the tube bundle. The area measured is the lesser area of the two heat-exchanger surfaces.

heat-exchanger tube plate

(also known as tube sheet), plate located at the end(s) of a multitubular shell-and-tube heat exchanger, into which the tubes are fixed.


process of adding heat energy causing a rise in temperature or a transfer of sensible heat into latent heat. See also district heating, heating system.

heating capacity

the rate of heat that the equipment adds to the conditioned space or heat transfer fluid in a defined interval of time, expressed in Btu/h (W).

heating coil

coil that uses a heat transfer fluid, condensing refrigerant, or direct electrical resistance elements to provide heating to heat fluids (air, gas, or liquids).

heating cycle

the period of operation including prepurge, primary heat-producing energy flow, and postpurge.

heating degree day

see degree day.

heating degree day (HDD)

[i.e., HDD65 (HDD18)]. For any one day, when the mean temperature is less than the local or country-specific common temperature base. Annual HDDs are the sum of the HDDs over a calendar year.

heating design temperature

the outdoor dry-bulb temperature equal to the temperature that is exceeded at least 99.6% of the number of hours during a typical weather year.

heating load

heating rate required to replace heat loss from the space being controlled.

heating load factor (HLF)

(1) ratio of the heating building load to the steady-state heating load. (2) ratio of the total heating of a complete cycle for a specified period, consisting of an on time and off time, to the steady-state heating done over the same period at constant ambient conditions.

heating seasonal performance factor (HSPF)

for the space-heating season, the ratio of the total space heating delivered to the total electrical energy input if the combined appliance operated exclusively in a space heating only mode. The quantity is expressed in units of Btu/Wh. For SI use, compare coefficient of performance.

heating stack loss

sensible heat carried away by the flue gas and the sensible and latent heat carried away by the water vapor in the flue gas.

heating system

one in which heat is transferred from a source of energy through a distribution network to spaces to be heated.

heating value

amount of heat produced by the complete combustion of a unit quantity of fuel. The gross or higher heating value is that obtained when all the products of combustion are cooled to the temperature existing before combustion, the water vapor formed during combustion is condensed, and all the necessary corrections have been made. The net or lower heating value is obtained by subtracting the latent heat of vaporization of the water vapor formed by the combustion of the hydrogen in the fuel from the gross or higher heating value.

hemispherical thermal emittance

average directional thermal emittance over a hemispherical envelope over the surface.

Henry’s law

mass of a slightly soluble gas that dissolves in a definite mass of liquid at a given temperature is nearly directly proportional to the partial pressure of that gas. This holds for gases that do not unite chemically with the solvent.

HEPA filter

(1) high-efficiency particulate air filter. (2) a ?lter with removal ef?ciencies of 99.97% or higher for a mass median particulate size of 0.30 µm (microns).

HEPA Filter (Merv16+), ULPA, Particle Air Filter

product that uses only airflow through a medium to remove contaminants from the air.

hermetic compressor

a motor compressor assembly contained within a gastight housing that is permanently sealed by welding or brazing, with no access for servicing internal parts in the field.

herringbone evaporator (v-coil)

evaporator in which the tubes, arranged in the vertical plane, are bent in the form of a V.


hydrofluorocarbon (HFC) a halocarbon that contains only fluorine, carbon, and hydrogen.

high discharge temperature cut-out

safety device that acts when an abnormal rise in the discharge temperature starts an alarm or stops the compressor or heat-producing device.

high event control

control action that occurs at the higher value of the activating variable being sensed.

high pressure

as applied to refrigeration systems, this term refers to gage pressure at room temperature (74°F [23.3°C]) that is typically more that 100 psig (689 kPa gage). Common high-pressure refrigerants include R-22, R-502, R-404A, R-407A, R-407C, R-410A, and R-507A.

high/low control

two level action in which both levels are controlled within defined limits.

higher heating value (HHV)

the amount of heat produced per unit of fuel when complete combustion takes place at constant pressure, the products of combustion are cooled to the initial temperature of the fuel and air, and the vapor formed during combustion is condensed. HHV is expressed in Btu/lb (J/kg) or Btu/ft3 (W/m3) for gaseous fuel and in Btu/lb (J/kg) or Btu/gal (J/l) for liquid fuel.

high-frequency thawing

dielectric thawing using suitable frequencies of a few megahertz or higher.

high-intensity discharge (HID) lamp

an electric discharge lamp in which light is produced when an electric arc is discharged through a vaporized metal such as mercury or sodium. Some HID lamps may also have a phosphor coating that contributes to the light produced or enhances the light color.

high-polluting events

isolated and occupant-controllable events that release pollutants in excess quantities. Typical cooking, bathing, and laundry activities are not considered high-polluting events.

high-pressure boiler

(1) boiler for generating steam at pressure in excess of 15 psig [103.4 kPa (gage)]. (2) hot-water boiler intended for operation at pressures exceeding 160 psig [1100 kPa (gage)] and/or temperatures exceeding 250°F (120°C).

high-pressure control

pressure-responsive device that cycles and/or stages condensers, cooling-tower fans, and pumps to control head pressure.

high-pressure float valve

(also known as high-side float valve), float-type expansion valve operated by changes in liquid level on the high-pressure side.

high-pressure refrigerant system

system whose gage pressure at room temperature (74°F [23.3°C]) is typically more than 100 psig (689 kPa). Common high-pressure refrigerants include R-22, R-502, and R-125.

high-pressure safety cutout

switch designed to stop the compressor or machinery when the discharge pressure reaches a predetermined high value.

high-pressure side

(also known as high side), that portion of a refrigerating system operating at approximately the condenser pressure.

high-stage condensers

found only in double-effect machines. This type of condenser is typically inside of the tubes of the second-stage generator.

high-temperature brazed joint

gastight joint obtained by joining metal parts with alloys that melt at temperatures higher than 1500°F (800°C) but at less than the melting temperatures of the joined parts. Compare to soldered joint.

high-temperature hot-water system

hydronic system intended for operation at pressures exceeding 160 psig [1100 kPa (gage)] and/or temperatures exceeding 250°F (120°C). Compare to low-temperature hot-water system.

high-vacuum thermal insulation

thermal resistance system that depends on a high vacuum for its performance. An example is a thermos or Dewar flask.

hipot test

electrical insulation test that consists of the application of a high voltage between a circuit and its frame.


describes a building or space that has been specifically designated as historically significant by the adopting authority or is listed in The National Register of Historic Places or has been determined to be eligible for such listing by the U.S. Secretary of the Interior.

hit-and-miss damper

two or more slotted slides in parallel arrangement and adjustable against each other.


deposit of ice crystals produced in a manner similar to dew but at a temperature below 32°F (0°C).

holding charge

(also known as service charge), reduced quantity of refrigerant or inert gas used to avoid the ingress of air and moisture into a component before start-up.

holdover coil

apparatus to store cold by fusion of ice or eutectic on a refrigerated coil.

home run wiring

wiring from the device all the way to the distribution panel. This wiring is usually run in the most direct paths but with standard wiring practices and the wiring does not contain any splices, branches, or devices.


a device designed to capture cooking and/or ware washing effluent. See Type I hood and Type II hood.

hood face

the plane of minimum area at the front portion of a laboratory fume hood through which air enters when the sash is fully opened, usually in the same plane as the sash when sash is present.

Hooke’s law

within the elastic limit of any body, the ratio of stress to strain produced is constant.

hoop stress

(also known as circumferential stress), stress imposed in the wall of a cylindrical tube in the circumferential direction by internal pressure.

horizontal axis of measurement

an axis of measurement that is parallel to the mounting base of a piece of equipment or the building foundation.

hot- and cold-type water cooler

a water cooler that, in addition to the primary function of cooling and dispensing potable water, includes means for heating and dispensing potable water.

hot deck

hot air chamber forming part of an air handler.

hot gas

refrigerant gas in the high-pressure side of the system.

hot/humid climate

climate in which the wet-bulb temperature is 67°F (19°C) or higher for 3500 h or more or 73°F (23°C) or higher for 1750 h or more during the warmest six consecutive months of a year that is typical for that geographic area.

hot-gas bypass regulator

automatic valve, operated by the suction pressure, which it maintains above a given value by diverting a certain quantity of high-side vapor to the low side of the system.

hot-gas defrost valve

solenoid valve located in a bypass line running from the outlet of the compressor to the evaporator.

hot-gas defrosting

(also known as internal defrosting), method that utilizes heat from inside the pipes of the evaporator, usually the highly superheated vaporized refrigerant from the compressor.

hot-gas line

a line used to convey discharge gas from the compressor to the evaporator for the purpose of defrosting.

hot-water heating system

(also known as hydronic heating system or wet heating system) heating system for a building in which the heat-conveying medium is hot water and the heat emitters are radiators, convectors, or panel coils.

hot-water storage tank

tank used to store water that is heated integral to or separate from the tank.

hot-water supply boiler

boiler completely filled with water and that furnishes hot water to be used externally to itself at pressures not exceeding 160 psig or at temperatures not exceeding 250°F (120°C) at or near the boiler outlet.

hourly free-floating zone air temperature

zone or space air temperature for a given hour when heating and cooling equipment is off or for an unconditioned space.

hourly heating load

heating load for a given hour.

hourly incident unshaded solar radiation

sum of direct solar radiation and diffuse solar radiation that strikes a given surface for a given hour.

humid heat

ratio of increase of enthalpy of moist air to the rise of temperature expressed per unit mass of the dry air component under conditions of constant pressure and humidity ratio.


device to add moisture to air or gases.


to add water vapor or moisture to any moisture-absorbing material, including the atmosphere.

humidifying effect

product of the mass of water evaporated times the latent heat at the evaporating temperature.


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

humidistatic controls

automatic controls used to maintain humidity at a fixed or adjustable set point.

humidity percentage

ratio in percent of the mass of moisture at a given temperature to the maximum possible at the same temperature. See humidity.

humidity ratio

ratio of the mass of water vapor to the mass of dry air in a sample of moist air. Also known as mixing ratio (or humidity value from Mollier’s diagram).

humidity saturation ratio

ratio of the specific humidity to that at saturation at the same temperature and pressure, usually expressed as a percentage.

hunting in a control system

condition that occurs when a controller, controlled device and system, individually or collectively, continuously overrides and undershoots or overshoots the control point with resulting fluctuation and loss of control of the condition to be maintained.

HVAC duct

duct and fittings used for conveying air in residential, commercial, institutional, and industrial heating, ventilating, and air-conditioning systems.

HVAC system

the equipment, distribution systems, and terminals that provide, either collectively or individually, the processes of heating, ventilating, or air conditioning to a building or portion of a building.

HVAC system end-to-end accuracy

combined end-to-end accuracy of the EMCS (energy monitoring and control system) and the accuracy with which the EMCS sensors represent the HVAC process.

HVAC zone

a space or group of spaces, within a building with heating, cooling, and ventilating requirements, that are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor).

hydraulic diameter

(1) for a fully filled duct or pipe whose cross section is a regular polygon, the hydraulic diameter is equivalent to the diameter of a circle inscribed within the wetted perimeter. For a fully filled duct or pipe whose cross section is round, the hydraulic diameter is equivalent to the diameter of the duct of pipe. (2) a commonly used approximation is to take four times the flow area divided by the perimeter of the solid boundary in contact with the fluid.

hydraulic lift

in a pumping system, the static height (head) to be overcome by the pump.

hydraulic shock

internal pressure stress imposed in piping systems by a sudden change in liquid velocity, as by the sudden stopping of flow.


a compound containing only the elements hydrogen and carbon.

hydrofluorocarbon (HFC)

a halocarbon that contains only fluorine, carbon, and hydrogen.

hydrogen peroxide, air cleaner gaseous

process creates hydrogen-peroxide from ambient water vapor and oxygen to create Reactive Oxygen Species (ROS). ROS may then reduce or deactivate bioaerosols like pathogens.


a chemical reaction during which molecules of water (H2O) are split into hydrogen cations (H+) (conventionally referred to as protons) and hydroxide anions (OH-) in the process of a chemical mechanism. The most common hydrolysis occurs when a salt of a weak acid or weak base (or both) is dissolved in water. Water autoionizes into negative hydroxyl ions and positive hydrogen ions. The salt breaks down into positive and negative ions.


an instrument used to measure the specific gravity (or relative density) of liquids, that is, the ratio of the density of the liquid to the density of water.

hydronic distribution system

a thermal distribution system that uses water or a mixture of water and additives as the distribution medium in a building.

hydronic system balancing

adjusting water flow rates through hydronic distribution system devices, such as pumps and coils, by manually adjusting the position valves or by using automatic control devices, such as automatic flow control valves.


science of heating and cooling water.


device for controlling the level of a liquid in a reservoir.

hydrostatic lockup potential

internal pressure stress in piping or vessels due to a temperature rise in liquids (such as refrigerants, water, or secondary coolants) when the containment volume is completely filled with liquid.

hydrostatic pressure

(1) normal force per unit area that would be exerted by a moving fluid on an infinitesimally small body immersed in it if the body were carried along with the fluid. (2) pressure exerted by a fluid at rest.

hydrotreated oil

a mineral-oil lubricant that has been treated with hydrogen to remove aromatic and olefinic components.


instrument responsive to relative humidity, usually relative humidity in the atmosphere. Compare to psychrometer.


branch of science that deals with the measurement of humidity


capable of absorbing and retaining/losing moisture.

hyperbolic tower

cooling tower of hyperbolic shape that depends on natural draft for air movement through the tower. The air movement can be either crossflow or counterflow.


in control systems, the difference between the response of a system to increasing and decreasing signals.