A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 165 terms in this directory beginning with the letter H.
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 or Group 17 of the periodic table of the elements (the group also includes fluorine, chlorine, bromine, iodine, and astatine.)

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.

Hospital & Healthsystem Association of Pennsylvania

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

harmonic content
The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.

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.

[[heating degree day]].

(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
see [[thermal energy 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.

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.

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]].

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.

Heat Exchange Institute

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 filter with removal efficiencies of 99.97% or higher for a mass median particulate size of 0.30 µm (microns).

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.

A region of electrical contact between two different materials.

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

Hydraulic Institute

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 voltage disconnect
The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.

high voltage disconnect hysteresis
The voltage difference between the high voltag disconnect set point and the voltage at which the full photovoltaic array current will be reapplied.

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

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.

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.

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.

[[heating load factor]]

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.

The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

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.

The region between an n-layer and a p-layer in a single material, photovoltaic cell.

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 deck
hot air chamber forming part of an air handler.

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

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-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
(1) line used to convey discharge gas from the compressor to the condenser. (2) 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.

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.

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.

Heat Transfer and Fluid Mechanics Institute

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.

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).

hybrid system
A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

Hydronics Institute

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.

hydrogenated amorphous silicon
Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.

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.