Word Definition

Absorbent A substance, usually solid, that is able to take up or absorb a gas or liquid.

Absorptance The ratio of absorbed to incident radiation on a surface. Usually denoted by a symbol α.

Absorption The process in which a substance in one state is incorporated into another substance of a different state (e.g. liquids being absorbed by a solid or gases being absorbed by a liquid).

Absorption chiller A machine that uses a heat source, such as steam, solar heat or waste heat, to generate chilled water. Unlike mechanical vapor compression chillers, absorption chillers use a thermal or chemical process, or both, to produce the refrigeration effect needed to provide the chilled water. No mechanical compression of the refrigerant takes place within the machine as occurring within more traditional vapor compression-type chillers.

AC Air conditioning

ACGIH American Conference of Governmental Industrial Hygienists (USA)

ACH See air changes per hour.

Active tracer gas release Controlled release of a tracer gas by a pressurized system or pump. (The term is used in ventilation rate measurements).

Adsorbent A substance that is able to hold molecules of gas or fluid without causing a chemical reaction.

Adsorption The capability of all solid substances to attract to their surfaces molecules of gases or solutions with which they are in contact. Solids that are used to adsorb gases or dissolved substances are called adsorbents; the adsorbed molecules are usually referred to collectively as the adsorbate. An example of an excellent adsorbent is the charcoal used in gas masks to remove poisons or impurities from a stream of air.

Adsorption, chemical The binding of gases to an adsorbent surface through chemical reaction after the physical adsorption.

Adsorption, physical A type of adsorption that resembles the condensation of gases to liquids and depends on the physical, or van der Waals, force of attraction between the solid adsorbent and the adsorbate molecules.

Aerosol A suspension of liquid or solid particles in air.

Age of air, local mean The mean time it takes for supply air to reach a certain indoor point.

Age of air, room mean Mean of all the local mean ages of air.

AHU See air handling unit.

Air change rate Ventilation air flow rate divided by room volume. It indicates the rate at which outdoor air replaces indoor air within a room during an hour, and is an important consideration for measuring indoor air quality. Also called air changes per hour.

Air change ratenominal The nominal air change rate is equal to the ventilation flow rate divided by the room volume.

Air changes per hour (ACH) Ventilation air flow divided by room volume. It indicates the rate at which outdoor air replaces indoor air within a room during an hour, and is an important consideration for measuring indoor air quality. Also called air changes per hour. Also called air change rate.

Air cleaner A device used to remove airborne particulates and gases from the air. Air cleaners may be added to HVAC systems or be stand-alone room units.

Air cleanerelectrostatic A device that uses an electrical charge to trap particles traveling in the air stream.

Air cleaning An indoor air quality (IAQ) controls strategy to remove various airborne particulates and gases from the air. The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation and gas sorption.

Air cleaning system A device or combination of devices applied to reduce the concentration of airborne contaminants, such as microorganisms, dusts, fumes, respirable particles, other particulate matter, gases, and vapours in air.

Air conditioning A form of air treatment in which temperature is controlled, possibly in combination with the control of ventilation, humidity and air cleanliness.

Air conditioning system A combination of all components required to provide a form of air treatment in which temperature is controlled or can be lowered, possibly in combination with the control of ventilation, humidity and air cleanliness. (EPBD, 2002/91/EC)

Air contaminant Any material in the atmosphere that affects persons and their environment (pollutant includes materials such as liquids, solids, aerosols, gases and odours). Also called an air pollutant.

Air curtain A planar jet that provides a climate separation between zones with different conditions of indoor air quality and climate.

Air diffusion Distribution of the air in a space by means of air terminal devices, in a way that meets specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.

Air diffusiondisplacement Air diffusion where the mixing of supply air and indoor air is very low. Usually supply air is a few degrees cooler that room air and supply velocity low.

Air diffusionmixing Air diffusion where the mixing of supply air and room air is intended.

Air douches A jet of air at specific conditions discharged at low velocity in a space in order to provide locally needed conditions.

Air extractmechanical The process of extracting air with the aid of powered air movement components, usually fans.

Air extractnatural The process of extracting air by means of wind forces or density differences or a combination of the two.

Air flow ratemass Mass flow of air over a specified time, usually expressed in kg/s or kg/h.

Air flow ratevolumetric Volumetric flow of air over a specified time, usually expressed in l/s or m3/h.

Air handling unit (AHU) An assembly consisting of sections containing a fan or fans and other necessary equipment to perform one or more of the following functions: air circulation, filtration, heating, cooling, heat recovery, humidification, dehumidification and mixing of air, and necessary controls functions.

Air leakage factor The air leakage per unit envelope area.

Air leakage, internal Air leakage between two air streams in an air handling component like heat recovery unit.

Air pollutant See air contaminant.

Air pollution Result of the presence of air pollutants in the atmosphere.

Air quality, indoor (IAQ) IAQ deals with the health and comfort of the air inside buildings and characterizes the indoor climate of a building, including the gaseous composition, temperature, relative humidity, and airborne contaminant levels. IAQ is the expression for both the concentration of impurities in the air and an expression of how people signify their perception of the air (perceived air quality) in the form of sensory measurements such as smell and irritation.

Air qualityperceived (PAQ) Indoor air quality as it is perceived by humans.

Air stratification The layering of air within a space, due to density differences caused by temperature distribution of the air.

Air supply, displacement Air supply where the mixing of supply air and indoor air is at a minimum.

Air supply, mechanical The process of supplying air with the aid of powered air movement components, usually fans.

Air supply, mixed The supply of mixed air. See also mixed air.

Air supply, natural The process of supplying air by means of wind forces or density differences or a combination of the two.

Air throw The distance an air jet travels upon leaving a diffuser before its velocity is reduced to a specific value, usually to the velocity which does not cause draft, 0.15-0.25 m/s depending on the temperature.

Air vent A valve, either manual or automatic, that is used to remove unwanted air from the highest point of a piping system.

Air, conditioned Air that has been heated, cooled, humidified or dehumidified to maintain an interior space within the "comfort zone". (Sometimes referred to as tempered air.)

Air, exhaust Air removed from a space and discharged to outside the building by means of mechanical or natural ventilation systems.

Air, indoor The air in an enclosed occupiable space.

Air, induced Air volume or flow that is set into motion by the primary air supplied to a space.

Air, mixed The mixture of outdoor air and recirculated return air.

Air, outdoor Air taken from outside the building which therefore has not previously circulated through a ventilation system.

Air, primary Conditioned and dehumidified outdoor air supplied to a terminal unit such as chilled beam, induction unit, etc. through a duct from the air handling unit.

Air, recirculation A part of extract air that is not exhausted from the building, but it is recirculated back to spaces.

Air, secondary Air volume flow rate extracted from a room and being supplied again to the same room after having been conditioned. (EN 13779). Also referred to as transfer air.

Air, supply Air delivered by mechanical or natural ventilation to a space, composed of any combination of outdoor air, recirculated air or transfer air.

Air, transfer Air moved from one indoor space to another.

Air, ventilated Outdoor air that is supplied to a room for ventilation purposes.

Airflow, induced The secondary airflow from the room induced into a terminal unit such as chilled beam, induction unit, etc. by the primary air.

Air-handling unit, decentralised Unlike a central air-handling unit, a decentralised air-handling unit is a unit that is allocated to a single room or group of rooms, supplying secondary air or outdoor air to that room.

Allergen A substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.

Area, gross floor The total area of all the floors of a building, including intermediately floored tiers, mezzanine, basements, etc., as measured from the exterior surfaces of the outside walls of the building.

Area, internal gross A term used in the United Kingdom, defined in the RICS Standard, for the area of a building measured to the internal face of perimeter walls at each floor level.

Area, net floor A term used in the ISO standard to express the interior gross area less the areas of all interior walls.

Area, occupied Area within a heated or cooled surface occupied for long periods. Normally the floor area within 1.0 m from external walls-windows and HVAC equipment and 0.5 m from internal walls.

Area, peripheral Area of a building, next to the exterior walls, which has a different heating or cooling load than the rest of the building.

Area/space, living floor Total area of rooms falling under the concept of rooms. (OECD Glossary of Statistical Terms)

Area/space, useful floor Floor space of dwellings measured inside the outer walls, excluding cellars, nonhabitable attics and, in multi-dwelling houses, common areas. (OECD Glossary of Statistical Terms)

Arrestance, ASHRAE A measure of the ability of a device to remove ASHRAE standard test dust from test air.

Arrestance, filter The amount of particles of non-specific size captured by a filter. The arrestance describes how well an air filter removes larger particles (total mass) such as dirt, lint, hair and dust.


BAC See building automation and control.

Background concentration The level of a contaminant present in the ambient air.

Balance point An outdoor temperature, usually between 0°C and 7°C, at which a heat pump's output exactly equals the heating needs of the heated building. Below the balance point, supplementary heat, or heat from other sources, is needed to maintain indoor comfort.

Barrier, radiant A thin, reflective foil sheet that exhibits low radiant energy transmission and under certain conditions can block radiant heat transfer; installed in attics to reduce heat flow through a roof assembly into the living space.

Barrier, vapour A moisture-impervious layer applied to the surfaces enclosing a humid space to prevent moisture travel to a point where it may condense due to lower temperature.

Bimetal Two metals with different rates of expansion fastened together. When heated or cooled they will warp and can be made to open or close a switch or valve.

BMS See building management system.

Boiler The combined boiler body and burner unit designed to transmit to water the heat released from combustion. (EPBD, 2002/91/EC)

Bouncing Particles that hit the fibres of a filtering media for air cleaning and then bounce back into the air stream.

Boundary conditions Values of physical parameters (e.g. temperature, heat flux, mass flux, velocity, etc.) that are specified at the boundaries of a solution domain and are required for solving the discretised equations in a computational fluid dynamics (CFD) solution or any other physical problem.

Breakthrough curve Curve of penetration vs. time for an adsorbent under a specified condition and for a specific pollutant.

British thermal unit (BTU) The amount of heat that must be added to one pound of water to raise its temperature one degree Fahrenheit. 1 BTU = 1055.06 J= 2.931 10-4 kWh.

BTU See British thermal unit.

Building automation and control (BAC) Products, software and engineering services for automatic controls, monitoring and optimisation, human intervention and management to achieve energy-efficient, economical and safe operation of building services equipment.

Building management system (BMS) A computer-based system that controls and monitors a building’s mechanical and electrical installations, fire alarms and security systems.

Building services Services provided by technical building systems and by appliances to provide indoor climate conditions, domestic hot water, illumination levels and other services related to the use of the building.

Building, commercial A building that is used for commercial use. Types can include office buildings, warehouses or retail buildings (convenience stores, big box stores, shopping malls, etc.).

Building, nearly net zero energy (nZEB) A building with a technically reasonable, achievable national energy use of > 0 kWh/(m2 a) of primary energy achieved with best-practice energy efficiency measures and renewable energy technologies which may or may not be cost optimal.

Building, nearly zero energy A building that has very high energy performance, as determined in accordance with Annex I of the EPBD recast. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby. (EPBD recast, 2010/31/EC)

Building, net zero energy (ZEB) Energy use of 0 kWh/(m2a) primary energy. NOTE 1_ A net ZEB is typically a grid connected building with very high energy performance. A net ZEB balances its primary energy use so that the primary energy feed-in to the grid or other energy network equals to the primary energy delivered to ZEB from energy networks. An annual balance of 0 kWh/(m2 a) of primary energy use typically leads to the situation where significant amount of the on-site energy generation will be exchanged with the grid. Therefore, a net ZEB produces energy when conditions are suitable and uses delivered energy during rest of the time.

Building, public Building owned or occupied by any public body.

Building, residential A structure used primarily as a dwelling for one or more households. Residential buildings include single-family houses (detached houses, semi-detached houses, terraced houses, row houses and multi-family houses (or apartment blocks), which include apartments/flats.

Buoyancy The vertical force exerted on a volume of air that has a density different from the ambient air caused by temperature differences.


Capacity, thermal The output or producing ability of a piece of cooling or heating equipment. Property of a material to hold heat. Measured usually in J/kgK.

CAV Constant air volume

CEN European Committee for Standardization

Certificate, energy performance A certificate recognised by an EU Member State or a legal person designated by it, which includes the energy performance of a building calculated according to a methodology based on the general framework set out in the Annex of Directive 2002/91/EC. (EPBD, 2002/91/EC)

Certificate, white A certificate issued by an independent certifying body confirming the energy savings of market actors as a consequence of energy efficiency improvement measures. (ESD, 2006/32/EC)

CFU See colony forming unit.

Chilled beam A cooled element or cooling coil situated in, above or under a ceiling which cools convectively using natural or induced air flows. The cooling medium is usually water.

Chilled beam, active (ventilated) The cooled element or cooling coil with integrated air supply where primary air, induced air or both pass on their exterior surface. The cooling medium in the coil is usually water.

Chilled beam, closed An active chilled beam where there is an integrated secondary air path directly from the room space. Closed chilled beams are usually installed within a suspended ceiling. The cooling medium is usually water.

Chilled beam, open An active chilled beam where secondary air is taken into the top of the beam. Open chilled beams are mainly used without a suspended ceiling. The cooling medium is usually water.

Chilled beam, passive (static beam) The cooled element or cooling coil fixed in, above or under a ceiling that cools mainly convectively using natural airflow.

Chilled ceiling (radiant ceiling) Ceiling panels that are made up of elements that connect together and cool primarily through radiation. The cooling medium is usually water.

Chimney effect The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure or building staircase, due to its lower density compared to the surrounding air or gas.

CHP See combined heat and power.

CHRV Central heat recovery ventilation

Cleanliness The condition of a ventilation system and/or the components in which the amount or concentration of contaminants is below a specified level.

CO2 emission coefficient For a given energy carrier, the quantity of carbon dioxide emitted to the atmosphere per unit of delivered energy. (EN 15603:2008)

Coefficient of performance (COP) The ratio between the output energy and the energy required to produce it. It is used for heat pumps in heating mode.

Cogeneration Simultaneous production of two or more forms of useable energy from a single fuel source, e.g. heat energy and electrical or mechanical power, in the same facility. Because a typical cogeneration facility uses thermal energy that is generally wasted in a traditional power plant, the process can be 50 to 70 percent more efficient. Fuels used in cogeneration facilities may take the form of natural gas, biomass, oil or coal. Cogeneration systems are designed to simultaneously produce electric power and thermal heat for industrial processes or the heating and cooling of buildings. Cogeneration plants can be any size, from 10 kilowatts to 1,000 megawatts or more. See also Combined heat and power (CHP).

Coil A cooling or heating element (heat exchanger) made of pipe or tubing, often including fins or plates, through which a fluid is passed, exchanging thermal energy with another fluid surrounding it for heating or cooling.

Coil, cooling Heat exchanger that extracts heat from the air stream by means of a heat transfer medium. See also Coil.

Coil, heating A heat exchanger that adds heat to the air stream by means of a heat transfer medium. See also Coil.

Collection efficiency The ratio of the mass of the particles collected in an electrostatic precipitator (ESP) to the mass of particles entering the ESP. It is often expressed as a percentage.

Colony forming unit (CFU) A laboratory measure of fungal concentration, indicating the quantity of viable organisms collected for a given unit sample.

Combined heat and power (CHP) The simultaneous conversion of primary fuels into mechanical or electrical and thermal energy, meeting certain quality criteria of energy efficiency. (EPBD, 2002/91/CE) see also Cogeneration

Comfort zone The range of temperatures, humidities and air velocities at which the greatest percentage of people feel comfortable.

Comfort, acoustical Sound pressure levels and frequency distribution of ambient noise and other acoustic conditions that do not cause unpleasantness.

Comfort, thermal The totality of conditions (air temperature, relative humidity, air velocity, pressure, clothing, activity) for which a person would not prefer a different thermal environment.

Commissioning The testing of HVAC systems prior to building occupancy to check whether the systems meet the operational needs of the building within the capabilities of the system design. Start-up of a building that includes testing and adjusting HVAC, electrical, plumbing and other systems to assure proper functioning and adherence to design criteria. Commissioning also includes the instruction of building representatives in the use of the building systems.

Compressor A reciprocating or rotary pump for raising the pressure of a fluid; this can be a single-stage or multistage unit. A reciprocating compressor is a machine that compresses gases, composed of one or several cylinders; each cylinder contains a piston that is moved by a crankshaft through a connecting rod. A rotary compressor is a machine having a rotating member that directly compresses fluid in an enclosed housing; the fluid pressure rises as the volume of the closed space decreases.

Concentration The quantity of one substance (gas or particles) dispersed in a defined amount of another substance (usually air or water).

Condenser A device that transfers unwanted heat out of a refrigeration system or a heat pump to a medium (either air, water, or a combination of air and water) that absorbs the heat and transfers it to a disposal point. There are three types of condensers: air-cooled condensers, water-cooled condensers and evaporative condensers. The evaporative condenser uses a combination of air and water as its condensing medium. Most residential systems have an air-cooled condenser.

Confidence interval The range of values around an estimate where the exact value of the estimate can be expected to be located with a given level of certainty, usually 95%.

Contaminant An unwanted airborne constituent that may increase health risks and reduce acceptability of the air.

Contaminant removal effectiveness (CRE) A measure of how effectively an airborne contaminant is removed from a room.

Convection The movement of heat by fluid flow (air or water).

Cooling capacity The quantity of heat that a cooling appliance is capable of removing from a room.

Cooling load The rate at which heat must be extracted from a space in order to maintain the desired temperature within the space.

Cooling system, free Typically a water-cooled or glycol-cooled system with an additional coil that provides chilled water cooling when the outdoor ambient is cold, thereby reducing or eliminating compressor operation.

Cooling tower A heat transfer device that cools warm water using outside air or water. Usually used to reject heat from the cooling process to the atmosphere.

Cooling, active Cooling process in which energy-consuming mechanical components like compressors, pumps and fans are used.

Cooling, district The distribution of thermal energy in the form of chilled liquids, from a central source of production through a network to multiple buildings or sites, for the use of space or process cooling. (EPBD 2010/31/EC)

Cooling, mechanical Cooling with a compressor cycle.

Cooling, passive Cooling process in which energy-consuming mechanical components like pumps and fans are not used.

COP See coefficient of performance.

Cost-benefit analysis A process in which a measure’s benefits are weighted against its costs. The term is often used when a measure is analysed from a socio–economic perspective, but also in engineering analyses when comparing alternatives.

Cost-effectiveness analysis An analysis in which the most cost-effective method of reaching a specific objective is calculated.

Cost-optimal level The energy performance level that leads to the lowest cost during the estimated economic lifecycle. (EPBD, recast, 2010/31/EC)

CRE See contaminant removal effectiveness.


Damper A movable device, placed in the ductwork, that opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms.

DCV Demand controlled ventilation

Decipol One decipol is the sensory pollution level in a room the caused by one standard person (one olf) when ventilated by 10 L/s of unpolluted air. It was developed to quantify how the strength of indoor pollution sources influence air quality as it is perceived by humans.

DEHS DiEthylHexyl Sebacate

Dehumidification The reduction of water content in the air.

Dehumidifier A device that removes moisture from the air.

Design criteria Values of parameters that define indoor air quality, thermal and acoustical comfort, energy efficiency and the associated system controls that should be achieved by the design.

Dew point The temperature at which the water vapour present in the air condenses.

Diffuse radiation Solar radiation received indirectly as a result of scattering due to clouds, fog, haze, dust or other obstructions in the atmosphere or on the ground.

Diffuser Air distribution device designed to direct airflow into desired patterns.

DIN Deutsches Institut für Normung (Germany)

Disinfection A method aiming to reduce the number of viable micro-organisms in a liquid or on a surface to such an extent that an infection hazard no longer exists.

Draught Human-perceived sensation of local cooling of body caused by air movement and its temperature.

Draught rating (DR-value) The percentage of people predicted to be dissatisfied due to draught in certain conditions.

Dual duct system An air conditioning system that has two ducts for supply air. One is with heated air and the other is with cooled air, so that air of the correct temperature is provided by mixing varying amounts of air from each duct.

Duct A pipe or closed conduit made of sheet metal, fiberglass board or other suitable material used for conducting air to and from an air handling unit or fan.

Duct, flex A duct usually installed in a single, continuous piece between the register and plenum box. A flexible duct usually has an inner lining and an insulated coating on the outside.

Ductwork Pipes or ducts that carry air throughout a building.

Dust, ASHRAE Synthetic dust used for loading air filters in laboratory tests. (ASHRAE Standard 52)

Dust, coarse Particles larger than 2.5 µm in diameter.

Dust, loading Synthetic test dust specifically formulated for determining the test dust capacity and arrestance of a filter.


Economizer, air A component of an air handling unit that increases the amount of outdoor air in the supply air when the outdoor air temperature is below the indoor temperature, in order to reduce the need for mechanical cooling.

EER See energy efficiency ratio.

Efficiency (filtration) Removal of dust in a filter, expressed as a percentage. (EN 779)

Emissivity The relative ability of its surface to emit energy by radiation in relation to black surface.

EN European Standard

Energy Broadly defined, the capability of doing work. More specifically, it is the capacity for doing work as measured by the capability of doing work (potential energy) or the conversion of this capability to motion (kinetic energy). Forms of energy include thermal, mechanical, electrical and chemical. Energy may be transformed from one form into another form useful for work. Most of the world's convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks. Electrical energy is usually measured in kilowatt-hours, while heat energy may be measured in joules (J) or kilowatt-hours (kWh).

Energy audit A systematic procedure to obtain adequate knowledge of the existing energy consumption profile of a building or group of buildings, of an industrial operation and/or installation, or of a private or public service, identify and quantify cost-effective energy savings opportunities, and report the findings. (ESD, 2006/32/EC)

Energy consumption The amount of energy consumed in the form in which it is acquired by the user. The term excludes electrical generation and distribution losses.

Energy demand, cooling The integrated cooling load over a total year in kWh or MJ. This is often expressed in terms of energy per square meter per annum (year): kWh/m²/a.

Energy demand, heating The integrated heating load over a total year in kWh or MJ. This is often expressed in terms of energy per square meter per annum (year): kWh/m²/a.

Energy efficiency ratio (EER) A ratio calculated by dividing the cooling capacity in watts by the power input in watts.

Energy management system A control system (often computerized) designed to regulate the energy consumption of a building by controlling the operation of energy consuming systems, such as the heating, ventilation and air conditioning (HVAC), lighting and water heating systems.

Energy performance of a building Calculated or measured amount of energy delivered and exported actually used or estimated to meet the different needs associated with a standardized use of the building, which may include energy used for heating, cooling, ventilation, domestic hot water, lighting and appliances.(EN 15316-1:2007)

Energy Performance of Buildings Directive (EPBD) An EU Directive of late 2002 aimed at improving the energy performance of buildings. It was strengthened and accelerated in 2010 by the ‘Recast EPBD’.

Energy performance requirement Minimum level of energy performance that must be achieved to obtain a right or an advantage, for example a building permit, lower interest rate or quality label. (CEN standard – En 15217 “Energy performance of buildings – “Methods for expressing energy performance and for the energy certification of buildings”)

Energy service company (ESCO) A natural or legal person that delivers energy services or other energy efficiency improvement measures to a user’s facility or premises, and accepts some degree of financial risk in so doing. The payment for the services delivered is based either wholly or in part on the achievement of energy efficiency improvements and on the meeting of the other agreed performance criteria. (ESD, 2006/32/EC)

Energy source Source from which useful energy can be extracted or recovered either directly or by means of a conversion or transformation process.

Energy source, renewable Energy from a source that is not depleted by extraction, such as solar energy (thermal and photovoltaic), wind, water power or renewed biomass.

Energy use for space heating or cooling Energy input to a heating or cooling system to satisfy the energy need for heating or cooling (including dehumidification), respectively.

Energy use for ventilation Energy input, in the form of electricity and heat, to the ventilation system for air transport, heat recovery and for the humidification system. (For some countries only the input of electricity is considered.)

Energy, delivered Energy, expressed per energy carrier, supplied to the technical building systems through the system boundary, to satisfy the uses taken into account (heating, cooling, ventilation, domestic hot water, lighting, appliances, etc.) or to produce electricity. (EN 15603:2008)

Energy, exported Energy, expressed per energy carrier, delivered by the technical building systems through the system boundary and used outside the system boundary. (EN 15603:2008)

Energy, final Energy supplied that is available to the consumer to be converted into useful energy (e.g. electricity at the wall outlet). (Intergovernmental Panel on Climate Change, IPCC)

Energy, incident solar (W/m2) The amount of solar radiation striking a surface per unit of time and area, expressed as W/m². Also referred to as irradiance.

Energy, net delivered Delivered minus exported energy, both expressed per energy carrier. (EN 15603:2008)

Energy, primary Energy from renewable and non-renewable sources that has not undergone any conversion or transformation process. (EPBD recast)

Energy, regulated Energy used in the home for heating, cooling, hot water and lighting.

Enthalpy Heat content or total heat, including both sensible and latent heat.

ENV European Prestandard

Envelope, building Integrated elements of a building that separate its interior from the outdoor environment. (IUPAC International Union of Pure and Applied Chemistry - Compendium of Chemical Terminology 2nd Edition 1997)

Environmental agents Conditions other than indoor air contaminants that cause stress, comfort, and/or health problems (e.g., humidity extremes, draughts, lack of air circulation, noise and over-crowding).

Environmental tobacco smoke (ETS) Mixture of smoke from the burning end of a cigarette, pipe, or cigar and smoke exhaled by the smoker. Also called second-hand smoke.

EPBD Energy Performance of Buildings Directive

ERV Energy recovery ventilator

ESCO See energy service company.

ESP Electrostatic precipitator

ETS See environmental tobacco smoke.

EUROVENT European Committee of Air Handling and Refrigerating Equipment Manufacturers

Evaporator A component of a thermodynamic refrigeration cycle where evaporation of the refrigerant takes place. The heat for the evaporation comes from the surrounding fluid. In practice, evaporator in air conditioning systems is found indoors and is known as cooling coil. In heat pumps systems, it is found outdoors and absorbs heat from the outdoor environment.

EVHA European Ventilation Hygiene Association (Europe)

Exfiltration The air flowing through the building envelope from inside to outside due to the pressure difference. In cold climates this may cause moisture damage in the construction due to condensation of moist indoor air in the structure.


Facilities management (FM) All services required for the management of buildings and real estate to maintain and increase their value.

Fan coil A component of an HVAC system containing a fan and heating or cooling coil, used to distribute heated or cooled air.

Fan power The electric power absorbed by a fan motor.

Fan power, specific (SFP) The combined amount of electric power consumed by all the fans in an air distribution system divided by the total airflow rate through the building under design load conditions, in Ws/m3.

Fan, duct A fan mounted in a section of duct to move conditioned air.

Filter A device for removing particulate material and gases from air.

Filter element, air A unit in the filtering system, comprised of filter material including framing, supporting parts and gaskets, inserted into a filter housing device.

Filter, average efficiency Weighted average of the efficiencies of filters to remove 0.4 µm particles for the different specified dust loading levels up to final the pressure drop (EN 779:2002).

Filter, charged Polymer fibre filter that is electrostatically charged or polarised.

Filter, coarse A filter that retains particles larger than 2.5 µm. It is classified in one of the classes G1 to G4 (based on removal of synthetic loading dust). (EN 779:2002)

Filter, fine A filter classified in one of the classes F5 to F9 (based on average efficiency of 0.4 µm particle). (EN 779:2002)

Filter, gas phase A filter to remove gases or vapour contaminants from an air stream.

Fire dampers Components that are installed in an air distribution system between two fire separating compartments and are designed to prevent propagation of fire and smoke. Generally are kept open by mechanical restraint, whose effect is cancelled under specific conditions. The valve is then closed automatically.

FiSIAQ Finnish Society of Indoor Air Quality and Climate (Finland)

Flow, counter In a heat exchanger, a pattern in which hot fluids and cold fluids travel in opposite directions. A counter flow heat exchanger is considered the most efficient flow pattern compared with cross flow and parallel flow.

Flow, cross In a heat exchanger, the exchange of thermal energy from one airstream to another in a cross pattern. Cross flow heat exchangers are typically used for heat transfer between a gas and a liquid. 

Flow, parallel In a heat exchanger, a pattern in which hot fluids and cold fluids travel in parallel directions.

Flow, piston A theoretical air flow pattern where supply air passes like a piston across a room and pushes old air out through the exhaust.

Flow, plug See Piston flow.

FM See facilities management

Formaldehyde A colourless water-soluble gas emitted from many building materials. It is frequently measured and evaluated separately from other volatile organic compounds (VOCs).

Free cooling, water side A system that uses either direct evaporative cooling, or a secondary evaporatively cooled water loop and cooling coil to satisfy cooling loads, to reduce energy use for mechanical cooling.

Freon A general term used to identify any of a group of partially or completely halogenated simple hydrocarbons containing fluorine, chlorine or bromine, which are used as refrigerants.

Fungi A large group of organisms including moulds, mildews, yeasts, mushrooms, rusts and smuts. Any of a group of parasitic lower plants that lack chlorophyll. Most fungi produce spores, which are broadcast through the air so that virtually all environmental surfaces will have some fungal material. Most health effects are associated with allergic responses to antigenic material or toxic effects from mycotoxins. Fungi also generate certain volatile organic compounds.

Fungicide A chemical substance that is used to get rid of fungi.


G-value A number between 0 and 1 that represents the sum of primary transmittance and secondary transmittance to a room. The secondary transmittance is the ratio between solar radiation and the part of the solar energy absorbed in the window/solar shade materials, which reaches the room through convection or as (thermal) radiation. The g-value is also referred to as the total solar energy transmittance or solar factor. In North America it is referred to as the solar heat gain coefficient (SHGC).

Gas, organic A chemical based on a structural framework of carbon atoms. Organic gases can present a flammable, toxic or asphyxiation risk.

Gas, tracer A detectable gas used in small concentrations to evaluate performance of ventilation such as airflows, local mean ages, air change efficiency, etc.

Greenhouse effect A phenomenon whereby the earth's atmosphere becomes thick with gases and substances that trap the sun’s radiation, making the earth warmer than it would be with direct sunlight alone. These gases (carbon dioxide [CO2], methane [CH4], nitrous oxide [N2O], tropospheric ozone [O3], water vapour [H2O], and chlorofluorocarbons) allow visible light and ultraviolet light (shortwave radiation) to pass through the atmosphere and heat the earth’s surface. This heat is re-radiated from the earth in form of infrared energy (long wave radiation). The greenhouse gases absorb part of that energy before it escapes into space. Thus the greenhouse effect allows solar radiation to penetrate but absorbs the infrared radiation returning to space.

Greenhouse effect (as related to buildings) The characteristic tendency of some transparent materials (such as glass) to transmit radiation with relatively short wavelengths (such as sunlight) and block radiation of longer wavelengths (such as heat). This tendency leads to a heat build-up within the space enclosed by such a material.

Grille A device for air openings or ducts that enables them to open to conditioned space. A grille is equipped with linear blades that control the airflow direction and thus the air distribution.

Grille, adjustable A grille with linear blades that can be adjusted to vary the direction of the supply air. The linear blades are normally either vertical or horizontal, or both horizontal and vertical.

Heat balance The equilibrium that is known to exist when all sources of heat gain and loss for a given region or body are accounted for.

Heat carrier A substance or fluid that can be used to produce or transport heat or to operate physical processes.

Heat exchanger A device in which heat is transferred between two mediums that do not come into contact. They are part of heat recovery systems and are used in air handling units, ductwork systems and process technology to lower energy consumption.

Heat exchanger, air to air plate A heat exchanger designed to transfer thermal energy from one air stream to another without moving parts. Heat transfer surfaces are in the form of plates. This exchanger may have a parallel flow, cross flow or counter flow construction or a combination of these.

Heat exchanger, air to air tube A heat exchanger designed to transfer thermal energy from one air stream to another without moving parts. Heat transfer surfaces are in the form of tubes. This exchanger may have parallel flow, cross flow or counter flow construction or a combination of these.

Heat exchanger, rotary A device incorporating a rotating cylinder or wheel for the purpose of transferring energy from one air stream to the other. It incorporates heat transfer material, a drive mechanism, a casing or frame, and includes any seals that are provided to retard the bypassing and leakage of air from one air stream to the other.

Heat gains Heat generated within or entering into a conditioned space from heat sources other than technical building thermal systems (heating, cooling or domestic hot water preparation, etc.).

Heat gains, internal Heat originating from within a building generated by occupants’ sensible metabolic heat and by appliances such as lighting, domestic appliances or office equipment, other than energy intentionally provided for heating, cooling or hot water preparation. Given in W or W/m².

Heat gains, solar Heat provided by solar radiation entering directly or indirectly (after absorption in building elements) into a building through windows, opaque walls and roofs, or passive solar devices such as sunspaces, transparent insulation and solar walls.

Heat loss The heat that flows from a building interior, through the building envelope to the outside environment or ground

Heat pump A machine, device or installation that transfers heat from natural surroundings such as air, water or ground to buildings or industrial applications by reversing the natural flow of heat such that it flows from a lower to a higher temperature. For reversible heat pumps, it may also move heat from the building to the natural surroundings. (EPBD 2010)

Heat recovery Heat utilized from a system that would otherwise be wasted (for example, heat transferred from exhaust air into supply air).

Heat source A body of fluid from which heat is collected for heating purposes (for example, in an air source heat pump, the air outside the house is used as a heat source during the heating cycle).

Heat transfer Flow of heat energy induced by a temperature difference. Heat flow through a building envelope flows from a heated, or hot, area to a cooled, or cold, area.

Heat transfer coefficient In surface heating and cooling, the combined convective and radiant heat transfer coefficient between the heated/cooled surface and the space operative temperature (design indoor temperature).

Heat, derived The total heat production in heating plants and in combined heat and power plants. It includes the heat used by the auxiliaries of the installation which use hot fluid (space heating, liquid fuel heating, etc.) and losses in the installation/network heat exchanges. For autoproducing entities (entities generating electricity and/or heat wholly or partially for their own use as an activity that supports their primary activity) the heat used by the undertaking for its own processes is not included. (Eurostat definition)

Heat, latent The heat released or absorbed by a substance during a process that occurs without a change in temperature and with the change of the state of matter.

Heat, sensible The heat released or absorbed by a substance during a process that occurs with a change in temperature.

Heater, demand (tank-less) water A type of water heater that has no storage tank, thus eliminating storage tank stand-by losses. Cold water travels through a pipe into the unit, and either a gas burner or an electric element heats the water only when needed.

Heater, electric resistance A device that produces heat through electric resistance.

Heater, vented A type of combustion heating appliance in which the combustion gases are vented to the outside, either with a fan (forced) or by natural convection.

Heating capacity The quantity of heat that a heating appliance is capable of supplying into a room within a time unit.

Heating load The instantaneous heating rate required to keep a building “in balance” at a specific minimum comfort temperature level (for example, a design temperature of 21.0°C), without taking into account the effectiveness of the heating system. Expressed in W or W/m².

Heating system, central A system where heat is supplied to areas of a building from a single appliance through a network of ducts or pipes.

Heating, district The distribution of thermal energy, in the form of steam or hot water, from a central source of production through a network to multiple buildings or sites, for the use of space or process heating. (EPBD, 2010/31/EC)

Heating, electric radiant A heating system in which electric resistance is used to produce heat that is mainly transferred by radiation to surfaces. There is no fan component to a radiant heating system.

Heating, hydronic A system that heats a space using hot water, which may be circulated through a convection or fan coil system or through a radiant baseboard or floor system.

Heating, intermittent Heating pattern in which normal heating periods alternate with periods of reduced or no heating.

Heating, tap water The heating of water for domestic use.

HEPA High Efficiency Particulate Air Filter

HR See heat recovery.

HRV Heat recovery ventilator

Humidification The addition of water vapour to room air or supply air.

Humidifier A device that is used for humidification.

Humidistat A device designed to regulate humidity input by reacting to changes in the moisture content of the air. Much like a thermostat but turns the system on and off by sensing the humidity level.

Humidity, absolute The absolute amount of water vapour in ambient air, expressed in g/kg or g/m³ dry air.

Humidity, relative A ratio expressing the amount of moisture in the air compared to what the air can hold at that temperature.

HVAC Heating, ventilation and air conditioning

HVACR Heating, ventilating, air conditioning, and refrigeration

IAQ See indoor air quality.

IDA Indoor air; the abbreviation of IAQ classes defined in EN13779.

IEQ See indoor environment quality.

Illuminance (lx) The total luminous flux, incident on a surface, per unit area. Expressed in lx = lm/m².

Indoor air quality (IAQ) The indoor air concentrations of pollutants that are known or suspected to affect people’s comfort, environmental satisfaction, health, or work or school performance. (ASHRAE, 2017)

Indoor climate Temperature, humidity, lighting, airflow and noise levels in a habitable structure or conveyance.

Indoor environment An environment within a building or an enclosed space.

Indoor environment quality (IEQ) IEQ encompasses all aspects of the indoor environment including air quality, thermal environment, lighting and acoustic environment.

Induction rate The total volume of air moved by induction, divided by the volume of primary air supplied.

Infiltration The transport of air through leakage paths in the envelope of a building, resulting from pressure (e.g. wind) and temperature differences.

Initial efficiency (filter) Efficiency of a clean filter operating at the air flow rate test.

Insulation Any material that is used to reduce heat flow or heat losses.

Insulation, clothing Resistance to sensible heat transfer provided by a clothing ensemble (i.e. more than one garment) Note: It is described as the intrinsic insulation from the skin to the clothing surface, not including the resistance provided by the air layer around the clothed body and is expressed in the clo unit or in m²K/W; 1 clo = 0.155 m²K/W.

Internal rate of revenue (IRR) A rate at which the accounting value of a security is equal to the present value of the future cash flow. (European Central Bank)

IRR See internal rate of revenue.

Isovel Boundary line of points of equal mean velocity.

kWh Kilowatt-hour, a unit of energy equal to 3600 kJ, or 3412 Btu. It is used to measure electrical energy that is used over time.

LCA See life cycle assessment.

LCC See life cycle cost.

Leakage If the duct and air handling system is not airtight, air will leak from, or into, the system depending on the pressure in the system, and reduce the air delivery efficiency of the system.

LHRV Local heat recovery ventilation

Life cycle assessment A standardised technique for assessing the environmental aspects and potential impacts associated with a product by quantifying all inputs and outputs.

Life cycle cost The total of all recurring and one-time costs over the full life of an asset.

Load calculation A process for determining the heat gain and heat loss in a building so that properly sized air conditioning and heating equipment can be installed.

Long wave infrared radiation Part of the electromagnetic spectrum with a wavelength between 8000 and 15000 nm, corresponding to the radiation of objects at room temperature. Normal glass or glazing is not transparent to this radiation.

Luminance The intensity of light emitted from a surface per unit area in a given direction, measured in candela per square metre (cd/m²). It is a property of extended (direct and indirect) light sources.

Manometer An instrument that measures air or water pressure differences between points.

MERV The minimum reported efficiency in specified particle size ranges during the test (ASHRAE 52.2-2007).

Metabolic rate Rate of energy production of the body. The metabolic rate varies by activity. It is expressed in the met unit or in W/m²; 1 met = 58.2 W/m². One met is the energy produced per unit surface area of a sedentary person at rest. The surface area of an average person is about 1.8 m².

Microbial volatile organic compound (mVOC) A chemical generated by a mould which may have a mouldy or musty odour.

Micro-organisms In the context of ventilation and air-conditioning systems, substances including bacteria (such as legionella), algae and moulds capable of multiplying in water or on humid surfaces (such as in the humidifier water or in condensate).

Most penetrating particle size The particle size for which penetration is a maximum for a given filter at a specified velocity.

MPPS See most penetrating particle size.

mVOC See microbial volatile organic compound.

Nanoparticles Ultrafine particles less than 100 nanometres in size. See also Particles, ultrafine.

Net present value (NPV) A standard method for the financial assessment of long-term projects. It measures the excess or shortfall of cash flows, calculated at their present value at the start of the project.

Noise rating (NR) A method for rating the acceptability of indoor environment for hearing preservation, speech communication and annoyance, as developed by the International Organization for Standardization (ISO). It represents the highest NR curve touched by the measured octave band spectrum.

NPV See net present value.

NR See noise rating.

nZEB Nearly zero energy building (EPBD recast, 2010/31/EC)

ODA Outdoor air; the abbreviation of IAQ classes defined in EN 13779.

Olf  A unit used to measure the strength of pollution sources that can be perceived by humans. One olf is the sensory pollution strength from a standard person defined as an average adult working in an office or similar non–industrial workplace, sedentary and in thermal comfort, with a hygienic standard equivalent of 0.7 bath/day.

Operation and maintenance Actions taken after construction to ensure that facilities constructed will be properly operated and maintained to achieve conditions and efficiency levels specified at the design level.

Organic compounds Chemicals that contain carbon. Volatile organic compounds vaporize at room temperature and pressure. They are found in many indoor sources, including many common household products and building materials.

PAH See polycyclic aromatic hydrocarbon.

PAQ Perceived air quality

Particles , nanoparticles Ultrafine particles less than 100 nanometres in size.

Particles, fine Particles less than 2.5 µm.

Particles, ultrafine See nanoparticles.

Particulates Small airborne particles found in indoor environments including fibrous materials, solid-state semi-volatile organic compounds, and biological materials.

Parts per million (PPM) A measure that expresses the concentration of a chemical or contaminate per unit volume of air or water. Generally used to establish safe levels of exposure to a substance.

Payback time The length of time required to recover the cost of an investment.

Penetration Ratio of the particle concentration downstream to upstream of the filter.

Performance A measure of the quantity and/or quality of the product or service of a worker.

Permeable Porous, allowing the passage of air.

Plenum A separate space in a building that provides for air circulation for heating, ventilation, and air-conditioning, typically located in the space between the structural ceiling and a drop-down ceiling.

Plume The air current rising from a hot body (or descending from a cold body).

PM10 Particulate matter that is 10 micrometers (10 µm) or less in diameter. Air quality measurements are typically reported in terms of daily or annual mean concentrations of PM10 particles per cubic meter of air volume. See also particulates.

PM2.5 Particulate matter that is 2.5 micrometers (2.5 µm) or less in diameter. See also Particulates.

PMV See predicted mean vote.

Pollutant See contaminant.

Pollutant removal effectiveness A measure of the relationship between the pollutant concentration in exhaust air and the pollutant concentration in a breathing zone.

Pollution The presence of undesired elements that are deteriorating to the comfort, health and welfare of persons or the environment (pollution includes noise, vibration, odours and gases).

Polycyclic aromatic hydrocarbon (PAH) A class of chemicals generated primarily during the incomplete combustion of organic materials.

Power The rate at which energy is transferred. Electricity for use as energy is also referred to as power. Electrical power is usually measured in watts (W). Also used for a measurement of capacity.

PPD See predicted percentage of dissatisfied.

PPM See parts per million.

Predicted mean vote (PMV) An index that predicts the mean value of the votes of a large group of persons on a 7-point thermal sensation scale with zero meaning thermal neutral state.

Predicted percentage of dissatisfied (PPD) An index that predicts the percentage of a large group of people likely to feel dissatisfied with their thermal comfort, i.e. that it is either too warm or too cool.  prEN Draft European Standard

Pressure drop - final Pressure drop up to which filtration performance is measured for classification purposes.

Pressure drop - initial Pressure drop of a clean filter operating at its test air flow rate.

Pressure, negative A condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurised space.

Pressure, positive A condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurised space, outward to surrounding areas.

Productivity The amount of output created (in terms of goods produced or services rendered) per unit of input used. It can be improved by increasing output (performance, etc.) or decreasing input (cost and other resources).

R-value A measure of resistance to heat flow through a given thickness of material. Is the inverse of the U-value. The higher the value, the better the material’s insulating properties.

Radiant ceiling panels, heating and cooling Panels, usually metal, suspended under a ceiling and insulated from the building structure. The primary cooling/heating agent temperature is close to the room's temperature.

Radiant floor A type of radiant heating system where the building floor contains channels or tubes through which hot fluids such as air or water are circulated. The whole floor is evenly heated. Thus, the room is heated from the bottom up. Radiant floor heating eliminates the draft and dust problems associated with forced air heating systems.

Radiation The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism).

Radiator A room heat delivery (or exchanger) component of a hydronic (hot water or steam) heating system; hot water or steam is delivered to it by natural convection or by a pump from a boiler.

Recovery ventilator, energy (ERV) A machine that draws outdoor air into a building and exhausts polluted air. It may preheat or pre-cool (depending on the season) to reduce energy costs associated with conditioning the air.

Reflectance The ratio of reflected to incident radiation. Usually denoted by the letter R or ρ.

Refrigerant Working fluid in the refrigeration cycle or heat pump cycle.

Refrigerant lines A set of two copper pipes connecting the outdoor unit and the indoor unit in a refrigeration system.

Register Covering of grill for air openings or the ducts where they open to the conditioned space.

Reversing valve A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating.

Room, habitable A room used for dwelling purposes but which is not solely a kitchen, utility room, bathroom, cellar or sanitary accommodation.

Sampling of tracer gas A process whereby a small amount of air is collected in order to measure the concentration of tracer gas. See also active sampling and passive sampling.

Sampling, active In ventilation measurement, the sampling of air by means of a pump.

Sampling, passive In ventilation measurement, sampling that depends on the diffusion of a contaminant into a solid sorbent.

SBS See sick building syndrome.

SC See shading coefficient.

Sensing element Component of a sensor that undergoes a measurable change in response to a change in the physical variable to be measured.

Sensitivity analysis A process that tests the extent to which a model’s results and predictions change when one or more assumptions change.

Sensor A device that converts a physical, chemical or biological property or quantity into a conveniently measurable effect or signal. In this context the term “sensor” is used to designate a “sensor system”, which may consist of several components. Based on the functional properties, these components can be grouped into three different units: a sensing element, a transducer and a transmitter.

Setback A reduction of climate control energy demand in HVAC controls when a building is unoccupied.

Setpoint The temperature to which a thermostat is set to result in a desired heated space temperature.

SFP See specific fan power.

Shading coefficient (SC) A measure of the ability of a window, or window with a solar shading device, to transmit solar heat relative to that ability for 3 mm clear, single glass. Is being phased out in favour of the g-value (in the US: solar heat gain coefficient or SHGC), and is approximately equal to the g-value multiplied by 1.15.

Short-cut or short-circuiting A situation that occurs when the supply air flows to return or exhaust grilles before entering the breathing zone (the area of a room where people are). To avoid short-circuiting, the supply air must be delivered at a temperature and velocity that results in mixing throughout the space.

Sick building syndrome (SBS) Non–specific symptoms experienced by building occupants that can include irritation of the eyes, nose and skin, headache, fatigue, and difficulty in breathing and are related to the characteristics of buildings and indoor environments. The symptoms improve when the occupant is away from the building and are not related to any known disease or exposure.

Solar heat gain coefficient The fraction of solar radiation transmitted through a window, or window with a solar shading device, both directly transmitted, and absorbed and subsequently released inward. The lower the number, the better the window is at blocking heat gain. Has replaced the shading coefficient as the standard indicator of a window's shading ability. In Europe this is the g-value.

Solar transmittance A number between 0 and 1 representing the ratio of the directly transmitted solar radiation to the incident solar radiation.

Sorbent A substance that has the property of collecting molecules of another substance by sorption.

Sound attenuators Components inserted into an air distribution system that reduce the airborne noise that is propagated along the ducts.

Source control A preventive strategy for reducing airborne contaminant levels in the air through removal of the material or activity generating the pollutants.

Sources of indoor air pollutants Indoor air pollutants can originate within a building or be drawn in from outdoors. Common sources include people, fixtures and furnishings, photocopiers, plants and food.

Space, conditioned An enclosed space that is provided with climate control (temperature and air quality).

Space, unconditioned A space that is neither a directly nor indirectly conditioned space, and which can be isolated from conditioned space by partitions or closeable doors.

Specific fan power (SFP) A measure of the electric power needed to drive a fan or fans relative to the amount of air that is circulated through the fans. Expressed in kW/m3 s.

Speech transmission index (STI) A measure of the intelligibility of speech, directly dependent of the background noise level, the reverberation time and the size of the room. STI values vary from 0 (completely unintelligible) to 1 (perfect intelligibility).

Split system A two-component heating and cooling (heat pump) or cooling only (air conditioner) system. The condensing unit is installed outside, and the air handling unit is installed inside (preferably in conditioned space). Refrigerant lines and wiring connect them together.

Stack effect A condition resulting from the rise of heated air, which creates positive pressure near the top of the building and negative pressure toward the bottom.

STI See speech transmission index.

System A combination of equipment and/or controls, accessories, interconnecting means and terminal elements by which energy is transformed to perform a specific function, such as climate control, service water heating or lighting.

System boundary A boundary that includes within it all areas associated with a building (both inside and outside of the building) where energy is used or produced. (EN 15603:2008)

TABS See thermally-active building system.

TBS See tight building syndrome.

Temperature asymmetry, radiant Difference between the plane radiant temperature of the two opposite sides of a small plane element.

Temperature difference, mean surface Difference between the average surface temperature and the design indoor temperature. It determines the heat flow density.

Temperature difference, vertical air Air temperature difference between head and ankles of a person. Note: 0.1 and 1.1 m for sedentary and 0.1 and 1.7 m above floor for standing.

Temperature drop Difference between the supply and return temperatures of the heating/cooling medium in a circuit.

Temperature, average surface Average value of all surface temperatures in the occupied or peripheral area.

Temperature, balance The outdoor temperature at which a building's internal heat gain (from people, lights and machines) is equal to the heat loss through windows, roof and walls.

Temperature, design indoor Operative temperature at the centre of a conditioned space used for calculation of the design load and capacity.

Temperature, excess The temperature difference between the supply air and the room temperature.

Temperature, mean radiant Uniform surface temperature of an enclosure in which an occupant would exchange the same amount of radiant heat as in the actual non-uniform enclosure.

Temperature, operative The uniform temperature of a radiant black body enclosure in which an occupant would exchange the same amount of heat as in the actual non-uniform environment (ISO 7730).

Temperature, plane radiant Uniform temperature of an enclosure where the radiance on one side of a small plane element is the same as in the non-uniform actual environment.

Temperature, room air The average of air temperatures measured at 1.1 m high, positioned out of the main air current from any heating or cooling device.

Temperature, set-back Minimum indoor temperature to be maintained during reduced heating periods, or maximum internal temperature to be maintained during reduced cooling periods.

Temperature, under- toz - ts: Difference between the room air temperature 1.1 metre above the floor and the temperature of the supply air.

Terminal device A device located in an opening provided at the boundaries of a ventilated space to ensure a predetermined motion of air in this space.

Test dust capacity Amount of test dust retained by a filter up to final pressure drop.

Thermal environment Characteristics of the environment that affect the heat exchange between the human body and the environment.

Thermal mass The ability of a material to store heat and energy, thereby slowing the temperature variation within a space. Typical thermal mass materials include concrete, brick, masonry, tile and mortar, water, and rock or other materials with a high capacity to store heat.

Thermally-active building system (TABS) Surface heating and cooling systems with pipes thermally coupled and embedded in a building structure (slabs, walls).

Thermostat A device that responds to changes in temperature and outputs a control signal. Usually mounted on the wall in a controlled space.

Thermostat, setback A device containing a timer mechanism that can automatically change the inside temperature maintained by the HVAC system according to a preset schedule. The heating or cooling requirements can be reduced when a building is unoccupied or when occupants are asleep.

Third-party financing A contractual arrangement involving a third party – in addition to the energy supplier and the beneficiary of the energy efficiency improvement measure – that provides the capital for that measure and charges the beneficiary a fee equivalent to a part of the energy savings achieved as a result of the energy efficiency improvement measure. That third party may or may not be an ESCO. (ESD, 2006/32/EC)

Threshold The contaminant dose or exposure level below which there is no expected significant effect.

Tight building syndrome (TBS) A condition in which a building is very tightly insulated against infiltration, its ventilation is reduced for energy conservation, and airborne contaminants are sufficiently elevated to cause health effects in occupants; often used synonymously with sick building syndrome (SBS).

Total volatile organic compounds (TVOCs) A measure representing the sum of all VOCs present in the air to provide an approximate indication of pollutant levels. Indoor air typically contains hundreds of different VOCs in very low concentrations, some of which can have additive effects.

Tracer step-down method A tracer gas technique used where an amount of gas is released into the room and the decay is registered.

Transducer An active device and component of a sensor that converts the raw, measured signal into a suitable signal, usually an electrical signal, which is a function of the change in the sensing element.

Transmittance The ratio of transmitted to incident energy. Usually denoted by the letter T or τ. A subscript e denotes energetic, i.e. solar transmittance (full solar spectrum). Subscript v denotes visual.

Transmittance, thermal See U-value.

Transmitter A device that converts the measured value to a standardized electrical signal that can be used as an input to a control module.

TSP Total suspended particles, a method of monitoring airborne particulate matter by total weight.

Turbulence intensity The ratio of the standard deviation of the air velocity to the mean air velocity. Used to measure variations in air velocity.

TVOCs See total volatile organic compounds.

U-value In the context of insulation, a measure of the rate of transfer of heat through a structure (a single material or a composite) divided by the difference in temperature across that structure. It is measured in watts per square metre per degree Kelvin (W/m²K). Generally, the lower the U-value the less energy is required to maintain comfortable conditions inside a building. Also called thermal transmittance. See also R-value.

Unit air cooler A refrigeration system component that transfers heat from air to a refrigerant or liquid consisting of one or more fans and a coil with refrigerant distributing and collecting headers.

UVC Ultraviolet light referring to light spectrum C(wavelength 280-100 nm).

Validation Procedure to test how accurately reality is represented.

Vapour A substance in a gaseous state, whose natural state is a liquid or solid form at normal atmospheric conditions.

Vapour seal A barrier that prevents air, moisture and contaminants from migrating through tiny cracks or pores in the walls, floor and ceiling into a critical space. Vapour barriers may be created using plastic film, vapour-retardant paint, vinyl wall coverings and vinyl floor systems, in combination with careful sealing of all openings (doors and windows) into the room.

Variable air volume system (VAV system) A ventilation system in which the airflow rates are continuously varied. The flow of a VAV system may vary according to a predetermined pattern or it may be determined by actual demand, e.g. demand-controlled ventilation.

Variable refrigerant flow (VRF)An HVAC system consisting of an outdoor condensing unit, multiple indoor evaporator units, refrigerant piping that transports the refrigerant between the units, and communication wiring. Each indoor unit can serve a different thermal zone in a building, and the refrigerant flow to each unit can be adjusted based on local requirements. It is considered a sophisticated, flexible and optimum-efficiency solution ideal for buildings with multiple spaces that require local control of both heating and cooling.

VAV See variable air volume.

VDI Verein Deutscher Ingenieure (The Association of German Engineers)

Velocity, face The discharge air flow rate divided by face area of air duct or terminal device.

Velocity, mean air The average value of the velocities.

Velocity, migration The velocity of a charged particle in an electric field. The average velocity of a particle migrating towards the collecting plate in the space between the high voltage and the grounded electrode in an ESP. In a standard ESP type this velocity is perpendicular to the gas direction of the gas flow.

Velocity, relative air Air velocity relative to the occupant, including body movements.

Vent A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.

Vent pipe A tube in which combustion gases from a combustion appliance are vented out of an appliance to the outdoors.

Ventilation The air exchange between the inside and the outside of a building through a specifically designed and installed ventilation system by means of a range of natural and/or mechanical devices. Depending on type of ventilation system, the air exchange rate is more or less controllable.

Ventilation effectiveness The relation between the pollution concentrations in the supply air, the extract air and the indoor air in the breathing zone (within the occupied zone). (EN 13779)

Ventilation flow rate The outdoor air flow supplied to a space to maintain acceptable indoor air quality.

Ventilation opening An intentional opening in a building envelope (e.g. trickle ventilator, louver, vent) designed to allow air to flow into and/or out of the ventilated building.

Ventilation rate Magnitude of outdoor air flow to a room or building either through the ventilation system or infiltration through building envelope. (EN 15251)

Ventilation system A combination of appliances designed to supply interior spaces with outdoor air and/or to extract polluted indoor air. (EN 15251)

Ventilation, balanced A ventilation system with mechanical supply and exhaust.

Ventilation, cross Natural ventilation in which the air flow mainly results from wind pressure effects on the building facades.

Ventilation, demand controlled (DCV) Ventilation system with feed-back and/or feed-forward control of the air flow rate according to a measure demand indicator. Demand is decided by set values affecting thermal comfort and/or air quality.

Ventilation, displacement Ventilation system with displacement air supply.

Ventilation, exhaust Mechanical removal of air from a building.

Ventilation, hybrid Ventilation in which natural ventilation may be at least in a certain period supported or replaced by mechanical ventilation.

Ventilation, mechanical Ventilation that is aided by powered air movement components.

Ventilation, natural Ventilation provided by thermal, wind or diffusion effects through doors, windows or other intentional openings in a building.

Ventilation, purge Manually controlled ventilation of rooms or spaces at a relatively high rate to rapidly dilute pollutants and/or water vapour. Purge ventilation may be provided by natural means (e.g. an open able window) or by mechanical means (e.g. a fan).

VOCs See volatile organic compounds.

Volatile organic compounds (VOCs) Chemical organic compounds that vaporize (become a gas) at room temperature. Common sources that can emit VOCs into indoor air include housekeeping and maintenance products, and building and furnishing materials.

Volume, space The total volume of an occupiable space enclosed by the building envelope, plus that of any spaces permanently open to the occupiable space.

VRF See variable refrigerant flow.

ZEB See net zero energy building.

Zone An area within the interior space of a building, such as an individual room, to be cooled, heated or ventilated. A zone has its own thermostat to control the flow of conditioned air into the space.

Zone, buffer A space between conditioned zones and the outside that is typically not conditioned. Examples include attics, attached garages, crawlspaces, basements and enclosed porches.

Zone, non-smoking That area or volume of a space within which smoking is not permitted.

Zone, occupied That part of space designed for human occupancy and where the design criteria of the indoor environment is required to be met. Normally the zone between floor and 1.8 m and 1.0 m from external walls-windows and HVAC equipment and 0.5 m from internal walls.

Zones, temperature Individual rooms or zones in a building where temperature is controlled separately from other rooms or zones.

Zoning The combining of rooms in a structure according to similar heating and cooling patterns.