Most Useful -Terms & definitions of HVAC

For the past 17 years, Holtop has been engaging on heat recovery ventilation equipment and I, personally by taking constant sips of industrial savvy from multiple experts for the past 6+ years, have had a good handle on the comprehensive knowledge of ERV/HRV.

During daily work and learning, the list below about terms and definitions is the one I most use, I hope it helps for you.

For easy checking, you can press “Ctrl+F” to search the terms you want.

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 e.g. smell and irritation (sensory measurements).

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

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.

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).

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

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

Ventilation rate. The magnitude of outdoor air flow to a room or building either through the ventilation system or infiltration through the building envelope.

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

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 feedback 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, exhaust. Mechanical removal of air from a building.

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

Ventilation, mechanical. Ventilation with the aid of powered air movement components.

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

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

Heat exchanger. A device in which heat is transferred between two mediums that don't come in contact.

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 which are provided to retard the bypassing and leakage of air from one air stream to the other.

Heat pump. A machine, a 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.

Heat recovery. Heat utilized from a system, which would otherwise be wasted. (E.g. Heat transferred from exhaust air into supply air)

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

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.

Aerosol. A suspension of liquid or solid particles in air.

Air change rate. Ventilation air flow rate divided by room volume. It indicates how many times, during a time interval, the air volume from space is replaced with outdoor air.

Air change rate, nominal. 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 how many times, during one hour, the air volume from a space is replaced with outdoor air.

Air cleaner. The device used for the removal of airborne particulates and/or gases from the air. Air cleaners may be added to HVAC systems or stand-alone room units.

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

Air contaminant. Any material in the atmosphere that affects persons and their environment (pollutant includes materials such as liquids, solids, aerosols, gases, and odors). The term is used interchangeably with air pollutants.

Air diffusion. Distribution of the air in a space by means of air terminal devices, in a manner, so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity, and noise level.

Air extract, mechanical. The process of extracting air with the aid of powered air movement components, usually fans. Air flow rate, mass. Mass flow of air over a specified time usually expressed in kg/s or kg/h.

Air flow rate, volumetric. The volumetric flow of air over a specified time usually expressed in l/s or m3/h.

Air handling unit. 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, humidifying, dehumidifying and mixing of air, and necessary controls functions.

Air leakage factor. The air leakage per unit envelope area.

Air pollutant.  Any material in the atmosphere that affects persons and their environment (pollutant includes materials such as liquids, solids, aerosols, gases, and odors). The term is used interchangeably with air pollutants.

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 occupied space.

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

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

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

The air-handling unit, decentralized. In contrast to the central air-handling units, these units are 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, net floor. A term used in the ISO standard to express the Interior Gross Area less the areas of all interior walls.

Area, occupied. The area within the 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/space, living floor. The total area of rooms falling under the concept of rooms. (OECD Glossary of statistical terms)

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

Boiler. The combined boiler body and burner-unit designed to transmit to water the heat released from combustion. 

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 discretized equations in a CFD (computational fluid dynamics) solution or any other physical problem.

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.

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

Certificate, energy performance. A certificate recognized by the 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.

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

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

Coil, heating. Heat exchanger, which ads heat to the air stream by means of a heat transfer medium.

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 system meets 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.

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

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

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

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

Damper. The damper is 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.

Dehumidification. The reduction of water content in 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.

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

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

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 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. 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.

Efficiency (filtration). Removal of dust in a filter and expressed in %.

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

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 performance of a building. The calculated or measured amount of energy delivered and exported actually used or estimated to meet the different needs associated with standardized use of the building, which may include, inter alia, energy used for heating, cooling, ventilation, domestic hot water, lighting, and appliances.

Energy Performance of Buildings Directive (EPBD). The Energy Performance of Buildings Directive, an EU Directive of late 2002 aiming at improving the energy performance of buildings, strengthened and accelerated in 2010 by the ‘Recast EPBD’.

Envelope, building. Integrated elements of a building that separate its interior from the outdoor environment.

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

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

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

Filter. Device for removing particulate material and gases from air.

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

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

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

Humidification. Addition of water vapor to room air or supply air.

Humidifier. A device used for humidification. Humidity, absolute. The absolute amount of water vapor in ambient air expressed in g/kg or g/m3 dry air.

Humidity, relative. The pressure of water vapor in the air by volume divided by pressure of water vapor by volume at saturation at the same temperature.

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

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

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.

Load calculation. A process to determine the heat gain and heat loss in a building so that properly sized air conditioning and heating equipment may be installed.

Noise rating (NR). The noise rating curves are developed by the International Organization for Standardization (ISO) to determine the acceptable indoor environment for hearing preservation, speech communication, and annoyance.

Operation and maintenance. Actions are 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.

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

Parts per million (ppm). The number of parts of a substance by volume in a million total parts.

PM10. The total mass of suspended particles with diameters less than 10 μm in m3 of air.

PM2,5. The total mass of suspended particles with diameters less than 2.5 μm in m3 of air.

Pollution. Presence of undesired elements, which are deteriorating to the comfort, health, and welfare of persons or the environment (pollution includes elements such as noise, vibration, odors and gases).

Predicted mean vote (PMV). Predicted Mean Vote is 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). The index that predicts the percentage of a large group of people likely to feel thermally dissatisfied for the body as a whole, i.e. either too warm or too cool.

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

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

Sensor. The device, which converts a physical, chemical, 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, a transmitter.

Sick Building Syndrome (SBS) symptoms. Non–specific symptoms experienced by building occupants may include irritation of 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 is not related to any known disease or exposure.

Sound attenuators. Components, which are inserted into the air distribution system and designed to reduce airborne noise propagated along the ducts.

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

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

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

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

Split system. A two-component heating and cooling (heat pump) or cooling only (air conditioner) system. The condensing unit is installed outside, 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.

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

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

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. 

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

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

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

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.

Validation. Procedure to test how accurately reality is represented.

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

Variable air volume system (VAV system). A ventilation system where 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.

Velocity, mean air. The average value of the velocities.

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

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

Zone, occupied. That part of space designed for human occupancy and where the design criteria of the indoor environments are 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.

*Source: REHVA – Federation of European HVAC associations