Glossary for Underfloor Air Distribution

Glossary for Underfloor Air Distribution

A platform structure typically consisting of 0.6 m x 0.6 m (2 ft x 2 ft) concrete-filled steel floor panels supported on pedestals 0.2 to 0.46 m (8 in. to 18 in.) above the concrete structural floor slab. Each panel can be independently removed for easy access to the underfloor plenum created below and can include openings for electrical outlets, grilles or any other floor accessory in its thickness. In most office installations, carpet tiles are laid on top to provide a finished floor surface. Raised floor systems provide maximum flexibility and significantly lower costs associated with reconfiguring building services.

Active diffuser

Any air supply outlet that relies on a local fan to deliver air from the plenum through the diffuser into the conditioned space of the building.

Air change effectiveness (ACE)

Air change effectiveness describes the ability of an air distribution system to provide ventilation (outside) air at the breathing zone (where occupants breathe).ACE is defined as the age of air that would occur throughout the space if the air was perfectly mixed, divided by the average age of air where occupants breathe.

Air changes per hour (ACH)

A measure of the air exchange rate of a building, or space, that gives the time unit in hours.

Air exchange rate

A measure of the rate at which the volume of air contained within a space is replaced by supply (outside, conditioned or re-circulated) air. This is expressed in terms of air changes per hour (ACH), and found by dividing the airflow rate (volume per hour) by the volume of the space, or building.

Air flow

The movement of air — typically defined as that within a defined volume such as a room, duct or plenum.

Air handling unit (AHU)

The component of an HVAC system that is responsible for conditioning and delivering air through the system. Within the AHU, a portion of the return air from the conditioned space is recirculated and mixed with incoming outside air for conditioning and delivery to the space, and the remainder is exhausted to the outside. The AHU typically contains one or more supply and return fans for maintaining air movement, and heating/cooling coils and filters to condition the air.The cooling coil and other equipment, as necessary, are used to control the moisture content of the air.

Air inlet (see also Air outlet)

Inlets are apertures through which air is intentionally drawn from a conditioned space. Grilles, diffusers and louvered openings can all serve as inlets. Examples are return inlets at ceiling level and floor diffusers that become return inlets for specially designed perimeter heating solutions for open plenum designs.

Air outlet (see also Air inlet)

Outlets are apertures through which air is intentionally delivered into a conditioned space. Grilles, diffusers and louvered openings can all serve as outlets. Examples are floor and ceiling diffusers.

Air supply volume

The volume of supply air flowing through a cross sectional plane of a duct per unit time. Found by multiplying air velocity by the cross sectional area of the duct, measured in cubic feet per minute (cfm) or liters per second (L/s).

Air velocity

The rate at which air travels in a given direction, measured as a distance per unit time. The units used vary according to the scale of the phenomenon, in the HVAC field, air velocity is commonly expressed as feet per minute (fpm) or meters per second (m/s).

Ambient air

Air in the general surroundings of the space in question, whether an external or internal space. Generally this refers to areas outside of work locations for the building occupants.

ASHRAE

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Cable Management

Addresses the distribution, routing and overall organization of cable networks installed in underfloor plenums. Raised floors came into widespread use as a means of containing and concealing the extensive cabling of typical voice, power and data systems, and are now a common feature in contemporary office buildings. In this respect, concerns that the installation of an UFAD system will entail additional construction costs can be mitigated — the decision to install a raised floor system is often made for communications purposes, regardless of the type of HVAC system chosen.

Ceiling-based systems

A ceiling-based air distribution system supplies air to, and removes air from, a conditioned space at ceiling level. Both supply and return grilles are located in the ceiling plane, above which there will be a ceiling plenum of sufficient depth to accommodate the extensive supply ductwork, as well as other building services. Relying on the principle of mixing-type air distribution, ceiling-based supply and return systems are designed to condition the entire volume of the space (floor-to-ceiling), thereby providing a single uniform thermal and ventilation environment. This control strategy provides no opportunity to satisfy different thermal preferences among the building occupants.

Ceiling plenum

The open space between the underside of a structural concrete slab and a suspended ceiling, through which supply (ceiling-based system) or return air (both ceiling- and floor-based systems) is delivered.

Churn rate

This term (%/year) is used to describe the annual percentage of workers and their associated work spaces in a building that are reconfigured or undergo significant changes. A recent IFMA survey found an average churn rate of 44% for U.S. office buildings. Although primarily addressing the reorganization of staff members, any changes within the personnel-structure of a company typically involve the relocation, upgrading, or expansion of equipment, office furniture and even space planning. With conventional ceiling-based HVAC systems changes in workspace configurations can be restricted by the location of ceiling grilles; due to the higher cost associated with reconfiguring overhead ducted systems, these changes are often not made, resulting in potentially poorer performance by the system. The flexibility of UFAD systems, in terms of quick replacement/relocation of diffusers and easy access to the underfloor plenum help reduce a company’s churn costs significantly. However, by being located on the floor, UFAD diffusers will generally need to be reconfigured in response to changes in the office layout more frequently than with overhead systems; this increases the likelihood of improved system performance.

Clear zone

During the placement of floor diffusers, a clear zone is typically defined as an imaginary cylinder of specified diameter around the center point of the diffuser. Clear zones are generally 0.9-1.8 m (3-6 ft) in diameter, depending on manufacturer’s data, and represent an area within which long-term occupancy is not recommended.

Although local thermal conditions may be acceptable for short-term occupancy, and when under direct individual control by the occupant, air velocities may be too high and temperatures too low (under cooling conditions) within clear zones to satisfy the thermal comfort preferences of a large majority of occupants ( 80%). Diffuser placement should take this into consideration and maintain a distance of at least half the diameter of the clear zone between occupants’ seating and their diffuser.

Conditioned air

Air that has been treated, typically in an AHU, by altering one or more of the following properties: temperature, humidity, cleanliness (filtering), or mixture of outside and recirculated air.

Conditioned space

A space within a building served by an HVAC system supplying conditioned air in order to achieve acceptable thermal comfort and indoor air quality conditions.

Constant air volume, variable temperature (CAV-VT)

A control strategy of an air supply system in which varying heating and cooling loads are met by adjusting the temperature of the supply air, keeping the air flow volume constant. Alternatively a variable air volume (VAV) system can be employed in which the air flow volume is varied, while the temperature remains constant.

Conventional systems

A typical, conventional air distribution system supplies air to, and removes air from, a conditioned space at ceiling level. Both supply and return grilles are located in the ceiling plane, above which there will be a ceiling plenum of sufficient depth to accommodate the extensive supply ductwork, as well as other building services. Relying on the principle of mixing-type air distribution, ceiling-based supply and return systems are designed to condition the entire volume of the space (floor-to-ceiling), thereby providing a single uniform thermal and ventilation environment. This control strategy provides no opportunity to satisfy different thermal preferences among the building occupants.

Cooling load

In the context of HVAC systems, the cooling load of a space is the amount of heat generated within that space (from any source) which the HVAC system must remove. Sources of heat in an office space typically include occupants, electrical equipment, artificial lighting and solar radiation through the building envelope.

Core zone

Typically the area at the center of the floor plan containing the services and circulation spaces — such as the elevator shaft, fire escape stairs and equipment room. The AHU is also often located in this zone.

Damper

A device that varies the volume of air flowing through a contained cross section (e.g. a duct, inlet, outlet or plenum) by varying the cross sectional area through which the air is routed.

Diffuser

An air supply outlet through which conditioned air is discharged into a space. A wide variety of diffusers can be located in the ceiling (ceiling-based HVAC system), floor (underfloor air distribution system), or integrated into the furniture (task/ambient condition system), and configured to deliver air in various directions and patterns. For more information, see our section on Diffusers in How Does It Work?

Displacement ventilation (DV)

In displacement ventilation systems (used for cooling only), low-velocity supply air at a temperature slightly below room temperature is introduced into the occupied zone of a space at low level — diffusers are usually configured as large-area floor pedestals or low side-wall. By extracting air from the space at ceiling level, an overall floor-to-ceiling air flow pattern is produced. This upward movement of air in the room takes advantage of the natural buoyancy of heat gain to the space. As air is heated and rises into the region above the occupied zone, some of it exits the space with only partial mixing with the room air. Space contaminants also migrate upwards producing higher concentrations in the warm stratified air near the ceiling. Displacement ventilation systems aim to minimize mixing of supply air with room air, instead maintaining conditions in the occupied zone as close as possible to that of the conditioned supply air, leading to an improved air change effectiveness.

Draft

Movement of air causing undesirable local cooling of a body due to one or more of the following factors: low air temperature, high velocity or inappropriate air flow direction.

Dry-bulb Temperature

The air temperature indicated by an ordinary thermometer.

Duct

A duct is an encased conduit, usually constructed of sheet metal and having a round, square, or rectangular cross-section, through which air moves around an HVAC system. Other types of duct construction include fibrous glass ducts (rigid fiberglass with aluminum facing) and flexible ducts (used to connect diffusers, mixing boxes, and other terminal units to the air distribution system).

Ductwork

The network of ducts comprising an HVAC system, typically connecting the AHU to supply, return, intake and exhaust grilles, and underfloor and ceiling plenums. Ductwork can be exposed or concealed within floor or ceiling plenums, services zones and plant rooms.

Economizer (see Outside air economizer)

Energy use

A term referring to the total energy used by a system in the course of its operation. In the context of HVAC this would include energy used by components such as fans, refrigeration and heating equipment, cooling towers, and pumps.

Entrainment (see also Secondary air motion)

Air discharged from an outlet creates a swirling, jet, or other air motion that pulls (entrains) the surrounding air into its path where it mixes with the supply air.

Exfiltration (see also Infiltration)

The uncontrolled, unintentional, flow of inside air out of a building. This can occur through cracks in any building component, around openings that are not airtight, and during the everyday use of windows and doors. Like natural ventilation, infiltration is caused by differences in air-pressure or density between inside and outside.

Exhaust air

The air extracted from a space and discharged to the outdoors. This is distinct from air extracted from one space and sent to another, or recirculated within the HVAC system.

Exchange rate (see Air exchange rate)

Exhaust opening, or inlet

Any opening, a grille for example, through which air is removed from a space.

Fan coil unit

A fan terminal unit with a heating (electric or hot water) and/or cooling (chilled water) coil on the discharge of the unit.

Fan-powered mixing box

A compartment containing an integral fan that mixes two air supplies before being discharged. In underfloor applications, these boxes may be configured as having one ducted inlet supplying room or return air, for example, to be mixed with plenum air entering the box through an unducted opening. A reheat coil can be added to the discharge of the unit.

Fan terminal unit

A compartment containing an integral fan that delivers a constant or variable volume of air to the space. These units are often used in perimeter and other special zones where large and rapid changes in cooling and/or heating load requirements occur.

First costs

The initial costs involved in a building project, typically incurred during the construction and installation stages.

Floor-to-floor height

The vertical height between the finished-floor level of a space in a multi-story building and that of the floor immediately above or below it.

Forced ventilation

A term used to describe the use of fans and intake and exhaust vents to mechanically distribute ventilation and other conditioned air throughout a building. Buildings operating forced ventilation systems are generally pressurized to reduce infiltration. This term is often contrasted with natural ventilation.

Grille

A perforated or louvered covering on any area that air passes through. Grilles can be placed in the ceiling, floor or wall and can be fixed, or adjustable.

HVAC system

An HVAC system is one that is able to provide heating, ventilating and air-conditioning to a building, either as a combined process or as individual operations.

Individual control

Used to describe a system incorporating individual, or occupant, control in which occupants are able to adjust the operating parameters according to their personal preferences. In the context of HVAC, underfloor systems can include grilles designed for easy occupant adjustment of the direction and volume of supply air serving their workspace.

Indoor air quality (IAQ)

This term generally refers to quantifiable properties of the respirable air inside a building. Chemical, biological and physical factors — such as the air temperature, humidity, gaseous composition, and concentrations of pollutants — are considered indicators of the quality of air occupants are exposed to. Providing a sufficient rate of ventilation to exhaust heat, moisture and pollutants generated inside a building is a key component of meeting IAQ standards such as those in ASHRAE Standard 62; which provides designers with guidelines for achieving acceptable ventilation rates and indoor air quality.

Infiltration (see also Exfiltration)

The uncontrolled, unintentional, flow of outdoor air into a building. This can occur through cracks in any building component, around openings that are not air-tight, and during the everyday use of windows and doors. Like natural ventilation, infiltration is caused by differences in air-pressure or density between inside and outside.

Interior zone

Spaces located further than 5 m (15 ft) from the faзade, which can be either high-occupancy (accommodating a number of work spaces) or low-occupancy (circulation or general meeting areas for example). Spaces within this zone are not directly affected by loads generated by the building envelope, such as solar heat gain or heat loss.

Isothermal

Of constant temperature (e.g. an isothermal air jet has the same temperature as the surrounding air).

Life-cycle costs

A measure of the total costs involved in a building project, calculated by including initial costs (e.g. construction and installation) and those estimated over the lifetime of the building (e.g. long-term operation and maintenance). Considerations of life-cycle costs are important when making decisions at the initial design stage.

Lower zone

The volume of a conditioned space below the stratification height produced by a DV or UFAD system

Mechanical ventilation

A term used to describe the use of fans and intake and exhaust vents to mechanically distribute ventilation and other conditioned air throughout a building. Buildings operating mechanical ventilation systems are generally pressurized to reduce infiltration. This term is often contrasted with natural ventilation.

Mixing systems (also known as mixing-type air distribution)

In mixing systems, conditioned air is delivered to the space at velocities much greater than those acceptable to occupants. Conventional overhead air distribution is an example of a mixing system. Supply air temperature may be above, lower, or equal to the air temperature in the occupied zone. The incoming high-velocity air mixes rapidly with the room air by entrainment so that by the time it enters the occupied zone its temperature and velocity are within an acceptable range. Mixing systems are designed to maintain the entire volume of air in the space (floor-to-ceiling) at a relatively uniform temperature, humidity, and air quality condition.

Natural ventilation

When air moves into and out of a building through intentional or planned routes, without the assistance of mechanical equipment, this is termed natural ventilation. Generally driven by pressure differences, inlets and outlets include windows, doors, grilles, roof-openings and other designed apertures. This is often contrasted with forced or mechanical ventilation.

Occupant control

Used to describe a system incorporating individual, or occupant, control in which occupants are able to adjust the operating parameters according to their personal preferences. In the context of HVAC, underfloor systems can include grilles designed for easy occupant adjustment of the direction and volume of supply air serving their workspace.

Occupied zone

The volume of a conditioned space containing the occupants of the space. Typically this is taken as extending from floor level up to a height of 1.8 m (6 ft), and sometimes considered as set in 0.6 m (2 ft), on plan, from external walls.

Outside air

This term can denote either the air outside a building, or air taken into a building that has not previously been circulating through the HVAC system.

Outside-air economizer

An HVAC control strategy that uses outside air under suitable climate conditions to reduce the required mechanical cooling. When the outside air temperature is less than the required supply air temperature during cooling periods, the economizer allows a building’s mechanical ventilation system to use up to 100% outside air, thereby reducing the energy required to cool the mixture of outside air and warm recirculated air under normal operating conditions. This method of cooling, often described as ‘free cooling’, is widely used in temperate climates where outside air temperatures rarely go above 21-24°C (70-75°F) during most days and will periodically be less than the supply air temperatures (night time economizer cycles are frequently employed, for example). As UFAD systems supply air at a higher temperature than that for ceiling-based systems (typically 18°C (65°F) for UFAD, 13°C (55°F) for ceiling HVAC), many North American temperate climates will have a significantly larger number of daytime hours during which the economizer can be used. Some method of variable volume relief must be provided to exhaust the extra outside air to the outside. In addition, enthalpy-based economizer control is recommended to maintain proper humidity levels (particularly during nighttime) and protect against condensation in the plenum.

Overhead systems

A typical, overhead air distribution system supplies air to, and removes air from, a conditioned space at ceiling level. Both supply and return grilles are located in the ceiling plane, above which there will be a ceiling plenum of sufficient depth to accommodate the extensive supply ductwork, as well as other building services. Relying on the principle of mixing-type air distribution, ceiling-based supply and return systems are designed to condition the entire volume of the space (floor-to-ceiling), thereby providing a single uniform thermal and ventilation environment. This control strategy provides no opportunity to satisfy different thermal preferences among the building occupants.

Passive diffuser

Any air supply outlet that relies on a pressurized underfloor plenum to deliver air from the plenum through the diffuser into the conditioned space of the building. Passive diffusers have no local fans associated with them, although they can be converted to an active diffuser by attaching a fan-powered outlet box to the underside of the diffuser.

Perimeter zone

This is the zone immediately adjacent to, and within 5 m (15 ft) of, the external faзade. Perimeter spaces require special consideration in terms of their heating and cooling loads, which are significantly different to those of internal/core zone areas due to the influence of factors such as solar gain and fabric heat loss through the building envelope.

Plenum (see also Service plenum)

Any defined space, typically above a suspended ceiling or beneath a raised floor, through which supply air and/or voice, power and data cabling and other building services can be distributed.

Plenum height

The vertical distance between the top surface of a structural floor slab and the top surface of the raised floor system above it, which contains, and defines, the underfloor plenum. Accounting for the typical thickness of raised floor panels, the clear space within the underfloor plenum will be 33 mm (1.3 in.) less than the plenum height.

Plenum inlet

Any location in an underfloor plenum where conditioned air that has been ducted from the air handler is discharged into the plenum.

Plenum partition

A partition, typically formed from vertically oriented sheet metal, erected within the plenum in order to divide up the plan of a conditioned space and create separate zones within the underfloor area.

Pressurized plenum

In this system configuration, the underfloor plenum is under a positive static pressure produced by the central AHU that drives the air along the plenum and up through the diffusers. Typical pressures are quite low (12.5-50 Pa [0.05-0.2 in. H2 O]).

Psychrometric

Relating to psychrometry, the study of atmospheric conditions –particularly the level of moisture in air. In terms of HVAC systems, psychrometric charts are useful for illustrating the relationship between properties such as wet-and dry-bulb temperatures, and absolute and relative humidities when determining the desired supply air conditions.

Raised floor

A platform structure typically consisting of 0.6 m x 0.6 m (2 ft x 2 ft) concrete-filled steel floor panels supported on pedestals 0.2 to 0.46 m (8 in. to 18 in.) above the concrete structural floor slab. Each panel can be independently removed for easy access to the underfloor plenum created below and can include openings for electrical outlets, grilles or any other floor accessory in its thickness. In most office installations, carpet tiles are laid on top to provide a finished floor surface. Raised floor systems provide maximum flexibility and significantly lower costs associated with reconfiguring building services.

Recirculated air

Return air that is diverted from the exhaust route, mixed with incoming outside air (in some systems, recirculated air bypasses the cooling coil and is mixed with the cool air leaving the coil to produce the warmer supply air temperatures used in UFAD designs), passed through the AHU for conditioning, and delivered to the conditioned space –- essentially a means of recycling the air circulating through an HVAC system for energy saving purposes.

Return air

The air extracted from a conditioned space (typically at ceiling level) and returned to the air-handling unit (AHU), where a portion is recirculated and the remainder is exhausted to the outside.

Secondary air motion (see also Entrainment)

Air discharged from an outlet creates a swirling, jet, or other air motion that pulls (entrains) the surrounding air into its path where it mixes with the supply air.

Sensor

A device that can detect and measure a variable, for example air temperature, velocity, humidity, or light levels.

Service plenum

Any defined space, typically above a suspended ceiling or beneath a raised floor, through which supply air and/or voice, power and data cabling and other building services can be distributed.

Stagnant zone

A volume of a space in which there is low air velocity and the potential for increased stratification and poorer air quality.

Static pressure (see also Total pressure; Velocity pressure)

Pressure is the force exerted per unit area by a gas or liquid. In air distribution systems, static pressure is equal to the total pressure minus velocity pressure and represents the pressure exerted by the air at rest. Air distribution pressures are typically measured in inches of water (in. H2O) or Pascals (Pa).

Stratification (see also Thermal stratification)

The creation of a series of horizontal layers of air with different characteristics (e.g. temperature, pollutant concentration) within a conditioned space. UFAD systems, and other displacement ventilation-based systems, rely on the upward convection of air driven by thermal plumes to remove heat loads and contaminants from a space. This results in both thermal and pollutant stratification in which a layer of warmer, more polluted air forms above the occupied zone where it will not affect the occupants.

Stratification height (see also Displacement ventilation)

In a displacement ventilation system, a horizontal interface, known as the stratification height, is established at the height in the room where the air flow rate in the thermal plumes equals the total supply air volume entering the room at or near the floor level. The stratification height divides the room into two zones (upper and lower) having distinct air flow conditions. The lower zone below the stratification level has no recirculation and is close to displacement flow. The upper zone above the stratification level is characterized by recirculating flow producing a fairly well mixed region. In a properly designed displacement ventilation system, the stratification height is maintained near the top of the occupied zone (1.8 m [6 ft]). In UFAD systems, a stratification height similar to that found in DV systems is formed, but the airflow conditions in the lower zone, and in some cases the upper zone, are changed due to the greater mixing provided by the turbulent floor diffusers.

Supply air

The air entering a space through an outlet, diffuser, or grille, having been delivered from the air-handling unit (AHU).

Supply duct

Any duct through which supply air is delivered to the conditioned space from the AHU, local fan, or other air movement device.

Task/ambient conditioning (TAC) system

Any space conditioning system that allows occupants to individually control the thermal environment in the localized zone of their work space while still maintaining acceptable environmental conditions in the building’s ambient spaces (circulation and open-use spaces for example). This is typically achieved by enabling occupants to adjust the volume and direction of the air supply serving their workspace, according to their personal preferences. TAC systems therefore generally include a large number of supply diffusers throughout a building, many located in close proximity to the occupants. Although not a requirement, most TAC systems are integrated with the use of underfloor air distribution.

Thermal comfort

That condition of mind that expresses satisfaction with the thermal environment. Thermal comfort is influenced by both subjective and objective factors. Heat transfer between the human body and the environment, and hence acceptance of the thermal environment is influenced by a combination of environmental factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activity level). There is also evidence that people who know they have control over their local thermal environment are more tolerant of temperature variations, making it easier to satisfy their comfort preferences.

Thermal plume

The upward movement of warm air due to buoyancy forces above a heat source (e.g. person, computer, lights) in a room. The air volume in a rising thermal plume increases with height as the plume entrains ambient air.

Thermal stratification (see also Stratification)

The creation of a series of horizontal layers of air having increasing temperature with height within a conditioned space. UFAD systems, and other displacement ventilation-based systems, rely on the upward convection of air driven by thermal plumes to remove heat loads and contaminants from a space. This results in both thermal and pollutant stratification in which a layer of warmer, more polluted air forms above the occupied zone where it will not affect the occupants.

Thermostat

An automatic control device that is responsive to temperature and used to control temperature in a conditioned space or zone. In the context of UFAD systems, thermostats located in an office space (typically installed on walls) register changes in ambient air temperature. These devices communicate information to the HVAC control unit, which adjusts the temperature, or air flow volume, of the supply air to maintain the temperature measured at the thermostat within a pre-programmed comfort range around a setpoint temperature.

Thermostatic control

A means of automatically controlling the operation of an HVAC system component, collection of components, or complete system in response to information about air temperatures as registered by one or more thermostats located within the conditioned space.

Total pressure (see also Static pressure; Velocity pressure)

Pressure is the force exerted per unit area by a gas or liquid. In air distribution systems, total pressure is equal to the sum of static pressure and velocity pressure. Air distribution pressures are typically measured in inches of water (in. H2O) or Pascals (Pa).

Underfloor air distribution (UFAD) system

An underfloor air distribution (UFAD) system uses an underfloor plenum (open space between the structural concrete slab and the underside of a raised floor system) to deliver conditioned air, from the AHU, directly into the occupied zone of the building. Air can be delivered through a variety of supply outlets typically located at floor level, or integrated as part of the office furniture and partitions. Return grilles are located at ceiling level, or at least above the occupied zone. Under cooling conditions, underfloor systems produce an overall floor-to-ceiling air flow pattern, similar in principle to displacement ventilation. This upward convection of warm air is used to efficiently remove heat loads and contaminants from the space. In contrast to true displacement ventilation systems, UFAD systems deliver supply air at higher volumes and higher velocities, enabling higher heat loads to be met. Although the supply air is delivered in close proximity to occupants, the risk of draft discomfort is minimized, as supply air temperatures are higher than those for conventional ceiling-based systems, and occupants have some amount of control (typically volume and sometimes direction and temperature) over their local air supply conditions.

Underfloor plenum

The open space between a structural concrete slab and the underside of a raised floor system. Commonly used as the access route for telecommunications cabling, in underfloor systems, the supply air is also delivered through this space.

Upper zone

The volume of a conditioned space above the stratification height produced by a DV or UFAD system.

Variable air volume (VAV)

A control strategy of an air supply system in which varying heating and cooling loads are met by adjusting the air flow volume, keeping the temperature of the air constant. Alternatively a constant air volume, variable temperature (CAV-VT) system can be employed in which the temperature of the air flow is varied, while the volume is kept constant.

VAV box

A variable air volume control box. Typically, a VAV box is ducted on its inlet and uses dampers to control the volume of air discharged from the unit.

Velocity pressure (see also Static pressure; Total pressure)

Pressure is the force exerted per unit area by a gas or liquid. In air distribution systems, velocity pressure is the pressure due to the velocity and density of the moving air. Air distribution pressures are typically measured in inches of water (in. H2 O) or Pascals (Pa).

Ventilation

The process of intentionally supplying outside air to a building achieved by either natural or mechanical (forced) means.

Ventilation effectiveness

Ventilation effectiveness describes the system’s ability to remove pollutants generated by internal sources in a space, zone, or building. In comparison, air change effectiveness describes the ability of an air distribution system to ventilate a space, zone, or building.

Zero-pressure plenum

In this system configuration, the underfloor plenum is maintained at very nearly the same static pressure as that of the conditioned space. Supply air is delivered to the plenum by the central AHU, and small fan-powered air outlets are used to discharge air from the plenum into the conditioned space. Some systems may create a slight negative pressure in the plenum to draw recirculated air (typically directly from the room through open floor grilles, or down from the ceiling through shafts) into the plenum where it is mixed with the supply air from the AHU.

Zone

Also known as a control zone for an HVAC system, a zone is defined as a space or group of spaces in a building having similar heating and cooling requirements throughout its occupied area so that comfort conditions may be controlled by a single thermostat.

Zoning (see also Interior zone; Perimeter zone)

The practice of dividing a building into smaller zones for control of the HVAC system. For example, buildings may be zoned into individual floors, rooms, or spaces with distinct loads, such as perimeter and interior zones.


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