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Glossary
of Terms
A
Access 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.
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.
C
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.
D
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.
E
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.
F
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.
G
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.
H
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.
I
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 façade, 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).
L
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
M
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.
N
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.
O
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.
P
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 façade. 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. H2O]).
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.
R
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.
S
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.
T
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).
U
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.
V
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. H2O) 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.
Z
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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