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Although the number of publications on underfloor technology has been on the
increase in recent years, until recently there were no standardized design
guidelines for use by the industry. Designers having experience with underfloor
systems had largely developed guidelines and practices of their own. To address
this situation, the American Society of Heating, Refrigerating, and
Air-Conditioning Engineers (ASHRAE) initiated a research project to develop and
publish a design guide on underfloor air distribution (UFAD) and task/ambient
conditioning (TAC) systems. CBE completed this project in December of 2003, and
the design guide is now available from ASHRAE.
This section was developed before the completion of the Underfloor
Air Distribution (UFAD) Design Guide published by ASHRAE. Please refer to
the design guide for the most comprehensive and up-to-date guidelines.
UFAD vs. Conventional Overhead System Design
UFAD systems are quite similar to conventional overhead systems in terms of the
types of equipment used at the cooling and heating plants and primary air
handling units (AHU). Key differences arise with UFAD systems in their use of
an underfloor air supply plenum, localized air distribution (with or without
individual control) and the resulting floor-to-ceiling air flow pattern, and
the solutions used for perimeter systems. Although the methods used to
calculate cooling and heating loads for a building with an UFAD system are
similar to those used for conventional overhead systems, the calculation of
design cooling air supply volumes requires different considerations. This is
primarily due to the fact that overhead systems assume a single well-mixed
temperature throughout the space (floor-to-ceiling), while UFAD systems assume
that some amount of stratification occurs. The stratified floor-to-ceiling air
flow pattern in UFAD systems allows most convective heat gains from sources
outside the occupied zone (up to 1.8 m [6 ft]) to be exhausted directly at
ceiling level, and therefore to not be included in the air-side load (see
further discussion below). In order to successfully employ an UFAD and raised
access floor system, it is essential that the implications of these differences
be considered, starting at an early stage in the design process. The following
is a step-by-step list of issues to be considered, and decisions to be made,
during the design process. Click on any step listed below for more information.
UFAD System Design Process:
1. Initial Building
Space Cooling and Heating Loads
Ventilation Air Requirements
Zone Supply Air Temperature and Flow Rate
Return Air Configuration
Cooling Coil Load
Ducts and Plenum Configuration
Primary HVAC Equipment
and Locate Diffusers
a Control Strategy
1. Initial Building Design Considerations
1A. Building Section
In new construction, underfloor air distribution can achieve a 5-10% reduction
in floor-to-floor heights compared to projects with ceiling-based air
distribution. This is accomplished by reducing the overall height of service
plenums and/or by changing from standard steel beam construction to a concrete
(flat slab) structural approach (see example table and figure below). A single
large overhead plenum to accommodate large supply ducts can be replaced with a
smaller ceiling plenum for air return combined with a lower height underfloor
plenum for unducted air flow and other building services. Concrete flat slab
construction is more expensive than steel beam construction, but is preferred
for underfloor systems due to thermal storage benefits as well as significantly
reduced vertical height requirements. In the example shown in the figure, the
underfloor/concrete configuration allows 0.25 m (10 in.) to be saved in
floor-to-floor height compared to overhead/steel beam system design. Even
greater savings (up to 0.56 m [22 in.]) can be realized if the ceiling plenum
is completely eliminated, exposing the concrete ceilings and providing an
opportunity for greater architectural creativity. In this case, proper acoustic
treatment must be incorporated into the new design.
Underfloor plenums accommodating both cable/electrical distribution and an UFAD
system are often deeper than those employed solely for cable management
purposes. However, the additional height required for acceptable air flow
performance is not large, based on recent CBE research results .
The following table presents a comparison of typical floor-to-floor
dimensions for a midsize (5-10 stories), high-tech class A office building
(assuming a 40-ft clear span between columns) :
Underfloor plenum heights are usually determined by:
(i) largest HVAC components (e.g., fan coil units, terminal boxes, ducts,
dampers) located under the floor;
(ii) requirements for underfloor cabling;
(iii) clear space for underfloor air flow (usually 76 mm (3 in.) minimum).
1B. Building Plan
The modularity of all components of raised access floor systems can be an
advantage in space planning, particularly over large open plan areas.
Consider the compatibility of anticipated building plan geometries with the
following typical system component dimensions.
Raised access floor panel dimensions: 610 mm (24 in.) square
Underfloor plenum pedestal spacing: as for floor panels, e.g. 610 mm (24 in.)
Typical perimeter zone size (occupied area): 4–5 m (12–15 ft) deep, from
the external wall.
The largest dimension of system components (e.g., ducts) that can reasonably
fit between underfloor pedestals is 560 mm (22 in.). Where structural
members/ducts cross the pedestal grid, metal bridging can be installed,
spanning from one panel to another in place of a pedestal.
2. Select System Configuration
When configuring an UFAD system, there are three basic approaches: (1)
pressurized plenum with a central air handler delivering air through the plenum
and into the space through passive grills/diffusers; (2) zero-pressure plenum
with air delivered into the conditioned space through local fan-powered
(active) supply outlets in combination with the central air handler; and (3) in
some cases, ducted air supply through the plenum to terminal devices and supply
outlets. In the discussion below, we will focus on the two plenum supply
configurations, as guidelines for fully ducted systems are well established.
2A. Pressurized Underfloor Plenum
Supply air that has been filtered and conditioned to the required temperature
and humidity, and includes at least the minimum required volume of outside air,
is delivered by a conventional air handling unit through a minimum amount of
ductwork to the underfloor plenum. The number of plenum inlet locations is a
function of the size of control zones, access points available in the building,
amount of distribution ductwork used under the floor, and other design issues.
Within the underfloor plenum, it is always desirable to the extent possible to
have the supply air flow freely to the supply outlets.
Pressurized plenums are designed to deliver supply air at constant temperature
and volume to all passive outlets of the same size and control setting within
the conditioned space.
Because the supply air in the underfloor plenum is in direct contact with the
concrete structural slab, a number of energy- and cost-saving strategies are
possible with UFAD systems. Such thermal storage strategies should be
implemented with great care. See further discussion under section 12 - Develop
a Control Strategy.
The underfloor thermal mass also has the effect of providing a consistent cool
air temperature reservoir (for cooling applications), making UFAD systems
extremely stable in their operation.
Almost all available floor diffusers allow some amount of individual control
(usually volume and in some cases direction) by nearby occupants.
There are several approaches to address zones with significantly different
partitioning with ducted VAV devices supplying air to each zone;
partitioning with fan-powered terminal devices supplying air to each zone;
controlled VAV diffusers may be used in both partitioned and open plenums;
fan-driven supply outlets may be used in both partitioned and open plenums;
plenums with mixing boxes and ducted outlets.
Zoning and partitioning of the underfloor plenum should be kept to the
minimum necessary to optimize UFAD performance and efficiency, as this helps
to maintain the plenum for it's intended purpose: to serve as a highly
flexible and accessible service plenum.
There is some evidence from completed projects that uncontrolled air leakage
from the pressurized plenum can impair system performance. It is important
that proper attention be given to the sealing of junctions between plenum
partitions, slab, access floor panels, and exterior or interior permanent
walls during the construction phase of the project. Due to the relatively low
pressure (12.5 - 50 Pa [0.05 - 0.2 in. H2O]) used in pressurized plenums,
proponents of pressurized plenums claim that leakage into adjacent zones is
minimal, and much of the leakage (between raised floor panels) will be into
the same conditioned zone of the building . This is a design issue that is
still in need of further investigation.
If access panels are removed for long periods of time, or the distance between
primary air inlets and supply diffusers is too great, control of air flow will
Although not a requirement, some designers recommend limiting the size of
underfloor zones (partitioned or otherwise) that are served by a single ducted
primary air inlet, from the air handler (see below). This ensures the system's
ability to avoid unacceptable variations in supply air temperature (due to
heat gain from or loss to the concrete slab and raised floor structure) and
quantity (due to pressure loss, obstructions, or friction) within the zone.
In some system designs, using multiple medium or small-sized (floor-by-floor)
AHU’s can minimize or totally eliminate ductwork; and improve zone control
when AHU capacities correspond to the specific requirements of each plenum
Some designs have resulted in higher pressures in the plenum on the order of
125 Pa (0.5 in. H2O); these have resulted in problems with over-conditioning,
lifting of carpets, and problems with diffusers.
Typical plenum pressure: 12.5 - 50 Pa (0.05 - 0.2 in. H2O)
In some cases, maximum floor area served by each
free-discharge supply air duct: 300 m2 (3,200 ft2)
2B. Zero-Pressure Underfloor Plenum
Primary supply air from the central air handler is delivered to the underfloor
plenum in much the same manner as with pressurized plenums. In this case,
since the plenum is maintained at very nearly the same pressure as the
conditioned space, local fan-powered supply outlets are required to supply the
air into the occupied zone of the space.
Zero-pressure plenums pose no risk of uncontrolled air leakage to the
conditioned space/adjacent zones.
The removal of floor panels does not disrupt overall supply-air flow.
Local fan-powered outlets under thermostatic or individual control allow
supply air conditions to be controlled over a wide range as necessary. This
controllability can be used to handle zones with significantly different
thermal loads without underfloor partitioning. The use of partitioning for
zone control can also be applied in a similar way as for pressurized plenums.
Zoning and partitioning of the underfloor plenum should be kept to the minimum
necessary to optimize UFAD performance and efficiency, as this helps to
maintain the plenum for it's intended purpose: to serve as a highly flexible
and accessible service plenum.
Since the supply air in the underfloor plenum is in direct contact with the
concrete structural slab, the same thermal storage strategies as with
pressurized systems can be used (see section 12 - Develop a Control Strategy).
Similarly, the frequency of ducted primary air inlets to the plenum must take
into consideration the heat exchange between the supply air and the underfloor
plenum structural mass.
By relying on both a primary air handler and local fan-powered outlets to draw
air from the plenum into the space, zero-pressure configurations can more
reliably maintain some amount of cooling effect if the chillers are off due to
the thermal inertia of the concrete slab.
The greater ability of zero-pressure systems to provide localized cooling
suggests their suitability in projects involving high and diversified heat
In some system designs, using multiple medium or small-sized (floor-by-floor)
AHU’s can minimize or totally eliminate ductwork; and improve zone control
when AHU capacities correspond to the specific requirements of each plenum
Compliance with local Fire Codes may require sprinklers and partitioning to be
installed in all plenum configurations.
3. Determine space cooling and heating loads
Cooling and heating loads for a building with an UFAD system are calculated in
much the same manner as for a conventional overhead system. However, the
determination of design cooling air quantities must take into account key
differences between these systems.
The stratified floor-to-ceiling air flow pattern in UFAD systems allows most
convective heat gains from sources outside the occupied zone (up to 1.8 m [6
ft]) to be exhausted directly at ceiling level, and therefore to not be
included in the air-side load. Air supply volumes therefore only consider heat
sources that enter and mix with air in the occupied zone. Heat sources must be
analyzed based on their convective and radiative components, a subject
addressed by . Depending on the location of the heat source in the space,
some amount of the convective portion can be neglected in this calculation. An
overview of this design approach is described in .
Other issues that can affect load calculations include the heat exchange
between the concrete slab and the supply air as it flows through the
underfloor plenum. If the slab has absorbed heat, particularly from warm
return air from the next floor down flowing along the underside of the slab,
then supply temperature will increase with distance from the primary air inlet
to the plenum. Supply air in the plenum will also be warmed by heat transfer
from the room through the raised floor panels. Estimates of supply air
temperature rise as a function of air travel distance in an underfloor plenum
are given in . This is also the subject of ongoing CBE research.
Another difference between UFAD design and overhead systems is consideration
of heat loss through the access floor. This adds another component to the
space cooling load calculation –- estimated to be as high as 0.1 W/m2 (1
W/ft2). Most of this heat transferred through the floor into the supply air
stream will reenter the conditioned space, although not instantaneously due to
the mass of the floor panels.
Research indicates that stratification for UFAD systems can result in overall
delta T's (return-supply temperature difference) in the range of 8-11°C
(15-20°F), for properly designed systems. However, these values are not what
determine the air flow requirement. The heat gain to the occupied zone and the
air flow required to maintain a given comfort condition in that zone is what
determines the actual air flow requirements, and consequently the overall
delta T that will be developed. Typically this results in air flow rates that
can be equal to or less than that for overhead systems, even for higher supply
In most applications, heating is primarily needed only near the building
envelope where cold downdrafts from perimeter glass may cause discomfort.
Heating may also be needed in some top floor interior zones and during periods
of low occupancy (e.g., nights and weekends).
Effective heating systems isolate the source of warm air from the thermal lag
effect of the concrete slab (which is usually slightly cooler than room
temperature). This can be done, for example, by ducting from an underfloor fan
coil unit, or by using baseboard radiation or convection units. Quick response
on heating can be very important during morning startup.
4A. Perimeter Zones
The largest loads typically occur near the skin of the building. Since these
areas are influenced by climatic variations, rapid fluctuations in heating and
cooling demands can happen, with peak loads often occurring only for several
hours per day and relatively few days of the year. Energy-efficient envelope
design is always the first stage of defense against excessive perimeter loads.
Under cooling conditions, the warm interior surface temperatures at the
perimeter (due to either highly absorptive glazing or intercepted solar
radiation if blinds are present) will form a strong vertical plume that can be
removed with only part of the load contributing to the air-side load.
Perimeter zone considerations often lead to hybrid system designs in which
active, fan-powered supply units are used to increase the rate at which the
system can respond to changes in load. Many perimeter zone solutions have been
successfully applied in practice (see Plan Views for a few examples). Some
manufacturers offer equipment and recommended configurations for perimeter
systems [10, 11].
4B. Interior Zones
Interior zones (defined as areas located further than 5 m (15 ft) from
exterior walls) are usually exposed to relatively constant and lower (compared
to perimeter zones) thermal loads (almost always cooling in typical office
buildings). However, with modern energy efficient office equipment and high
diversity rates of personnel, these loads can fluctuate significantly; control
strategies and system design need to be well thought out to accommodate these
These zones can often be well served by a constant volume, or constant
pressure in a pressurized system, control strategy. The need for dynamic
control of these (typically) large zones is minimized due to the ability of
occupants to make small local adjustments to individual diffusers. This
configuration with a minimum amount of underfloor partitioning helps to
maintain flexibility in the relocation of other services (e.g., cabling).
The interaction between interior and perimeter systems needs careful
consideration. For example, if plenum air is used to supply cooling for
perimeter zones, reset of SAT for the core zones may militate against being
able to satisfy perimeter load conditions.
4C. Other Special Zones
Other special zones having large and rapid changes in cooling load
requirements, such as conference rooms or lecture halls, should incorporate
fan-powered or VAV air supply solutions. This can require underfloor
partitioning for these areas. Automatic controls to these zones should be
capable of meeting both peak demand and significant turndown during periods of
little or no occupancy. Manual control of these zones has also been used in
Partition construction: Vertical sheet metal dividers
Perimeter zone dimensions: Typically extending 4 – 5 m (12 – 15 ft) from
5. Determine ventilation air requirements
Minimum outside air requirements should be determined according to applicable
codes (e.g., ASHRAE Standard 62-1999).
Some improvement in ventilation effectiveness is expected by delivering the
fresh supply air near the occupant at floor or desktop level, allowing an
overall floor-to-ceiling air flow pattern to more efficiently remove
contaminants from the occupied zone of the space. An optimized strategy is to
control supply outlets to allow mixing of supply air with room air up to a
height no higher than head level (6 ft [1.8 m]). Above this height, stratified
and more polluted air is allowed to occur. The air that the occupant breathes
will have a lower percentage of pollutants compared to conventional uniformly
If an enhanced air change effectiveness can be shown to exist in comparison to
well-mixed overhead systems (see ASHRAE Standard 129-1997) future versions of
ASHRAE Standard 62 may allow some credit to be taken, thereby allowing reduced
ventilation air quantities. The magnitude of this improved air change
effectiveness will be largest during times of outside-air economizer use. The
fact that the number of hours of economizer operation is typically greater for
UFAD systems also contributes to overall increased ventilation effectiveness.
6. Determine zone supply air temperature and flow rate
Because the air is supplied directly into the occupied zone, supply air
temperatures must be warmer than that used for conventional overhead system
design. For cooling applications, supply air temperatures at the diffusers
should be maintained no lower than in the range of 17 – 20°C (63 – 68°F)
to avoid overcooling nearby occupants. This supply temperature can be reset
even higher under partial load conditions.
Mixed air temperature after the cooling coil, or plenum inlet temperature,
must be determined by taking into account temperature increase (or decrease,
depending on the slab temperature) as the air flows through the underfloor
plenum. Current estimates for typical air flow rates in an underfloor plenum
with a slab that is 3°C (5°F) warmer than the plenum inlet air temperature
call for a 1°C (2°F) increase for every 10 m (33 ft) of distance traveled
through the plenum.
In temperate climates, where high humidity is not a problem, these warmer
supply air temperatures increase the potential for economizer use, and allow
higher cooling coil temperatures to be set, if desired.
Cooling air quantities for UFAD systems should be carefully determined. Higher
supply air temperatures would suggest that higher supply air volumes are
required, but the higher return temperatures created by stratification reduce
the required increase in volume. Finally, as previously described, a
calculation of the portion of heat sources that bypass the occupied zone in
the space allows cooling air quantities to be further reduced. The net effect
is that for most designs, controlled stratification in the space allows
cooling air quantities for UFAD systems to be equal to or less than those
required under the same conditions using overhead air distribution.
As discussed further below under controls, control strategies for temperature
and flow rate will vary depending on the magnitude and variability of loads in
each control zone, as well as other system design issues.
Supply air temperature - Return air temperature
UFAD 17-20°C (63-68°F) - 25-30°C (77-86°F)
Overhead 13°C (55°F) - 24°C (75°F)
7. Determine return air configuration
For optimal cooling operation of a UFAD system, it is important to locate
return grills at ceiling level, or at a minimum, above the occupied zone (1.8
m [6 ft]). Air is typically returned through grilles located in a suspended
ceiling or through high side-wall grilles if no ceiling plenum is present.
This supports an overall floor-to-ceiling air flow pattern that takes
advantage of the natural buoyancy produced by heat sources in the office and
more efficiently removes heat loads and contaminants from the space.
A certain portion of return air is mixed with primary air from the AHU to
achieve desired air temperatures and humidity, and enable reduced energy
costs. In many climates to achieve proper humidity control, conventional
cooling coil temperatures must be used (producing a coil leaving temperature
of 12.8°C (55°F)). In this situation, a return air bypass control strategy
can be employed in which a portion of the return air is bypassed around the
cooling coil and then mixed with the air leaving the coil to produce the
desired warmer supply air temperature (17-20°C [63-68°F]).
In some cases, a percentage of return air can be re-circulated directly back
into the underfloor plenum via return shafts near the ceiling or from the
ceiling plenum. Room air flowing back into the plenum through open floor
grills can also serve as make-up air for zero pressure plenum designs when
local fan-powered outlets require more air than that being supplied from the
If re-circulation takes place directly in the underfloor plenum, the supply
and return air streams must be well mixed within the underfloor plenum before
delivery to the conditioned space. This can usually be achieved by
distributing the primary air at regularly spaced intervals throughout the
plenum, and/or employing fan-powered local supply units to aid mixing of
primary supply air with the return air.
8. Calculate cooling coil load
The calculation of cooling coil load is done in much the same manner as for a
conventional mixing-type air distribution system. All of the loads in the
space (some of which have bypassed the occupied zone, allowing a reduction in
cooling air-side loads) must be handled at the coil.
Humidity control considerations require that, in more humid climates the
cooling coil be operated to produce the necessary dehumidification of outside
air resulting in a coil leaving air temperature of 12.8°C (55°F). This air
is then mixed with warmer bypassed return air to produce the desired plenum
inlet air temperature. It is important that these systems use enthalpy based
economizer control to ensure proper supply air humidity control.
In temperate climates, the warmer supply air temperatures of UFAD systems
increase the potential for economizer use and may also allow higher cooling
coil temperatures than for conventional overhead systems, thereby increasing
9. Layout ducts and plenum configuration
Underfloor ducts serving specific zones should be sized to accommodate peak
cooling loads. The capacity of the central chiller plant, air handlers and
main duct risers can generally be reduced by accounting for time variations
and load diversity (up to 50%).
The design and layout of main ducts from the central plant to plenum inlet
locations is similar to that of conventional overhead systems except that
access must be provided for the ducts to reach the underfloor plenum. The
amount of main ductwork can be reduced in designs using medium to small-sized
air handlers (floor-by-floor units) that are located closer to the point of
use. However, ductwork for ventilation air is still required and must be sized
accordingly if the use of an outside-air economizer will be an important
The amount of re-circulation ductwork can be reduced by taking some of the
return air at ceiling level directly back into the underfloor plenum without
returning to the air handler. For example, return air can be brought down
induction shafts formed with furring spaces along structural columns. This
alternative configuration of bypass control can be used as long as proper
dehumidification is maintained back at the air handler and complete blending
of return and supply air is achieved within the underfloor plenum.
The height of underfloor plenums is generally determined by the largest HVAC
components located under the floor. These components can typically be
distribution ductwork, fire dampers at plenum inlets, fan coil units, and
terminal boxes. Commercially available HVAC products allow plenum
configurations as low as 0.2 m (8 in.), although heights in the range of
0.31-0.46 m (12-18 in.) are more common.
In pressurized plenums as low as 0.2 m (8 in.), full-scale experiments have
demonstrated that static pressure remains very uniform across the entire
plenum, making balancing of individual diffusers unnecessary . These same
experiments also showed that solid obstructions, even with only 38 mm (1.5
in.) of clear space above them, may be located in a 0.2-m (8-in.) or higher
plenum and have very little impact on the uniformity of the overall pressure
and air flow distribution.
In both zero-pressure and pressurized plenums, the delivery of air through
fan-powered outlets is even more reliable than that through passive diffusers
in pressurized plenums. Active diffusers are less susceptible to pressure
variations (such as when access floor panels are removed) and other flow
10. Select primary HVAC equipment
Due to the low operational static pressures in underfloor air supply plenums
(typical pressures are around 25 Pa (0.1 in. H2O)), central fan energy use and
sizing can potentially be reduced relative to traditional ducted overhead air
distribution systems depending on the design strategy adopted (see  for a
more complete discussion of this issue).
As discussed earlier, cooling air quantities, and therefore air handler
capacities, for UFAD systems should be carefully determined and may be equal
to or less than those required under the same conditions using overhead
Although the total space cooling load is similar for both UFAD and
conventional overhead systems, higher supply and return air temperatures in
UFAD systems allow higher efficiency chillers to be used, when suitable
weather conditions, and in particular humidity conditions, prevail.
Thermal storage strategies using nighttime precooling of the concrete slab in
the plenum can reduce peak demand for mechanical cooling during the following
day, thereby allowing refrigeration equipment to be downsized.
11. Select and locate diffusers
The flexibility of mounting supply diffusers in movable raised access floor
panels is a major advantage for UFAD systems. The inherent ability to easily
move diffusers to more closely match the distribution of loads in the space
makes the placement of diffusers a much easier task. In fact, initial layout
can be done quite crudely. Final placement can take place after the location
of furniture and loads, as well as the preferences of individual occupants,
are more accurately determined.
11A. Passive Diffusers
Passive diffusers are defined as air supply outlets that rely on a pressurized
underfloor plenum to deliver air from the plenum through the diffuser into the
conditioned space of the building. Swirl floor diffusers, constant velocity
floor diffusers, and linear floor grills are the primary types of floor
diffusers (see below).
Airflow pattern - Swirling upwards, rapid mixing.
Adjustability - Rotate grill or bucket to adjust air flow volume.
Ideal Location - Interior and perimeter zones.
Airflow pattern - Multi directional, air jet.
Adjustability - Adjust grill for changes to air flow direction, thermostat for
air flow volume.
Ideal Location - Interior and perimeter zones.
Airflow pattern - Planar sheet, air jet.
Adjustability - Multi-blade damper is used to adjust air flow volume.
Ideal Location - Perimeter zones.
diffuser constant velocity floor
linear floor grill
Swirl Floor Diffusers. This is the most commonly installed type of
diffuser in UFAD systems; more models are commercially available than any
other design. The swirling air flow pattern of air discharged from this round
floor diffuser provides rapid mixing of supply air with the room air in the
occupied zone. Swirl diffusers are generally installed as passive diffusers,
requiring a pressurized underfloor plenum, although fan-driven models are
available. Occupants have limited control of the amount of air being delivered
by rotating the face of the diffuser, or by opening the diffuser and adjusting
a volume control damper.
Constant Velocity Floor Diffusers. This recently introduced diffuser is
designed for variable-air-volume operation. It uses an automatic internal
damper to maintain a constant discharge velocity, even at reduced supply air
volumes. This is a passive diffuser (power is needed for the damper motor
only), requiring a pressurized plenum, but fan-driven configurations are also
available. Air is supplied through a slotted square floor grill in a jet-type
air flow pattern. Occupants can adjust the direction of the supply jets by
changing the orientation of the grill. Supply volume is controlled by a
thermostat on a zone basis, or if available, as adjusted by an individual
Linear Floor Grills. Linear grills have been used for many years,
particularly in computer room applications. Air is supplied in a jet-type
planar sheet making them well matched for placement in perimeter zones
adjacent to exterior windows. Although linear grills often have multi-blade
dampers, they are not designed for frequent adjustment by individuals, and are
therefore not typically used in densely occupied office space.
It is recommended that you contact the diffuser manufacturers directly to
obtain the most up-to-date product information on passive diffusers.
11B. Active Diffusers
Active diffusers are defined as air supply outlets that rely on a local fan to
deliver air from the plenum through the diffuser into the conditioned space of
the building. Passive diffusers can generally be converted to an active
diffuser by simply attaching a fan-powered outlet box to the underside of the
diffuser or grill. Most manufacturers provide both passive and active
In addition to the three types of diffusers described above for passive
diffusers, several different designs for active diffusers are available.
Fan-powered diffusers include:
Floor supply module, consisting of four round discharge grills (jet-type)
mounted in a single raised floor panel. Fixed vanes in the grills are inclined
at 40°, so that air flow direction can be adjusted by rotating the grills. A
rotary speed control knob recessed into one grill allows air supply volume
(fan speed) to be controlled [13-19]. Please check on the availability of this
unit with the manufacturer.
Desktop air supply pedestals (typically two), which are fully adjustable for
air flow direction, as well as air supply volume by adjusting a control panel
on the desk. Air is supplied from a mixing box that is typically hung in the
back or corner of the knee space of the desk and connected by flexible duct to
the two desktop supply nozzles. The mixing box uses a small variable-speed fan
to pull air from the underfloor plenum and deliver a free-jet-type air flow
from the nozzles [15, 18-22].
Underdesk diffusers, which consist of one or more fully adjustable (for air
flow direction) grills, similar to a car's dashboard. A fan unit located
either adjacent to the desk or in the underfloor plenum delivers air through
flexible duct to the grills (jet-type) mounted just below and even with the
front edge of the desk surface (other positions are possible). Two primary
models are available. One allows adjustment of the fan speed to change air
supply volume [18, 19], and the other uses a damper to adjust air supply
volume towards the occupant while maintaining a thermostatically controlled
total volume (through other supply outlets) into the space .
Partition-based diffusers, mounted in the partitions immediately adjacent to
the desk. Air is delivered through passageways that are integrated into the
partition design to controllable supply grills (jet-type) that may be located
just above desk level or just below the top of the panel. Although uncommon in
the U.S., some of these systems have been installed in Japan .
It is recommended that you contact the diffuser manufacturers directly to
obtain the most up-to-date product information on active diffusers.
12. Develop control strategy
12a Temperature, humidity, and volume control
As described earlier, since air is supplied directly into the occupied zone
near floor level, supply outlet temperatures should be maintained no cooler
than in the range of 17-20°C (63-68°F) to avoid overcooling nearby
To achieve the required higher supply air temperatures while still maintaining
humidity control the following approach, called side-stream bypass, is often
used. Cooling coil temperatures are typically in the range of 10-13°C (50-55°F)
for dehumidification purposes. Only the incoming outside air and a portion of
the return air is dehumidified (minimum amount needed for humidity control).
The remaining return air is bypassed around the coil, if done at the air
handler, and mixed with the cool primary air to produce supply air of the
proper temperature and humidity before being delivered directly into the
underfloor plenum. In this configuration, a range of coil temperatures can be
produced, including low temperature air systems with or without ice storage.
An alternative bypass configuration involves re-circulating some of the return
air directly back into the underfloor plenum where it is mixed with the cool
primary air from the air handler. In this case, it is very important to ensure
thorough mixing of primary and return air in the plenum.
In less humid climates, the warmer supply air temperatures increase the
potential for economizer use and may also allow higher cooling coil
temperatures to be used, thereby increasing chiller efficiencies.
A variety of operating strategies have been used in existing installations. A
preferred method for interior zones is constant air volume, variable
temperature (CAV-VT). Supply temperature is controlled based on zone
thermostats. Occupants can make minor changes to local comfort conditions by
adjusting a diffuser.
Due to temperature stratification that naturally occurs with UFAD systems,
thermostats should be carefully placed and their readings correlated with
acceptable comfort conditions in the occupied zone. For example, if
stratification is large, an acceptable temperature at the five-foot level may
in fact coincide with uncomfortably cool temperatures at the ankle level.
While the floor supply system is very well matched to the stable load profiles
in most interior building zones, it is incompatible with the more rapidly
varying loads found in perimeter and other special zones. These zones require
a separate system, or at least a hybrid solution in which fan-powered outlets
and/or additional equipment are used to address the extra heating and cooling
requirements of these zones.
Perimeter zone operation can be either variable air volume (VAV) or CAV-VT,
but in either case must be able to respond to the special cooling and heating
demands of these zones. Perimeter zone solutions have not been standardized in
existing UFAD installations; many different approaches have been used.
12B. Thermal storage control strategies
Since supply air flowing through the underfloor plenum is in direct contact
with the concrete floor slab of the building, control strategies must consider
thermal storage in the slab as well as other mass in the plenum (e.g., floor
panels). UFAD systems are very stable in operation with only gradual (usually
unnoticeable to occupants) changes in supply temperature over time, unless the
supply air is isolated from the mass.
Energy and operating cost savings can be achieved using a thermal storage
strategy in the concrete slab. In temperate climates, cool nighttime air can
be brought into the underfloor plenum where it effectively cools the slab
overnight. During the following day's cooling operation, higher supply air
temperatures can be used to meet the cooling demand, thereby reducing
refrigeration loads for at least part of the day. This 24-hour thermal storage
strategy benefits from lower off-peak utility rates and extends the hours of
economizer operation. For this strategy to be successful, the following issues
must be addressed:
night setback must be used.
- Since a
precooled slab will be at its coolest temperature in the morning, the
design, capacity, and response rate of the heating system, if needed, will
be particularly important under morning startup conditions.
economizer control must be used to maintain proper humidity levels of the
incoming nighttime air and to protect against condensation in the plenum.
limit control switches for both slab and space temperatures must be
activated to prevent overcooling.
estimates indicate that a precooled slab is most effective at reducing
daytime cooling loads during morning hours only.
- If the
slab is not pre-cooled at night, then supply outlet temperatures will likely
increase with distance from the primary air inlet to the plenum due to the
effects of stored heat in the slab (particularly from warm return air from
the next floor down flowing along the underside of the slab).
currently conducting research to investigate the thermal storage performance of
underfloor air supply plenums.
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