Teledesic Broadband Center
Figure 1: Teledesic Broadband Center
Underfloor Air Distribution System Characteristics
The Teledesic Broadband Center is a high-tech single story office
building retrofitted from a food distribution warehouse.
- San Francisco
- San Francisco: Electrical, Structural, Mechanical
Michael Yantis: Acoustical
Bellevue (Seattle), WA
Main Floor = 6,600 m2
Mezzanine = 1,770 m2
The building has been occupied since fall of 1999.
Building Design Features
project, shown and depicted in Figures 1-10, involved the conversion of a food
distribution warehouse to office space for a technology company. Constructed from tilt-up concrete walls, the tall
floor-to-ceiling height of 7.6 m (25 ft) enabled a mezzanine area of 1,770 m2
(19,000 ft2) to be added above the main floor.
areas of glazing are kept to a minimum on the north and south elevations at
10-15%, the west elevation comprises 40-50% glazing. Throughout the main floor plan, workstations are provided with task
lighting to augment suspended HID lighting, while suspended direct/indirect
fluorescent fixtures provide background lighting on the mezzanine.
the overall ‘modern industrial’ interior design theme, modular furniture
configurations define workstations in the open-plan office areas, which also
include open team spaces and traditional closed conference rooms.
Figure 2: Main Floor Interior
Figure 3: Typical Workstation
Underfloor Air Distribution System Characteristics
Specifying a UFAD system was somewhat of an obvious choice for this particular project. While the high ceiling-to-floor height would have presented a number of
problems in terms of installing a conventional overhead system, it provides the
opportunity to promote vertical stratification of room air in the upper levels
(above the occupied zone), a characteristic of UFAD systems. Architecturally, concealing of HVAC components within the underfloor
plenum contributes to creating an expansive, uncluttered interior, improving
the visual and spatial character of the workplace.
m (18 in.)
diffuser (core/interior zone), supplied by Trox; linear grille
(perimeter zone), supplied by Titus.
m x 0.61 m (24 in. x 24 in.) concrete-core panel, supplied by Interface.
18°C (65°F), varies with load
volume, variable temperature (CAV-VT) in perimeter and interior zones. yes">
Passive floor diffuser in the interior, fan powered mixing boxes
most of the building, the UFAD system serving the various zones is similar in
design and operation. In order to
minimize the ductwork required, the east and west ends of the building each
accommodate an air handling unit (AHU) supplied with chilled water from a roof
mounted air-cooled chiller. The AHU delivers air at a nominal 18°C (65°F) to
the plenum serving the entire office space, as shown in Figure 4, with the
exception of two zones:
1. The north
perimeter zone accommodating a combination lounge, library and reception area;
computer room equipped with a separate ‘computer room A/C’ unit.
Teledesic building features a 0.45 m (18 in.) high plenum with concrete core
raised floor system throughout. Although
the conference rooms are the only area for which the plenum is partitioned, the
various spatial zones are defined both by different floor diffuser types, and a
variety of access floor surface finishes. Within workplace areas, 0.61 m square (24 in.) carpet tiles, laid
non-coincident with the floor panels, are used with swirl diffusers in the
interior/core zones and linear bar grille diffusers in perimeter zones. Circulation areas retain the concrete floor panel as a finished floor
surface, although at least one corridor has a poured concrete surface on top of
the access floor. Within the
plenum, cabling is substantial due to the high degree of computer related work
being done at Teledesic, and plenum cable trays are elevated to the top most
portion of the plenum (See Figure 5). Figure
6 shows electrical distribution junction boxes in the plenum from which
flexible whips emanate to feed floor panel mounted electric and telecom access
Figure 5: Cable Tray
Figure 6: Electrical Distribution
partitioning has only been used for the large conference room, however
‘air-highways’ made from sheet metal troughs are used to help evenly
distribute air through the plenum. In
the mezzanine area these air-highways are insulated to de-couple them from the
floor slab. The mezzanine area has its own air-handling unit, which
allows this area to be supplied at a slightly lower temperature. A concrete filled metal deck separates the ceiling space of
the ground floor from the mezzanine plenum. Stratified warm air in the ground floor ceiling space will transfer heat
into the mezzanine plenum so the supply air needs to be cooler to compensate;
thus the need for insulating the mezzanine level air highways.
core area is operated as a constant volume, variable temperature (CAV-VT)
system whose supply air temperature is controlled by a number of sensors
located in the interior zone and linked to the AHU via an Alerton
energy-management system. As the
structure is essentially single story, significant heat loss through the roof
in the winter causes a variation in the required supply air temperature between
18-21°C (65-70°F) throughout the year.
Figure 7: West Perimeter Diffusers
Figure 8: North Lounge Area
normal operating conditions air is returned to the AHU via return grilles
located near the ceiling. Alternatively,
when the system is using outside air, in an economizer mode, return air is
exhausted via a roof relief damper.
air is supplied to the north perimeter zone through linear diffusers located in
the kick plate of the cabinets; these diffusers incorporate a two-position
damper that minimizes plenum air delivery during heating. Finned tube
convectors located along the external wall provide heating for this exposure.
perimeter zones are operated as a CAV-VT system by the use of variable air
volume (VAV) fan-powered mixing boxes. During intermediate load conditions, room air is drawn into
the plenum through linear diffusers located towards the interior of the
perimeter zone, mixed with fresh plenum supply air and re-emitted through
diffusers at the perimeter of the zone. Note
that although each zone operates as CAV, for the system overall, the supply air
volume varies to some degree as the fan-powered boxes reduce the percentage of
plenum air used.
winter, the percentage of plenum air used is the minimum necessary to satisfy
fresh air requirements, with re-circulated room air comprising the majority of
the volume of supply air.
contrast to the open plan work areas, the conference rooms are enclosed spaces. yes">
Two large conference rooms are located peripherally, adjacent to the
west external wall; two small rooms are located towards the interior of the
plan. The difference between large and small is not only attributed to their
size and location, but also in the configuration of UFAD systems serving each.
9: Large Conference Room
large conference room, shown in Figure 9, is the only space in which the plenum
is partitioned (with the exception of the computer room, which is partitioned
as a separate system). Operated as
a VAV system in response to signals from a room thermostat, supply air is
ducted directly from the AHU to a VAV box serving this plenum zone.
10: Small Conference Rooms
small conference room, illustrated in Figure 10, uses a method similar to that
used in the core areas where plenum air is supplied to the space via a set of
swirl diffusers. However, ducting
connects another set of diffusers to a small variable speed fan, operated by a
multi-step manual control switch. This
enables occupant control over the level of cooling provided, a unique feature
for a relatively intermittently occupied space such as a conference room.
UFAD System Performance
the building is not fully occupied, our case study efforts confirm that this
building operates extremely well. The relatively simple design, high degree of temperature
uniformity, lack of complaints, low noise, and the minimum need for adjustment
all attest to the success of this design.
comments reflect the views of CBE researchers based on a limited study of the
building and do not necessarily represent those of the designers and/or
installation of air-highways, or ducting, in the plenum was considered
necessary to evenly distribute air in large floor plates.
to the use of fan powered VAV boxes in the perimeter zones, terminal fan
energy is consumed during both cooling and heating periods. Overall, this can increase total fan energy consumption. yes">
However, winter re-heating is minimized by using air that is drawn from
the room and mixed with the minimum amount of fresh plenum supply air
necessary to satisfy ventilation requirements.
variable airflow within the system due to the operation of the perimeter VAV
boxes, no plenum pressure control is used. This results in some fluctuation of the plenum pressure as the system
‘rides the fan curve’ during decreases in the cooling load.
sensors were installed in the slab with the intent of using night purging to
cool the slab in summer, thereby offsetting some of the cooling load. It was intended that the slab temperature would be monitored and used
to coordinate reset of the supply air temperature on a daily basis.
Unfortunately, controls associated with using the floor slab for
thermal storage were not programmed properly and thus were disbanded for the
main floor area. However, it appears that this strategy was unnecessary for
the ground floor areas since the loads in the occupied portion of the space
are smaller than anticipated. In
fact the supply air temperature has to be raised in cool weather to compensate
for the lack of heat gain. The
mezzanine level does use night cooling and, as discussed above, requires a
lower supply temperature than the ground floor.
the highly variable loads of conference areas by using manual control is an
interesting feature. In general
it is desirable to place intermittently occupied zones under automatic control
in order to minimize energy use and occupant interaction, as not all the
persons in such a room will be familiar with the building. However, people working in the building primarily use the
small conference rooms. The
occupants are familiar with being able to adjust their own floor diffusers, so
the conference room control is just an extension of user control. Long-term monitoring of the conference areas would indicate
the feasibility of such a departure from conventional control methods.
far, the large conference room has experienced some episodes of overheating
when occupied in the wintertime, possibly related to using relatively warm
plenum air (needed for core areas) when the cooling demand is high in the
conference room. This might also
be related to discrepancies between reliance on a fixed height thermostat and
actual variations in the temperature profile created when operating in VAV
mode. There also may be a time
delay due to the thermal mass of the floor or sluggish control system
response, which may require that a manual override be installed to open the
VAV box on demand. Monitoring of
the system would help determine the cause of this problem.
Reviewed: December 1999 – March 2001
(Case Study, October 2001)