Figure 2: Main Floor Interior

Figure 3: Typical Workstation

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Underfloor Air Distribution System Characteristics

Design Intent
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.

Design/Configuration

2. A computer room equipped with a separate ‘computer room A/C’ unit.

The 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 modules.

Figure 5: Cable Tray

Figure 6: Electrical Distribution

Plenum 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.

Operation
The 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

Under 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.

Cooling 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.

Other 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.

In 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.

In 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.

Figure 9: Large Conference Room

The 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.


Figure 10: Small Conference Rooms

The 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.

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UFAD System Performance

Although 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.

CBE Findings
(These 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 owners.)

The installation of air-highways, or ducting, in the plenum was considered necessary to evenly distribute air in large floor plates.

Due 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.

Despite 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.

Temperature 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.

Accommodating 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.

So 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.

Date Reviewed: December 1999 – March 2001 (Case Study, October 2001)