Print Version: Underfloor Case Study FNBO

Figure 1: First National Bank of Omaha Technology Center, South Exposure

Building Design Features

Underfloor Air Distribution System Characteristics

UFAD System Performance

Building Use:
The Technology Center houses the corporate offices and check processing center for the First National Bank of Omaha. The building is occupied for 24/7. Under normal operations, it is expected that 500 people will occupy it.

Location:
16^th and Capitol
Omaha, NE

Design Team:

Owner/developer:

First National Bank of Omaha

Architect:

HDR Architecture, Inc.

Mechanical design:

HDR Architecture, Inc.

Contractors:

Hawkins Construction Company

Size:
Three stories
18,580 m² (200,000 ft²)
61 m by 114 m (200 ft by 375 ft) footprint

Construction Status:
Completed in September 1999.

Two views of the technology center are shown in Figures 1 and 2. The building facade is constructed of glass curtain walls on the south end commons areas and offices, and is of precast concrete panel construction with six-foot high strip windows and granite panel veneer on the north two-thirds of the building. Approximately 45% of the building exterior skin is glass curtain wall or window.
Wall overall U-value = 0.26 W/m²^.K (0.046 Btu/hr^.ft²^.°F). Window overall U-value = 1.64 W/m²^.K (0.29 Btu/hr^.ft^2.°F) with a 0.43 shading coefficient.

An underfloor air distribution (UFAD) system serves 3,340 m² (36,000 ft²⁾ of office space and check processing equipment on the middle level and 2,790 m² (30,000 ft²) of office space and computer environment on the top level, for a total of 33% of the floor areas. The remaining occupied spaces (67%) are served by traditional overhead VAV systems.

Interior spaces are primarily open plan using modular walls and furniture except at the perimeter where private offices are located. Lighting is provided by overhead recessed ceiling lighting fixtures. Underfloor electrical consists of hard conduit to distribution boxes from which flexible whips emanate to serve workstations.

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Design Intent
Issues considered during the design of this state of the art facility included integration of diverse building functions, critical HVAC requirements for certain areas, and responsiveness to ‘green’ building design practices and energy consciousness. These design issues were to be achieved in conjunction with, and not at the expense of, the functional demands of the Technology Center. The overall goal of this design was to produce an integrated, straightforward, and efficient mechanical system that optimizes the performance of the people and building it serves. Key design parameters were interior reconfiguration flexibility and avoidance of equipment and power outages by providing redundancy in mechanical and electric power systems.

Design/Configuration

Plenum Height:	0.46 m (18 in.) in all areas except computer and card processing areas which are 0.61 m (24 in.).
Diffuser Types:	Open areas and cubicles: Constant volume swirl, Titus TAF-R and Titus passive linear bar grilles (aisles). Private offices: Constant volume swirl diffusers plus active fan-powered diffusers, Tate-TAM with linear bar grille diffusers.
Raised Floor:	0.61 m (24 in.) Tate supplied concrete-core panels using a stringer-less post mounting system.
Supply Air Temperature:	Nominal 17°C (63°F), varies with load
UFAD System Types:	All UFAD systems are constant air volume-variable temperature (CAV-VT), however volume can be adjusted manually at the AHU as needed.

HVAC systems - Conditioned air is supplied through fourteen modular packaged air-handling units (AHU’s) located within the top-level mechanical penthouse area and a ground level mechanical room. Constant volume, variable temperature systems are used to serve the computer and equipment intensive areas of the building. Variable volume systems with hot water reheat are provided to serve the more standard office areas throughout the building. The forced air systems maintain space and overall building pressurization to control airflow and contaminate movement within the building.

Primary cooling and heating for the Technology Center is provided by chilled water and steam purchased from Energy Systems Company (ESC). The chilled water system is sized to provide approximately 800 tons of cooling capacity to the building. In addition, 250 tons of back-up cooling capacity is available via on-site chillers to serve critical cooling requirements in the event of a failure of the primary cooling source.

For heating, medium pressure steam is piped directly to integral face and bypass preheat coils serving the AHU’s, and to heat exchangers serving hot water heating and humidification systems. The hot water heat exchanger serves a hot water loop that supplies the perimeter heating terminal units and hydronic radiant ceiling panels. The heating system is sized to provide approximately 1,052 MW (3,600 MBtuh) of heating capacity to the building.

UFAD systems – The basic configuration typical of these systems is shown in Figure 3. There are seven UFAD pressurized underfloor plenums zoned to accommodate individual space requirements and different exterior exposures. Walls between adjacent functional areas partition the plenums between one another. Supply air is ducted to multiple plenum inlets from five separate air handling units through the return air space above the ceiling. This was necessary because of the remote location of the AHUs relative to the underfloor plenums. Air is supplied to the underfloor plenum at multiple locations to facilitate even air distribution. Ducting through the UFAD plenum was ruled out to reduce obstructions and congestion for the cabling system. Reheat coils in the branch ducts allow individual temperature control of each underfloor plenum. Although the underfloor supply air plenums are operated as CAV-VT systems, each individual plenum system is capable of delivering varying amounts of air via a variable speed drive on the AHU. Return air flows freely through the ceiling return air plenum and then is ducted back to the air handling units.

Air is delivered from the pressurized underfloor plenums into the space through three types of diffusers. In computer environments and equipment spaces typical perforated raised access floor panels are used. Approximately half of the panels have adjustable dampers to accommodate airflow balancing. In open/common areas with carpeted access flooring heavy-duty rectangular floor registers are provided, as shown in Figure 4. Small round floor diffusers are used to serve individual cubical workspaces, as shown in Figure 5.

The majority of spaces served by the underfloor air distribution systems are ‘open’ plan areas. However, the enclosed private offices at the perimeter require special attention due to varying loads. Conference rooms and perimeter offices are equipped with fan powered rectangular diffusers located on the interior side of the room in addition to the passive (no local fan) round floor diffusers (Figure 6). In the underfloor areas typical ceiling return air grilles are used. Hydronic radiant ceiling panels provide heating in perimeter areas. Fan powered diffusers are also provided at high heat producing equipment (large production laser printers) to deliver ‘spot cooling’ from the underfloor supply plenum.

Operation
Each individual AHU supply air temperature is controlled by a voting procedure based on individual zone cooling calls (zone temperature greater than set point); i.e., SAT is reset over a specified range (typically 59-62°F (15-16.7°C) based on number of calls for cooling. Reheat coils for each plenum provide variable temperature control in response to space temperature sensors located in the interior of the zone (some are located at the ceiling level). The east and west office area zones are controlled on an exposure basis and encompass both the interior and perimeter portions of the zone. AHU supply volume is controlled by duct static pressure in a manner similar to traditional VAV systems. Space pressurization is maintained by controlling the return fans to maintain an offset from supply volume.

Perimeter offices and conference rooms accommodate variations in load by allowing occupants to manually control a floor fan and adjust a passive floor diffuser. The fans are controlled by a wall switch and may be activated to deliver increased cooling and ventilation air as needed due to interior load or solar gain. A hydronic radiant ceiling panel provides heat.

The humidity levels are controlled and adjustable at each AHU primarily to meet the needs of the computer or paper processing equipment served. The hot water distribution system is designed to provide a variable supply water temperature to the floor reheat coils and radiant ceiling panels based on a signal from a solar compensated outside air sensor. This allows the supply temperature to be reset to efficiently match the building heating load.

All building mechanical systems are monitored and controlled by an integrated building automation system (BAS). This computerized system operates over a dedicated network using distributed controllers and sensors. The BAS provides scheduling, trend logging, and energy management capabilities along with system and maintenance alarms. This system can also connect to other control systems such as security and fire protection to monitor status and receive alarms. A PC-based operators workstation is located in the facilities management area.

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Although it will take another cooling season and some final “wringing out” to fully determine how well the system is performing, indications are that overall performance is good. Facility management (FM) personnel prefer the simplicity of the UFAD system and the fact that it has fewer active mechanical components to service than the overhead VAV systems.

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.)

Interior diffusers – During commissioning it was discovered that many interior diffusers were located too close to occupants. However, it was considered to be too much effort to move the panels because of the necessity of working with glued down carpet tiles. Therefore, most diffusers located in cubicles were either shutoff or reduced to about 20% flow. Leakage from electrical floor plates in many cases was enough to compensate for the reduced diffuser flow, along with aisle diffusers that were left at full flow. Reducing the overall airflow may ameliorate the low stratification that was found during initial operation. Furthermore, it is not clear how the use of linear diffusers in the aisles impacts room air distribution. These diffusers generally have lower induction rates than swirl diffusers and the effect of this on stratification performance is not well understood.

Diffuser adjustment – The FMs do not encourage occupants to adjust diffusers. However, since there are multiple shifts some diffusers are adjusted frequently due to individual differences. The diffusers can get stuck due to dirt, which makes adjustment difficult. In other cases the diffuser flow plates were too loose usually due to the baskets having been removed.

Perimeter systems – It is too early to tell how well this configuration works; there are no automatic temperature controls in the perimeter offices since the original design did not include perimeter offices. During the first cooling season many perimeter offices were too hot during the peak solar gain part of the day. In some zones the return air was found to be restricted, thus reducing the airflow from the small fan-driven diffusers. The FM reports that few occupants actively switch the fans off and on so most are left on at all times. However, energy performance could be significantly impacted by this strategy due to small fans running continuously and from simultaneous heating and cooling in the perimeter zones. It is estimated that these offices require from 117.5 to 136.3 l/s (250 to 290 cfm) cooling air for about 9.3 m² (100 ft²) offices during peak solar conditions. This is somewhat greater than expected based on previous experience and results of laboratory experiments. Locating the fan-powered diffusers on the inside wall and operating them at high volume may result in a high degree of mixing that might compromise the stratification performance.

The extensive use of ductwork in this design seems be a conservative solution, at least for the plenum supply. This extensive ductwork reduces the fan energy savings potential for UFAD systems. CBE research has demonstrated that multiple plenum inlets may not be necessary since even distribution can be achieved in large plenums supplied at one end.

Underfloor dirt is a maintenance issue, especially in the processing areas where a significant amount of debris is generated. The plenum is cleaned routinely every six months along with the fire detectors.

Local building codes dictated that hard conduit be used in the plenum instead of a modular wiring system. Flexible conduit (armored cable) whips are connected to the hard-wired junction boxes to provide power to floor connection boxes.

Date Reviewed: September 2000 – September 2001 (Case Study, April 2002)