SMUD Customer Service Center (CSC)

Image 1: Sacramento Municipal Utility District (SMUD) Customer Service Center (CSC)

Building Design Features Underfloor Air Distribution System Characteristics UFAD System Performance

Building Use: The CSC houses SMUD corporate offices as well as a Payment (Call) Center, the Energy Technology and Education Center, and conference facilities. The building is designed to accommodate 550 people Design Team: Owner/developer: SMUD Architect: Williams + Paddon, Roseville, CA Mechanical design: Peters Engineering Contractors: Hensel Phelps - Sacramento Consultants: Robert Bein, William Frost & Associates

Location: Sacramento, CA Size: Four wings, two or four stories connected to a central entry atrium Total floor area is 19,320 m2 (208,000 ft2) Construction Status: Construction was completed in 1996.

The CSC was intended as a showcase of low-energy design featuring an underfloor air distribution system, a high-efficiency central chiller plant with thermal energy (cold water) storage, and efficient electric lighting integrated within a sophisticated daylighting scheme. Exterior shading devices and interior light shelves contribute to keeping loads low. SMUD management placed a priority on giving employees personal control over the indoor environment, and this has been achieved by providing ergonomic workstations, individual task lights, operable windows, adjustable window coverings and the accessibility of floor diffusers – a key component of the underfloor air distribution system.

Each floor of a wing (except the SE wing) is divided into a number of underfloor plenum zones by partitions in the plenum that form an air dam (i.e., not tightly sealed). Each zone has a volume control damper so an AHU can supply more than one zone.

The interior is largely open plan with only a small number of private offices located mostly in the perimeter. The raised floor system consists of 0.61 m x 0.61 m (24 in. x 24 in.) panels and houses a 0.3 m (12 in.) high plenum. Floor panels are covered with 0.46 m x 0.46 m (18 in. x 18 in.) carpet tiles glued down and overlapping the floor panel joints. A modular cubicle and furniture system is used throughout. The ceiling is made up of suspended acoustical panels.  Suspended direct/indirect fluorescent fixtures provide lighting.

Electrical distribution consists of hard conduits that feed junction boxes (J-box) in the plenum from a floor electrical closet. Each floor has one closet that contains electrical and telecommunications main distribution circuits. From each J-box, each of 6-8 cubicles are fed by a metal shielded whip to a utility box mounted on the underside of the desk. This box contains surge protection switches and connection ports and is served by two separate circuits, one for computers and the other for small appliances (i.e., each of the two circuits’ serves 6-8 cubicles). Two printer stations per 10 cubicles are supplied from floor panel mounted modules. The computer circuit is isolated back to distribution. Oversized grounds are used for safety. Telecommunication cables are direct wired from main floor distribution through 1.18 cm (3 in.) grommets mounted in the floor panels to cubicle appliances.

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Underfloor Air Distribution (UFAD) System Characteristics

Design Intent
The decision to use a raised floor for cable management and HVAC in the office area was the result of a cost-effectiveness analysis that identified first cost offsets by eliminating overhead ductwork and diffusers and labor savings during future office reconfigurations. The potential beneficial effects on employee productivity were discussed but not quantified. The UFAD system was a key element for realizing the combination of low energy use and occupant personal comfort control.

Design/Configuration

As indicated by Figure 2, the SMUD CSC building contains a number of different roof mounted penthouse AHUs that serve both constant volume and variable volume (VAV) system types. Perimeter zones are served by dual duct VAV systems with air supplied by linear bar grills in the window sills as well as by floor diffusers. All UFAD areas are served by air handlers with hydronic heating and cooling coils and an economizer. Each AHU is equipped with a variable speed drive to maintain constant pressurization in the supply ducts. Each underfloor plenum zone is supplied through a control damper that maintains constant plenum pressure. Variable temperature conditioned air is delivered to the occupied spaces via occupant-controlled, passive floor diffusers. By design, each cubicle is equipped with at least one floor diffuser. Return air grilles are located at ceiling level. The return fan is shared between the interior and perimeter systems.

Operation
Since the building is equipped with operable windows, the HVAC systems were designed to maintain slightly positive pressure in the building to ensure exfiltration; thus only filtered, conditioned air is supplied to the occupied space. The operable windows are equipped with micro-switch sensors that reset operation of the adjoining VAV box to night setback mode (i.e., increased cooling and decreased heating setpoints and minimum volume damper position) anytime a window is opened.

A separate dual-duct VAV system, with registers in the window sills, provides the necessary heating and cooling to accommodate skin loads at the perimeter of the building when required. The hot deck of the VAV system is supplied from the return plenum of the office space, and in mild winter conditions, the warm return air can be used to heat the perimeter without energizing the heating coil.

For UFAD systems the supply fan is modulated to control supply duct pressure to 125 Pa (0.5 in. w.c.). Although the plenum supply dampers are modulated to maintain constant pressure differential across the access floor panel, the system from the perspective of the delivery of room air operates as a CAV-VT system. Variations in plenum pressure would result only from changes in floor diffuser opening by the occupants. A fan tracking system that sets return volume equal to 85% of total supply flow controls the return fans. As indicated in Figure 2, each plenum zone is provided with a corresponding room air temperature sensor (all located on interior spaces; the perimeter system is controlled from wall sensors located on exterior walls). All zone sensors for an AHU are used to compute an average space temperature for the AHU. During cooling the SAT is varied from 12.8°C to 21.1°C (55°F to 70°F) as the deviation of the average from room setpoint varies between –1 to +2°F. During heating the SAT is varied between 35°C and 21°C (95°F and 70°F) for the same deviation range, respectively.

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UFAD System Performance

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.)

System design and control - This design represents somewhat of a hybrid: the perimeter zones are not overhead systems but also are not true UFAD; air is supplied at variable volume to window sill diffusers along with passive floor diffusers. Interior zone sensors control the supply air temperature for the floor diffusers while separate sensors on perimeter exterior walls control the perimeter system. Thus, the perimeter system augments the cooling supplied by the UFAD system in these zones. The interior areas are more typical CAV-VT UFAD systems. It is unclear as to the impact on stratification and comfort of operating the perimeter zones with VAV sill diffusers.

Problems related to control have occurred due to the averaging process used to control the AHU SAT. Using the average for all zones will tend to leave some zones unsatisfied. However, occupant adjustment of the floor diffusers should help to accommodate these variations.

The operable windows were designed with micro-switches to disable the local VAV box when the window was opened. However, these switches were also tied to the alarm system resulting in false alarms. For this reason the window switches have been disconnected. The windows are not frequently opened primarily because of outside noise (the CSC is near a freeway).

Economizer mode – This system uses an outside air economizer (i.e., changeover controlled by outside temperature alone rather than by return temperature) but this may not be the most energy efficient solution for this system, especially with a 21°C (70°F) changeover temperature. Since stratification will cause the return temperature to be significantly greater than normal the changeover point could be set higher allowing a greater range of economizer operation. In addition, when the economizer is in wide open mode both return and outside air dampers are opened 100%; this will most likely result in greater mixed air temperature than if the return damper was closed.

Occupant survey - A review of occupant survey data revealed a surprising number of comments related to dust problems with diffusers, poor location of diffusers, difficulties with adjustment, and inadequate airflow (too much or too little). Half of the respondents’ (159) who had access to a floor diffuser submitted comments. The majority (77%) of those comments indicated one or more of the problems indicated above. However, of these comments 36% register dissatisfaction with air movement and/or temperature. This is approximately consistent with the overall temperature and airflow dissatisfaction levels; i.e., 24% overall reported temperature dissatisfaction of some sort and 31% reported air movement dissatisfaction. These results do, however, contrast with views expressed by facility managers who cite a low number of complaints. One possible explanation is that occupants may not know that the diffusers are adjustable and/or movable and therefore do not consider filing a complaint. Many of these issues appear to be resolvable by simple relocation and/or better diffuser design, or by improved system control. Facility personnel report that the majority of complaints are derived from either equipment failures or stuffy conditions because too many diffusers are turned off.

The survey results also indicate that 27% of occupants with diffusers never adjusted them while the remainder changed them from 1-4 times a month (49%) to more than once per day (5%).

Despite a 24% dissatisfaction rating for the floor diffusers, 70% of the occupants with diffusers rated their importance as high vs. only 7% for low importance. This tends to corroborate research findings that occupants prefer to have control over their environment.

Stratification – Facility engineers report that the system exhibits very little stratification. This is based on EMCS data and not measurements in the space. However, the return configuration is complicated by the fact that all wings open into the central atrium and the return system supports both the perimeter overhead VAV system as well as underfloor zones. This makes judgments about stratification based on EMCS return air temperatures difficult. Also, it is possible that this system is over-aired. The fact that a significant number of diffusers are turned off due to occupants feeling too cool and the supply air temperature operates in a narrow range of 65°F and above (even on hot Sacramento days the SAT never is less than 65°F) suggests that the overall supply volume is too great. High airflow relative to load will decrease stratification. Constant volume systems inevitably will be over-aired at light load conditions since the peak volume is being used to support a lower load condition, which will result in less stratification. This whole situation can sometimes be exacerbated by the difficulty associated with adjusting the floor diffusers due to sticking and non-linear airflow performance as the basket damper is adjusted; occupants will sometimes default to the least disturbing condition and close the damper.

Date Reviewed: December 1999 - September 2000

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