Excessive-Efficiency HVAC Helps Carry LEED Platinum to 350 Mission

Part 1 of a two-part article on the high performance characteristics of 350 Mission in San Francisco.

A very high level of sustainable design characterizes 350 Mission, a 30-story office tower on Mission Street in San Francisco. The building is LEED v2.2 certified LEED Platinum and also meets the 2011 California Green Building Code (CALGreen) and the San Francisco Green Building Code. To achieve this goal, the owner, Kilroy Realty Corp., and the design team explored a wide range of innovative, high-performance MEP solutions.

The mechanical and lighting systems incorporate numerous energy saving measures. As with all mechanical structures, the primary design criteria were occupant comfort, space, resource efficiency and cost. All occupied spaces must meet the thermal comfort criteria of ASHRAE Standard 55, which sets guidelines for temperature and air velocity under various installation conditions and recognizes the benefits of occupant control over their local thermal environment.

The high-performance HVAC solution is an underfloor air distribution (UFAD) strategy. The main reasons for this decision were lower operating energy consumption, improved indoor air quality (IAQ), individual comfort and controllability of the occupants, longer operating times of the air saver and flexibility in space use. The UFAD system, in conjunction with a high-efficiency chiller and an air saver, offers an excellent HVAC solution. In addition to air distribution, the UFAD plenum also provides power, voice and data distribution.

While other high-performance HVAC alternatives were considered, project leaders pointed to UFAD. Some system alternatives enabled even greater energy savings, such as chilled beams with a dedicated outside air system (DOAS). Another option investigated was an optimized overhead VAV system.

Ultimately, the comparative studies showed that the UFAD system achieved all project objectives, including the owner's vision for flexible office space, improved IAQ and energy savings. The system also achieved the required cooling capacity to offset the thermal loads on the building envelope.

The building's direct digital control (DDC) building management system is fully interoperable and controls both the base building's core and shell systems and tenant systems, including the underfloor fan boxes, chilled water temperature sensors, lighting system, carbon dioxide occupancy sensors and demand-controlled ventilation.

The HVAC system maximizes energy efficiency by delivering conditioned air directly to occupants at floor level, where it is then stratified and ducted above the occupancy zone (via a ceiling void or open ceiling) back to units on each floor.

With a floor distribution, the supply air can be warmer (60 to 62°F) than with a ceiling distribution (55°F) because only the area where people are present needs to be cooled, not the entire room up to the ceiling. The air temperature can increase closer to the ceiling as it leaves the occupied room. The reduced need to cool the supply air results in less energy demand from the chiller, which does not need to produce cooler chilled water.

In addition, UFAD is a low-pressure system. Since the air is not forced through a network of ducts and dampers, the system can achieve significant fan energy savings. The interior zone of each room is a large open air space. UFAD requires only peripheral fan-driven boxes in the external areas.

In terms of IAQ, the floor-mounted air supply and ceiling-mounted return take advantage of the natural tendency of air to rise when heated, pulling it through the occupants' breathing zone. This is the most effective ventilation distribution strategy according to ASHRAE Standard 62.1. Manually adjustable floor diffusers are also provided, allowing occupants to adjust the air volume and optimize their individual thermal comfort.

Other key energy saving measures include energy-saving LED lighting, daylight control, light occupancy sensors and a high-efficiency water heater. In addition to being used in the cooling tower, reclaimed rainwater is also used to flush toilets.

Photo Caption: 350 Mission is a LEED Platinum-certified, 30-story office tower on Mission Street in San Francisco, directly across from the new Transbay Transit Center in the rapidly developing South of Market district. • Completed in 2015, the $250 million project sits on a scant 19,000-square-foot site. The 420,000-square-foot building is a Type 1 concrete structure with a total height of approximately 420 feet. • The tower is clad in a high-performance, unitized curtain wall with alternating inward and outward leaning panels, giving the facade a “woven” appearance as light reflects off the surface. • The tower features a 50-foot-tall lobby that also functions as a free-flowing, open public space. The space utilizes a unique folding glass wall system that opens the lobby to the street while maintaining the interior climate with an energy-efficient radiant floor heating and cooling system. A real-time digital video screen displays art. • All of the building's prime office space has been pre-leased to Salesforce, a global cloud computing company.

Photo credit: © Skidmore, Owings & Merrill LLP | AJSNY, 2016. All rights reserved.

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