Visitors to the newly rebuilt California Academy of Sciences will find it a paragon of efficient, sustainable design. Nestled within San Francisco’s lush Golden Gate Park and featuring an undulating 2.5-acre “living roof” planted with vegetation native to the region, the 410,000-square-foot natural history museum earned a Leadership in Energy and Environmental Design Platinum certification from the U.S. Green Building Council—making it the largest public facility to garner the certification, according to the Academy’s Web site at www.calacademy.org. 
   Completed in October 2007 after breaking ground two years earlier, the new building started garnering awards for sustainable design before it was even open, including the Environmental Protection Agency’s regional 2006 Environmental Award. A year earlier, its construction barely begun, the new Academy was the sole U.S. winner of the silver Holcim Award for Sustainable Construction, an international competition involving 1,500 projects in 118 countries.

Radiant Heating, Cooling
A key aspect of the project’s commitment to green values is a state-of-the-art radiant floor heating and cooling system that provides energy-efficient comfort to 38,000 square feet of the main exhibition level. 
   Engineered by the San Francisco office of engineering and design firm Arup and installed by O’Brien Mechanical, Inc., also of San Francisco, this radiant system consists of 100,000 linear feet of Wirsbo hePEX plus tubing from Uponor connected to a half dozen 2,000,000-BTU/hour KN-20 gas-fired direct-vent condensing cast-iron boilers from HydroTherm. Also installed was a trio of 4-ton centrifugal chillers from McQuay. 
   “What makes this building so special is its long life cycle—at least 50 years,” said Paul Switenki, a project engineer at Arup. “Given that longevity, we were motivated to choose systems that may be more costly initially, but that will pay for themselves with energy savings over time. We know from experience at Arup that radiant is a very energy-efficient way of heating and cooling the space.”
   The Academy’s main exhibit area is a “bare-box, high-ceiling space with well-shaded glass exterior walls,” Switenki said. That, along with San Francisco’s mild climate, makes it an ideal application for radiant, which keeps the heating (or cooling) near the floor—where museum visitors and personnel are situated—not blowing around near the ceiling, as with a conventional forced-air system. 
   “Instead of using large, mega-horsepower fans that consume energy all day long, we’re using low-horsepower pumps” to circulate water through the PEX tubing in the floor, Switenki said, adding that radiant offered another major advantage over forced-air—invisibility. “We had a mandate—no ductwork hanging from the ceilings,” Switenki said. “With all of the radiant tubing buried in the slab, no one ever sees it.”

How it Works
To warm the slab and heat the space, water is heated in the boilers on the lower level of the building and circulated through a series of pumps. They push the water through heat exchangers that transfer the heat to manifolds connected to the PEX tubing. The water then circulates back through the system to the boilers for reheating, and the cycle begins anew. 
   “At night, when the space is shut and empty, the glass will let a lot of the heat out, so the system will keep the indoor temperatures up at a reasonable level,” Switenki said. “In the morning, the system will heat the space early enough to make it comfortable as people begin to arrive. During the day, you won’t see the heat used too much, depending upon the occupancy load.”
   The radiant cooling process works in a similar fashion, but uses chillers to provide lower-temperature water. Because the local climate is so mild year-round, the demand for cooling is neither frequent nor substantial. But striving to connect the interior of the new Academy with its natural surroundings, architect Renzo Piano made extensive use of clear glass for many of the exterior walls. As a result, the potential for some heat gain on warm days is inevitable. 
   “Since we were using the PEX tubing for heating, we figured we might as well use it for cooling,” Switenki said. “On those days when the ambient outdoor temperature is 80 degrees Fahrenheit or warmer, the chilled-water cooling may be used to ‘top off’, or augment, the natural ventilation system that is part of the design.” 
   It is also likely that different parts of the structure may demand heating and cooling simultaneously no matter what the outdoor temperatures. For example, even on colder days, the server rooms with computer equipment will still need cooling. Depending on whether heating or cooling is required in any of the building’s nine radiant zones, the temperature of the circulating fluid will be adjusted accordingly by opening or closing a heating or cooling valve.

Innovative Installation
Easily the biggest design and installation hurdle was determining the depth of the PEX tubing to optimize the heat transfer to the concrete slab. Typically, the installer places panels of foam insulation over the slab, covers it with wire mesh, and then staples PEX tubing to that deck insulation or ties it to the mesh. Insulation and the tubing are then covered with another layer of concrete. 
   “We wanted to expose more of the circumference of the tubing to the concrete thermal mass, so the concrete absorbs and spreads the heat more uniformly than if the tubing were merely sitting on top of it,” Switenki said. “This necessitated that the tubing be raised off the deck insulation a little bit—roughly an inch.”
   To create that extra inch, Arup, O’Brien Mechanical and Uponor devised a cost-effective solution. Extra foam insulation panels, made of rigid polystyrene, were cut into inch-thick strips and then attached with a spray adhesive to the deck panels already in place. Plastic tracking rails were then positioned atop these polystyrene risers, and the loops of PEX tubing were threaded through them. Wire mesh was then laid on top of the tubing, not under it. To finish the installation, another two inches of concrete was poured. 
   The technique sounds simple enough in hindsight. But before attempting a “live” install inside the new Academy, O’Brien Mechanical conservatively chose to experiment with the novel idea in a series of smaller-scale installs, mocked-up in temporary buildings adjacent to the main job site.
   “Installation started slowly, as we climbed the learning curve,” said O’Brien Mechanical project foreman Randy Payne. “But once our crews mastered the cut-and-glue process—particularly the spacing of the tracks across the space—we began picking up time until the work became almost second nature.” Payne estimated the tubing installation took roughly seven weeks with a crew ranging in size between eight and 10 installers.
   “To tackle a radiant project of this magnitude was a big undertaking,” Payne said. “We had to sit back and plan carefully, testing our ideas before going all-out. But in the end, it didn’t prove super-difficult—just another mechanical piping system, which is what we do at O’Brien. After a project of this size, I would foresee the next job being a lot easier.”

Really Big
The radiant installation at the California Academy of Sciences, in terms of sheer square footage, is currently one of the largest in North America. Given the international prestige of this project, it is also among the most important. The energy efficiency and sustainability of radiant heating and cooling were an excellent fit with the institution’s stated mission of, in the words of Academy executive director Dr. Gregory Farrington, “inspiring visitors to conserve natural resources and help sustain the diversity of life on Earth.” 
   “Radiant is a system we like to specify,” said Arup’s Switenki, “But sometimes owners will balk, especially if they’re looking at the short-term. Maybe it’s because they’re new to it, or because there’s a premium attached to it. But once a building with radiant is up and running, this technology really does a good job of saving on energy costs.”