Radiant Heating, Cooling Critical Component
March 2, 2009
The newly rebuilt California Academy of Sciences is a paragon of efficient, sustainable design. Nestled within San Francisco’s lush Golden Gate Park and featuring a 2.5-acre “living roof” planted with vegetation native to the region, the 410,000-square-foot project earned a Leadership in Energy and Environmental Design (LEED) Platinum certification from the U.S. Green Building Council (USGBC), the first museum ever to do so.
The new building, which opened to the public late last September, continues to garner awards for sustainable design, including the Environmental Protection Agency’s regional Environmental Award. The new academy is also the sole U.S. winner of the silver Holcim Award for Sustainable Construction, an international competition involving 1,500 projects in 118 countries.
LONG LIFE CYCLEA 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 Arup and installed by O’Brien Mechanical, also of San Francisco, this radiant system consists of 100,000 linear feet of tubing - Uponor Wirsbo hePEX™ plus tubing, to be exact - connected to six, 20,000-Btuh boilers and three, 4-ton chillers.
“What makes this building so special is its long life cycle - at least 50 years,” said Paul Switenki, one of Arup’s project engineers. “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, as Switenki described, a “bare-box, high-ceiling space with well-shaded glass exterior walls.” 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.
“Instead of using large, mega-horsepower fans that consume energy all day long,” said Switenki, “we’re using low-horsepower pumps” to circulate water through the PEX tubing in the floor.
Radiant offered another major advantage over forced-air: invisibility. “We had a mandate: No ductwork hanging from the ceilings,” said Switenki. “With all of the radiant tubing buried in the slab, no one ever sees it.”
HOW THE SYSTEM WORKSTo warm the slab to heat the space, water is heated in the boilers on the lower level of the building and circulated through a series of pumps. The latter 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,” explained Switenki. “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,” said Switenki. “On those days when the ambient outdoor temperature is 80°F 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, regardless of 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 INSTALLEasily 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.
Such was not the case here.
“We wanted to expose more of the circumference of the tubing to the concrete thermal mass, so that the concrete absorbs and spreads the heat more uniformly than if the tubing were merely sitting on top of it,” said Switenki. “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, and the manufacturer Uponor devised a clever and cost-effective solution. Extra foam insulation panels, made of rigid polystyrene, were cut into 1-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 install, another 2 inches of concrete was poured.
The technique sounds simple enough in hindsight. But before attempting a “live” install inside the new academy, O’Brien conservatively chose to experiment with the novel idea in a series of smaller-scale installs, mocked-up in temporary buildings adjacent to the main jobsite.
“Installation started slowly as we climbed the learning curve,” said O’Brien project manager 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 that the tubing installation itself took roughly seven weeks with a crew of eight to 10 installers.
“To tackle a radiant project of this magnitude was a big undertaking,” he 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.”
For more information, visit www.uponor-usa.com.
Publication date: 03/02/2009