San Francisco, California
Natural History Museum
Radiant Heating and Cooling
Arup & Partners
Uponor products mentioned in this case study
COMMERCIAL: Radiant Heating and Cooling
CALIFORNIA ACADEMY OF SCIENCES
Completed in October 2007 after breaking ground two years earlier, the new building won’t open to the public until October 2008. But the project is already garnering awards for sustainable design, 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 AND COOLING
“What makes this building so special is its long life cycle — at least 50 years,” says 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 a “bare-box, high-ceiling space with well-shaded glass exterior walls,” says Switenki. 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,” Switenki continues, “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, says Switenki. “We had a mandate — no ductwork hanging from the ceilings. With all of the radiant tubing buried in the slab, no one ever sees it.”
HOW THE SYSTEM WORKS
“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 explains. “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,” says 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.
“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. 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 Uponor devised a clever and 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 install, 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 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,” recalls 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 estimates 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,” says Payne. “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.”
ONE OF THE BIGGEST
“Radiant is a system we like to specify,” says Arup’s Switenki, “but sometimes owners will balk, especially if they are looking at the short-term. Maybe it’s because they are 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 energy costs.”
The design information in this case study is provided for illustrative purposes only. The actual requirements of similar projects will depend on regional climatic conditions, project-specific heat loss, owner expectations, applicable building codes, etc. Please contact your Uponor representative for assistance in designing your specific projects.
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For more information about the benefits of cross-linked polyethylene (PEX) tubing, contact a reputable manufacturer, such as Uponor North America (www.uponor-usa.com).
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