HVAC Technology Report: Greening schools
Read the other articles in the HVAC Technology Report series:
" Systems design and performance tips for packaged rooftop units (RTUs )" March 2009
" BAS design issues " June 2009
When planning a green building, designers and engineers face a chicken-and-egg question: What comes first, the building envelope or the mechanical systems? In the case of green schools, the building envelope comes first. If the building's envelope and siting are considered before any mechanical systems are put in place, engineering the building is easier and less costly.
The experts interviewed for this article agree that the building envelope—not the mechanical systems—must be the starting point when designing green schools. For example, to achieve greater control of a school's indoor environment, Donald Clem, LEED AP, senior architect with Steven Winter Assocs. Inc ., Norwalk, Conn., tries to convince his clients not to order operable windows. By considering the physical aspects of the building first, they can save money on engineering the HVAC system.
Peter D'Antonio, PE, president of PCD Engineering Services Inc ., Longmont, Colo., expands on the concept: “We've found that putting your money in the building envelope gives the best return on your investment because then your mechanical systems will be as small as possible,” he said.
D'Antonio advises design teams to consider three passive ways to take advantage of solar energy:
Building orientation: “Can we open the building to the south to allow heat in when it's needed in the winter, and yet keep heat out with shading devices in the summer?”
Daylighting: “Because lighting is typically about half of the electric energy load of a school, we look for the possibility of long, narrow buildings, open to the south, to maximize daylighting opportunities in all rooms.”
High-efficiency building envelope: “Can we tune the windows—with glass on the south side that allows more solar gain in the winter but keeps it out in the summer? And on the north, can we use a different type of glass that keeps the energy in and supplements daylighting?”
“School administrators are becoming aware of the type of roofing, the color of the roof, roof gardens, insulation on the roof, the type and number of windows, the insulation on the exterior walls, the type of lighting, and the lighting controls,” adds Anil Ahuja, PE, president of CCJM Engineers Ltd . in Chicago. “All play a role in making the building more efficient. We do energy modeling with every [U.S. Green Building Council] LEED school. That's when we advise the architects to change the envelope parameters.”
Does LEED really work?
Though LEED has recently received considerable attention from popular media, there are still nagging concerns about the system's effectiveness. Many commentators point out the obvious: LEED assigns credits before a building has been operated, but the only real way to know a building is green is to gather data about energy use and environmental stewardship over time.
In fact, in a study of 100 LEED certified buildings, published in August 2009 by Canada's National Research Council, Guy R. Newsham, Sandra Mancini, and Benjamin J. Birt concluded that “on average, LEED buildings used 18% to 39% less energy per floor area than their conventional counterparts. However, 28% to 35% of LEED buildings used more energy than their conventional counterparts. Further, the measured energy performance of LEED buildings had little correlation with certification level of the building, or the number of energy credits achieved by the building at design time.”
Furthermore, D'Antonio points out that “less than 10% of buildings going up today are green. 'Green' has become greatly overused and watered down.”
Bearg agrees: “A lot of people are just going through the motions; there's a lot of green-washing going on!”
Nonetheless, everyone interviewed for this article agrees that LEED is a step in the green direction. As D'Antonio said, “It's important that we define what green means, and that's what LEED is all about.”
Decouple ventilation, thermal comfort systems
We asked the experts what kinds of energy-related problems they find in existing buildings. Bearg said a common problem stems from the fact that variable air volume (VAV) systems combine cooling and ventilation functions. In his article, “Achieving and Maintaining Healthy Green Buildings,” in the Journal of Green Building (Vol. 4, No. 1), Bearg pointed out that, while retrofitting a school with more energy-efficient lighting equipment will reduce the waste heat produced, this can have a negative effect on ventilation: “Since the quantity of supply air provided by a VAV system is directly proportional to the amount of cooling required, this lighting retrofit can directly reduce the quantity of supply air delivered, leading to a… degradation in IAQ due to [the] lowered ventilation rate.” For this reason, Bearg and others recommend a variety of strategies to decouple ventilation and thermal comfort systems in schools.
Pat Dolan, project manager for Chicago-based CCJM Engineers, said, “We try to avoid high-velocity air speed from diffusers. With school kids of all ages—but especially with the youngest children—it is so uncomfortable to be close to the discharge from the supply diffusers.”
Dolan expects variable refrigerant (VRF) systems to make strong in-roads in the school market. He said that with VRF, “you are continually cooling the ambient air in the space, rather than the ventilation air introduced into the space. But because you're not necessarily using ceiling diffusion to mix tempered air with ventilation air, you can provide a lot of cooling with a relatively low amount of airflow compared with ceiling diffusers.”
When asked to examine the HVAC system, Bearg discovered two things: First, “the air came in at the ceiling, hugged the ceiling, and left at the ceiling. There was a lot of air movement across the ceiling, but there was no movement in the occupied zone down where the children were sitting.” Second, the heat recovery system had been greatly compromised and was delivering only 40% outside air rather than the 100% that had been expected.
The lessons learned were:
1. Commissioning is essential.
2. Demand control ventilation (DCV) with diffusion at or near floor level is a better solution. Because the delivered air volume in a DCV system is based on occupancy determined by CO2 sensing, DCV systems save on fan energy.
However, there are also some good reasons to be wary of CO2 sensing. For one thing, recent studies, including one conducted by the Lawrence Berkeley National Laboratory in 2006, have revealed that several tested CO2 sensors were unreliable. One way to solve this problem, said D'Antonio, is not to rely on CO2 sensors. “Because we specify energy recovery ventilators that reduce the ventilation load by up to 75%, we might not need demand-control ventilation,” he said.
On the other hand, Bearg said choosing a higher-quality shared CO2 sensing system can overcome the reliability issue. He added that the centralized shared-sensor approach allows simultaneous monitoring of multiple IAQ parameters, including absolute humidity.
Another emerging technology that allows separation of ventilation from thermal comfort systems is the chilled beam, which was developed in the 1980s in Scandinavia. Since the “green light bulb” has only recently been switched on for large numbers of American politicians, executives, and school officials, the United States is just now catching on to the value of chilled beams.
Like VRF systems, chilled beams save energy compared to VAV and fan coil units for two reasons. First, in a chilled beam system, cooling is accomplished by circulating a concentrated thermal agent—water—rather than air, which requires less energy. Second, chilled beams condition the space via convection rather than by pushing air with energy-guzzling fans.
The downside of chilled beams, said Bearg, “is that you've now got this cold surface, and if you don't do a good enough job of controlling humidity in the building, you run the risk of having condensation on the cooled surface. So that means chilled beam systems require a highly effective air barrier so you're not letting warm, humid air leak into the building during the summer months. And if there is condensation, you need to provide some way to drain the water away harmlessly.”
Ahuja cited a different kind of barrier to specifying chilled beams in schools: unions. “Most of the mechanical contractors are experienced only in installing traditional mechanical systems,” said Ahuja. “The sheet metal workers are not really trained [to install chilled beams] and their union is concerned that you will eliminate their workers—and they're very powerful.”
Given the importance of mechanical insulation in energy and acoustical performance of school buildings, Clem stated that mechanical insulation is a factor. “There are the whole acoustic requirements of LEED for Schools that affect mechanical [systems] with sound attenuation concerns (e.g., duct lining, sizing and duct placement, and fan discharge),” he said.
Clem also noted that some green building tax credit programs specifically address insulation/mechanical use. For example, un-faced batts or blankets and blown- or sprayed-in insulation materials are not allowed in plenum spaces above ceilings or in areas where air handling equipment is located due to potential of respirable fibers.
Commissioning and O&M: The final frontiers
No matter how energy-efficient an HVAC system is on paper, it may never save a nickel's worth of energy if it is not properly commissioned and if the school's O&M staff members don't understand how to run it.
Clem emphasizes the importance of having the right players on the design team. “The owner has to really be involved, and you want your commissioning agent (CxA) at the table early in the process,” he said. He also stressed the importance of training for the commissioning and operations staffs, which is required in LEED.
D'Antonio agrees that the staff must be trained in and committed to energy conservation. “Energy conservation deals not only with design, but with the operational phase—the teaching element, training, and behavioral practices at schools,” said D'Antonio. “We've seen schools with very basic systems achieve the Energy Star rating because they were practicing good energy management.”
Ahuja points out that many old school buildings are just now receiving their first air conditioning systems because of the resurgence of summer school programs. “The maintenance staff came from janitorial ranks,” he said. “When we go back to schools where we installed direct digital control (DDC) in the 1990s, we see that the digital controls have been bypassed or not kept up because the guys are intimidated by the system. Or they've incorrectly reset the setpoints. Or they open the ceiling and jam the damper open.” Ahuja said his company educates the superintendents to ensure they understand these issues.
Ahuja also notes that high-performance systems such as geothermal require commissioning to ensure that they are working per specification. Most geothermal systems include multiple pumps, two-way valves, de-superheaters, flow controls, and extensive piping. The CxA must verify that those components were installed correctly and are operating as intended.
“We have been asked to review recently constructed buildings in which the geothermal system is so complicated that no one knows if the system is working correctly,” Ahuja said. “In fact, in one case, the building engineer operated the building in manual mode in order to avoid a complicated control system. The system was running around the clock 365 days a year. In that case, a CxA would have recognized the problems right away and saved the school district thousands of dollars.”
So after all of the high-tech modeling and designing, and after the super-efficient systems have been installed, it still comes down to the most basic issue: How do you convince everyone at a school—superintendents, teachers, students, and on down to the janitor—that the goal of energy-efficiency is worth their time and attention?
Read more about greening schools in this month's MEP Roundtable " Improving school systems ".
LEED the way
The cut-off date for registering a building under U.S. Green Building Council LEED version 2.2 was June 26, 2009. Since then, LEED accreditation must be conducted under LEED v3. And until the end of 2009, projects that were registered under v2.2 can migrate to v3 with no additional fees.
Donald Clem, LEED AP, senior architect with Steven Winter Assocs. Inc., Norwalk, Conn., whose firm has been involved in LEED since the USGBC began the LEED program, listed some of the major differences between versions 2.2 and 3:
• USGBC has standardized LEED so that all rating systems, including LEED for Schools, use the same credits and all are 100-point systems. However, in LEED for Schools v3, buildings can earn 10 additional points for either innovation or special measures that are unique to each region. The regional point idea addresses complaints that previous LEED versions forced a single standard on buildings in widely varying climatic and regulatory environments.
• There is greater emphasis on sustainability in v3. For example, there are more points in v3 in the Water Efficiency and Energy and Atmosphere categories.
• LEED v3 reflects a ratcheting upward of requirements in ASHRAE 90.1. For example, walls that were required to be R-8 under ASHRAE 90.1-2004 must be R-15 under ASHRAE 90.1-2007.
• LEED v3 provides more flexibility insofar as which system to use. For example, non-academic buildings on a school campus can be registered either under LEED for New Construction or LEED for Schools.
• LEED for Schools v3 puts new emphasis on acoustics. There are points for duct linings, larger ducts, and placing machinery away from classrooms.
Geothermal systems: The good earth
Compared with chilled beams, geothermal heat pumps are better understood and more generally accepted in the United States. CCJM Engineers' Pat Dolan and Anil Ahuja discussed their experience with geothermal systems in two schools: Prairie Crossing Charter School in Grayslake, Ill., and Peterson Elementary, the first Chicago Public School building to have a geothermal system.
At Prairie Crossing, the geothermal system is unique in that it substitutes for a cooling tower. The system was installed in two phases for the school's two-building campus. Dolan and Ahuja said that, because geothermal systems are expensive compared to other thermal comfort systems, payback is always an important consideration. They estimate the payback for Prairie Crossing at about 5.1 years for Phase I and about 4.5 years for Phase II. (Peter D'Antonio, president of PCD Engineering Services Inc., on the other hand, said he usually sees paybacks in the range of 10 to 15 years in his Colorado projects.)
Dolan said that for Peterson School, another important consideration was the availability of space for the geothermal well field. “One of the criteria that prevents us from using geothermal in every case, especially in the city of Chicago, is that you need to provide generous spacing of the wells to maintain long-term permits and to avoid temperature drift in the ground,” he said. “You also have to be sure of constructability. Any well deeper than 500 ft becomes problematic because of static head produced by the well. The ideal is to limit the depth to 300 ft and to place the wells 12 to 15 ft apart on center—so the well field can handle the tonnage needed by peak cooling. We'd prefer that to be green space, but we've also used ground source under paved areas for light vehicle parking. The problem is that you want the ground to remain saturated during peak load times to get the best heat transfer between the wells and the ground. But with paved areas, you get very little percolation of storm water, so you rely mostly on ground moisture to provide the conductivity.”
D'Antonio also is involved in an innovative geothermal system. His company is designing a geothermal loop that is coupled with active solar thermal collectors for a school in Silverton, Colo., high in the Rockies. “We used a combination of energy analysis programs—eQUEST, fChart, and some in-house calculations—to model the system,” he said. “The solar collectors charge the earth with heat, which ultimately increases the geothermal system's efficiency. We can do this all through the summer when we're getting solar gain and school's not in session—and any other time when we're not in heating mode. The concept is made possible because there's no cooling load at 9,000 feet above sea level.”
He questioned Dolan's statement that, ideally, wells should be limited to 300 ft with 12- to 15-ft spacing. He said that in Colorado, his company routinely gets good results with 400-ft wells and 20- to 25-ft spacing.