A Clearview Of The Future
Clearview Elementary School in Hanover, PA, was designed first and foremost to help children succeed academically. School district officials were well aware of the scientific studies conducted by the California Board for Energy Efficiency, which showed learning rates, as measured by test scores, were 16% to 26% higher in classrooms with daylighting. In addition, they knew that proper ventilation was crucial in helping students and teachers stay healthy, alert, and focused on learning.
That being said, constructing a "green" school was not originally on the drawing board, but school officials were definitely open to the idea. It's a good thing they were, because the 43,000-sq-ft school, which was completed in 2003, has become a showcase for integrated green design. Indeed, Clearview, designed by L. Robert Kimball & Associates (Harrisburg, PA), earned Gold-level certification through the USGBC's LEED® Green Building Rating System.
Among the reasons for Clearview's acclaim is the fact that it is expected to use 40% less energy than a comparable conventional school building. This savings is achieved through the use of insulated concrete forms, triple-glazed windows, optimized solar orientation, daylighting, indirect lighting, ground-source heat pumps, and an underfloor air system. These types of components are often considered to be budget-busters, but thanks to the integrated design used at Clearview, the first cost was no more than that of a traditionally built school.
INTEGRATED BENEFITSThe Hanover School District had definite ideas as to what it wanted for Clearview. These included good IAQ, energy efficiency, high levels of ventilation, and flexibility. The last point was important, because they wanted a school that could easily incorporate any future IT systems without having to alter the school's infrastructure and without having a large impact in terms of cost and disruption. The best way of accomplishing their list of requirements was to employ integrated design.
"Quite simply, the key to understanding cost-effective, green buildings is integrated design," said John Boecker, AIA, LEED AP, vice president and director of high performance green design for L. Robert Kimball and Associates.
Boecker described integrated design as understanding the impact of each system upon all others and the syntheses between them, much like understanding the systems within your body. "Think of the building more as an organism, then analyze the direct and indirect impacts of all these systems and energy-efficiency measures by modeling different combinations of systems in order to find the combina-tion that indeed represents the most energy efficient and highest performing building."
At Clearview, a critical component of the integrated design process was the underfloor supply air plenum distribution system. These types of systems augment ventilation effectiveness by using nature's principles as their model. In a typical overhead air distribution system, fresh air is blown down, in opposition to natural convection, and spent hot air is pushed down into the breathing zone rather than allowing it to rise according to the laws of physics.
In an underfloor supply air plenum system, the fresh air coming from floor diffusers reaches the breathing zone far more effectively. "With this type of system, you're participating with natural convection," said Boecker, "as opposed to trying to mechanically fight against it. Very simply, it is a minimally pressurized plenum that is designed to create a vortex of air that stratifies at about two meters, thereby delivering fresh air down at the level where kids breathe."
Underfloor air distribution systems are more efficient because the air is more easily accessible to the breathing zone and the temperature of the air leaving the cooling coil can be increased by several degrees. "You have to be careful, though," cautioned Boecker. "Because if you increase this temperature too much, you can approach the dewpoint of air in the plenum under certain conditions. This could, if you're not designing it properly, create condensation on the steel pedestals that support the raised floor panels. That moisture can lead to mold growth. You just have to understand that temperature, relative humidity, and dewpoint are important components here and design accordingly."
The ability to control the airflow from the underfloor air distribution system was also part of Clearview's integrated design. Rather than a single thermostat on the wall, each one of the school's floor diffusers operates as a metering device. It's possible to open and close the apertures in the baskets beneath the diffusers, thereby controlling the volume of air being disseminated into the space at each diffuser location. This means there is more control over the thermal comfort levels in each space.
The underfloor air distribution system also met the school's requirement for flexibility, because if there is a need to reconfigure, it is not necessary to move any ductwork, a fairly expensive and time-consuming proposition. Rather, the floor diffusers can be relocated simply by picking up the carpet tile, unscrewing the four corners of the panels that have the diffusers in them, and moving them around wherever they are needed.
This flexibility is possible, said Boecker, because the whole area beneath each classroom can be thought of as a huge VAV box. "The key is that the plenum must remain pressurized, which means that you have to be sure that it is pretty well sealed, not exactly air-tight, but tight enough to hold pressure relative to the slightly lower pressure in the occupied space. That's how it works."
Minimizing ductwork also played into the green nature of the school, because no ceiling was required. Without a ceiling, there was no need to mine, manufacture, or transport additional building materials, which further reduced environmental impacts.
There were additional benefits to not having a ceiling in the school: With only lights, sprinklers, and the structure overhead, the ceiling could be much higher, thereby allowing indirect lighting to be used more effectively. In addition, the higher volume in the classrooms made these spaces more comfortable in terms of proportion. And the teachers loved the exposed joists, since they could hang artwork and other educational materials from them.
GROUND-SOURCE SELECTIONWhat integrated design typically means for HVAC systems is that the size of the supply and distribution systems can be minimized, and therefore, first cost can be reduced. This reduction often more than offsets the additional cost of all the energy efficiency measures that are installed. This is exactly what happened at Clearview.
After carefully considering all options, ground-source heat pumps were chosen for the school. This type of equipment is not typically found at schools, since boilers and/or rooftop equipment are usually less expensive. The more energy-efficient ground-source heat pump systems could be used here, however, because the school's loads were reduced due to a more robust thermal envelope, lower lighting power densities, building orientation, etc. Thus, the system and all of its associated components could be smaller.
When installing ground-source heat pumps, the most expensive part of the system is the well field. "The idea is that you can reduce the size of that well field by reducing the loads, resulting in a much more efficient and much less costly system to operate over the life of the building. These operations costs, by the way, will dwarf the initial construction costs. And, of course, all the associated environmental impacts are reduced as well. In the end, smaller, more efficient systems allow us to build these buildings at the same or less cost than those with conventional systems," said Boecker.
The best laid plans can sometimes go awry, however, and that happened at Clearview as well. During construction, it was discovered that the location of the well field was untenable, which meant that the wells had to be moved to another part of the site. The depth of the wells also needed to be changed, and in turn, the number of wells changed as well. As a result of these modifications, the well field ended up being undersized.
This was discovered during a cold spell, which occurred in the second month of occupancy, when the outdoor temperature dropped to approximately 0°F for a week. The temperature inside the school hovered in the mid-60s, which was a bit chilly. The undersized well field resulted in the water temperature returning from the wells in the morning at around 33°, when the system wanted to recharge with all the heat pumps demanding warm water from the wells simultaneously. The closed loop well piping simply didn't have enough surface area for heat exchanging with the earth to raise the temperature of the water high enough to allow the heat pumps to provide comfortable air temperatures.
While this situation created some short-term discomfort and concern, it also provided a wonderful learning opportunity. "This taught us something that we will now use on every project: The solution was to install a supplemental hot water boiler," said Boecker. "It was a small capacity boiler, and we looped it into the system with proper controls. That cost about $20,000 extra, but it solved the problem."
The extra cost of the boiler actually ended up being less than what it would have cost to install a larger well field capable of handling peak heating capacity under the coldest exterior temperature conditions. Boecker said that they learned to intentionally not design well fields for peak capacity but instead, these systems should be sized to accommodate less-than-peak loads and be equipped with a supplemental boiler.
"If we do that well, maybe all we really need to do is oversize the domestic hot water system and connect that into the ground-source loop in a very efficient, cost-effective way," said Boecker. "In essence, this equates philosophically to not building the church for Easter Sunday. It was a useful discovery, although it was a lesson learned the hard way."
IAQ MEASURESAs can be expected, the design of Clearview emphasized superior IAQ. Each classroom has a CO2 monitor, which is tied into the building's DDC system. If a sensor rises above an adjustable setpoint, which initially was established at 800 ppm, then the DDC energizes the heat recovery unit's supply and exhaust fans.
Boecker said that while heat recovery was incorporated into the Clearview design, he would have to consider it carefully on future projects. That's because the energy saved by recovering the heat from the exhaust can be offset by the fan power required to run those supply and exhaust fans.
"Oftentimes, the energy used to run the fans is more than that which can be saved by recovering the heat. We're finding that more frequently, but this absolutely must be analyzed on an individual project basis. In short, it's possible that a ‘free cooling' economizer cycle might be more energy efficient than heat recovery. It's project-specific, though."
The filters were also upgraded, but not to the MERV 13 level required by LEED credit for IAQ construction management. The reason behind this decision was that each one of the distributed ground-source heat pump units would have required modification post-market in order for them to accommodate the MERV 13 filtration requirements.
By this point, many are probably wondering just how much this school cost to build. Surprisingly enough, even with its many innovative features, Clearview's $6.3 million price tag for hard construction costs was ten cents less per square foot than the average cost for all elementary schools built in Pennsylvania during the same year. Additionally, an annual energy cost savings of $34,000 is projected. Additional design fees were needed, though, to engage in the modeling analysis required, but the school district procured grants to cover most of the extra design fees.
"It was unusual to suggest at the beginning of the design process that we would be able to build a green school at no additional construction cost," said Boecker. "We were hoping we could get there, and we did, because the school was willing to spend a little more time and money during design if it meant improving the health, welfare, and performance of its students."