Grade A HVAC

The Chicago Public School system, which serves 410,000 students in 650 schools, likes to design its new buildings the old-fashioned way. That's not to say that its new schools are antiquated or out of date—quite the contrary. The Chicago Public Schools, or CPS, has gone back to basics in its MEP system design.

03/01/2008


The Chicago Public School system, which serves 410,000 students in 650 schools, likes to design its new buildings the old-fashioned way. That's not to say that its new schools are antiquated or out of date—quite the contrary. The Chicago Public Schools, or CPS, has gone back to basics in its MEP system design.

Joseph Clair, PE, managing engineer in the CPS department of operations, and his team of engineers realized—based on anecdotal evidence from the stationary engineers—that some of the oldest schools, those built around 1900, were well-lit and well-ventilated, and required only basic system upgrades. The buildings also lived up to standards that the USGBC touts today, though the 100-something year-old facilities didn't have the U.S. Green Buildings Council to guide them when they were built.

“We've found that these schools, with their good wall construction and daylighting, meet or exceed standards set forth today by organizations like ASHRAE or USGBC,” Clair said.

The CPS mirrored the older buildings' design, and selected brick and concrete block construction for its new schools. And similar to old schools, such as Von Steuben High School, newer buildings have larger windows. Though the older buildings had leaky windows, the CPS now has replaced nearly 90% of them, increasing energy efficiency and overall performance.

In 2007, green building was a significant portion of the construction industry. According to RSMeans Business Solutions, Kingston, Mass., owned by Reed Construction Data, about $21.6 billion was spent on new education construction projects. About one-fifth of that—279 projects totaling 25 million sq. ft—has RFPs or bidding documents that stated contractually that they were requiring these buildings to be submitted for USGBC LEED at some level.

So CPS is not alone. It's actually ahead of the curve.

Many older buildings in Chicago met these green standards because the city already was forward-thinking in its building goals, but in 2003, the City of Chicago declared that all newly built public buildings had to meet USGBC LEED 2.0 Certified standards.

CPS recently has investigated setting a target that its buildings use no more energy than what would fall on the site as renewable energy. So each new facility is built greener, with the whole site in mind, accounting for water efficiency (via water-efficient toilets), IAQ, integrated design (including the engineers, architects, and construction team from the beginning), and energy efficiency.

CPS' energy metric looks to ASHRAE 90.1-2007 and Energy Star building benchmarking as its energy design standard, and aims to either exceed the ASHRAE standard by 35% or achieve the Energy Star benchmark in the 80th percentile in all cases based on models during the design phase. Many of the CPS's old buildings—those built more than 100 years ago—have more thermal mass than the newer structures, so they were already coming close to this number.

But the school system is growing, and new buildings are springing up around the city. Welcome to Albany Park Multicultural Elementary School, the second school built under Chicago's new “green schools” umbrella. Construction started in April 2005 and was completed in September 2006, just in time for the beginning of the school year. Albany Park serves 720 students, though the building is only half-occupied right now, though it should be full within the next couple of school years. The school embraces all aspects of green and recently was certified at USGBC's LEED Certification level.

The 104,000-sq.-ft building has 26 classrooms and sits on a 4.5-acre site. In support of Mayor Richard M. Daley's effort to make Chicago the most environmentally friendly city in the nation, green building strategies are integrated into the design phase of each city project, including this school.

Building excellence

The HVAC system uses three condensing boilers, and each is sized for half of the load such that the failure of one will allow for 100% of the load to be serviced on a design day. The boilers operate at a range of temperatures from 160 F to 180 F based upon load, and cycle on and off as needed.

These boilers are the primary part of the HVAC system, because the school is heated more than cooled throughout the school year. The boiler room, which houses three units, is on the third floor, reducing the stacks outside the building, set in a densely populated neighborhood.

The school also has three variable air volume systems and two constant volume systems, with building automation systems attached to them. All five air-handling units (AHU) are capable of bringing in 100% outside air and are full economizing. Heat recovery was considered, but does not work due to the low amount of exhaust air at the system level caused by the general and toilet exhaust requirements. This system is not demand-controlled, but school designs from here on out will be, according to Clair. Occupant comfort is the primary concern in system design, so the engineers are controlling to temperature, with a dehumidification strategy if the space humidity is above ASHRAE 55 comfort levels. There are occupancy sensors, but they are currently only for lighting. CPS is moving toward incorporating them into Albany Park and future schools' HVAC system as well as part of an effective ventilation and energy-efficiency strategy.

Elementary schools like Albany Park within the CPS system do not get cooling towers, but the large high schools do. The package at Albany Park does not have variable primary chiller, as the system was designed in 2003, but future systems will, as well as variable-drive chillers.

Commissioning made the big difference in efficiency of these systems. Though the engineers don't have enough significant final performance data yet, the preliminary data suggest that commissioning has been the biggest boost to energy efficiency certification through the local gas and electric utilities.

Energy models predicted that the facility would use around 700,000 kWh of electricity each year (for non-plug-loads) and 38,000 therms per year. The first full year of data suggests the usage at around 1 million kWh of electricity (including plug-loads) and 32,000 therms. The cooling system commissioning was not able to be completed during the first cooling season, so with further refinement, the expectation is that the electricity usage will come down. Compared with previous prototypes of similar size and use, Albany Park is performing at about 75% of the electricity usage and 75% to 80% of the natural gas usage. It is expected that the next generation of buildings will be able to significantly surpass this performance as design integration and technology improve the overall building performance.

“Our commissioning authority puts together a measurement and verification plan consistent with the International Performance Measurement & Verification Protocol tracking the first year of performance against baseline, and gives us the tool to continue to do so for subsequent years,” Clair said.



How green is your school?

Though geared mostly toward school officials, MEP engineers should take note of the new “Advanced Energy Design Guide for K-12 School Buildings,” published in early 2008 by ASHRAE and other key developers.

The 174-page tome outlines how HVAC, commissioning, water efficiency, and a host of other engineering-related factors can have a big impact on our nation's youngest scholars. It links academic performance to students' surroundings, and offers both statistics and case studies for background.

For example, the report indicates that favorable light, sound, and temperature can help students learn better. In many cases, improving these attributes also can reduce energy use. Much of the background of this report is based on “Greening America's Schools: Costs and Benefits” by Greg Kats, in which he provides 17 studies that demonstrate productivity increases of 2% to 25% from improved IAQ, acoustically designed indoor environments, and high-performance lighting systems.

Advanced, energy-efficient heating and cooling systems create cleaner, healthier indoor environments that lower student and staff absentee rates and improve teacher retention. This translates into higher test scores and lower staff costs. Overall, HVAC really does play a role in education.

Download the guide for free at

Reference: Greening America's Schools Costs and Benefits, A Capital E Report, October 2006. Report prepared by Gregory Kats. Sponsoring organizations include American Federation of Teachers, American Institute of Architects, American Lung Association, Federation of American Scientists, and the U.S. Green Building Council.

Warming students' soles

To help combat the cold northern Illinois climate, the Prairie Crossing Charter School, a public K-8 school in Grayslake, Ill., is heated and cooled by heat pumps using a geothermal vertical-loop as a heat exchanger. The building has an in-slab radiant loop connected to the geothermal loop for heat in the classrooms and corridors.

This unusual system was somewhat costly. The Illinois Clean Energy Community Fund provided a $62,500 grant for the geothermal heat pump system, or about $4.55/sq. ft. Originally, the project team considered several high-performance options, but with the additional grant money, settled on adding the geothermal system.

The school, which serves 360 students in approximately 13,800 sq. ft, just achieved USGBC LEED NC Gold version 2.1 for its phase one building. There are three new buildings at the school, two classroom buildings and a gym, that were built with the same environmental principles. The one-level building project began in March 2004, with phase one consisting of classroom space completed in December 2004, and phase two completed in 2007. The building is wood-framed, with modular-built 2x6 ft walls and residential-style wood roof trusses. The exterior walls have both R-19 batt insulation and a layer of rigid polystyrene insulation. The roof has two layers of 2-in rigid polystyrene and asphalt shingles.

The tight building envelope helped make it more energy-efficient. The architects incorporated continuous rigid insulation on the outside of wall framing with taped seams and caulked joints between the slab and wall sills. They added sealing around windows and roof eave joints. The envelope was design to approximately 1 to 1.5 air changes/hour at 50 Pascal (ACH50). No blower door or pressure tests were performed for the final construction.

To help the IAQ, the two-pane windows are a mix of operable and inoperable wood windows with gray-tinted, low-E glazing with argon gas fill. The teachers (and other staff members) can open windows and modulate interior temperatures with exterior temperatures without complex economizer control strategies. The central corridor has automatically controlled windows up high, which helps passively exhaust air from the classrooms. All south-facing windows have deep roof overhangs, which reduce solar heat gain.

The school does not have active humidity controls because a psychrometric analysis of the school's HVAC system showed that the heat pump system, along with two enthalpy energy-recovery wheels, kept RH within ASHRAE 55 comfort ranges. Because the school faculty actively use the operable windows in each classroom, they also have a reasonable expectation of RH levels in the classroom. Each classroom has a thermostat for both the ventilation air and the radiant heating slab.

The school uses approximately 7.15 kWh/sq. ft/year. The building performs 60% better than ASHRAE 90.1-1999 requirements using the USGBC LEED Energy Cost Budget (ECB) methodology. Compared to 90.1-2004, the system is about 40% better. Because the energy-analysis software cannot model the radiant loop, it was ignored for their analysis.

The analysis was based on climactic data for Chicago (Table D-1 of ASHRAE 90.1-1999) and the building envelope requirements prescribed in Table B-17 of ASHRAE 90.1-1999.

Additional insulation: The U-value for exterior walls is 0.038 versus 0.089 allowed.

Energy-recovery ventilation units: The building uses two units that have enthalpy wheels for air tempering, with two-speed control motors. The units are rated at 76% efficient.

High-efficiency glazing: The assembly U-value for the windows and glass doors is 0.31 versus 0.63 allowed.

High-efficiency heat exchanger: The building uses a geothermal well consisting of 36 vertical wells at an average of 175 ft deep. The energy cost budget model uses an air-cooled condenser for heat rejection. The cooling capacity of the system is estimated at 45 tons.

High-efficiency terminal equipment: The classroom building's heat pumps have an energy-efficiency ratio (EER) of 17.1 and a coefficient of performance (COP) of 3.5, versus code at EER of 9.3 and COP of 3.8. It also has a boiler efficiency of 80% for the baseline high-performance equipment.

Variable speed drives: The fan motors on the classroom heat pumps have variable-speed electronically commutated motors.



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