More than 45 years after its construction, Benilde-St. Margaret's School (BSM) in the Minneapolis suburb of St. Louis Park, MN, recently completed a $12 million renovation and expansion project. In addition to new classrooms and administrative offices that serve 1,100 students in grades seven through 12, the 57,000-sq-ft addition to the private Catholic school includes a new 250-seat chapel, a technology-enhanced library/media center, and a fine and performing arts center that features a 480-seat theater.

Prior to the construction project, just three classrooms in the school were air conditioned, with cooling provided by individual window units, while three rooftop units provided A/C to the cafeteria and gym.

Today, six new York rooftop A/C units are helping students enjoy the comfort of A/C in 17 classrooms and throughout other areas of the new construction. "As long as we were creating new space, it seemed only natural to include air conditioning in the project," explained Bob Tift, president of the school. "It also affords us the opportunity to increase our summer program offerings by providing a cool, comfortable environment during the warm summer months."

Existing building fails test - no room for ductwork

Natural though it seemed to include A/C in the project, it was, in fact, a bit unnatural. According to Scott Krumm, project manager with Egan Mechanical Contractors, Inc., "The original building was not built for a central ventilation system or with a plan to add a system in the future. As a result, existing areas that were designated for the addition of air conditioning had no ceiling space for the delivery of conditioned air."

Added Matt Jacobsen, owner of Midwest Mechanical Supply (MMS), "The layout provided the biggest challenge to the installation of air conditioning. We had to take an existing building and somehow get air into the building using a range of rooftop units whose capacity varied according to the cooling demands of the areas they served."

After considering a number of cooling options, Egan Mechanical recommended the installation of six York rooftop units: two Sunline Ultra(r) units (4 tons and 20 tons), three Millennium(r) large commercial rooftop units (two 30 tons and one 40 tons) and one new 60-ton Eco2 rooftop unit. "School officials also liked the idea that a CO2 sensor adjusts the ventilation rate on demand to maintain air quality at a healthy level while minimizing operating costs," Krumm added.

High grades for efficiency

BSM officials also specified a cooling system that was energy efficient, and the 9.5 and 10.1 energy efficiency ratings (EER) of the Millennium and Eco2 units respectively qualified the school for rebates through Xcel Energy, the local power provider. "When you talk about energy efficiency," said Jacobsen, "it's important to note that the high-efficiency Eco2 unit is optimized for HFC-407C, and is the first environmentally responsible unit that meets the ASHRAE 90.1-2001 efficiency standards. And, like the Millennium units, the Eco2 unit uses an outdoor air economizer to provide 'free' cooling, with the compressors turned off, when the ambient air is sufficiently cool and dry to air condition the building."

Eco2 and Millennium units also respond to environmental concerns with their energy-efficient operation. Lower kilowatt consumption means fossil-fuel plants produce less CO2. According to Krumm, the units' multiple steps of capacity control also attracted the attention of school officials. "With these units, they have the ability to select from four different stages to deliver the most efficient off-design energy performance and maintain better control of occupant comfort."

Easy installation earns extra credit

The ease with which the units were installed further convinced Krumm they made the right choice. "The installation of the rooftop units went smoothly," he recalled. "Their compact footprint and easy-to-assemble curbs made it simply a matter of getting the crane, lifting the units to the roof, and dropping them into place."

Added Jacobsen, "The convertible airflow design of the units was also important during installation. It was easy to convert the units to horizontal discharge and run ducts along the roof before entering the building, compensating for the lack of existing interior ductwork."ES