©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 5, May 2012.
By Ryan Thorson, P.E., Member ASHRAE; and David Williams, P.E., Member ASHRAE
About the Authors
Ryan Thorson is a mechanical engineer and David Williams is a high performance projects principal at LHB in Duluth, Minn. Williams is a member of SSPC 189.1. Both are members of ASHRAE's Minnesota chapter.
The School District of Maple, located in the northernmost part of Wisconsin along the south shore of Lake Superior, covers a rural area of approximately 500 square miles (1295 km). The district's aging facilities needed upgrading, including extensive remodeling and additions. All construction had to occur without disrupting classes. The largest of the projects was Northwestern High School in Maple, Wis., which was built in 1937. The oldest portions of the building were removed and other sections were repurposed to give them new life.
The high school project consisted of the renovation and addition to a grade nine to 12 high school in northern Wisconsin (Climate Zone 7) with the goal of improving student services and available programs. The original Northwestern High School was built in 1937 with additions in 1949, 1965 and 1980.
The team used integrated design to create systems that provide operational benefits without an increase in construction capital costs. The project involved large two- and three-story additions, with the remainder of the building being remodeled or demolished to accommodate ADA accessibility throughout the facility.
The steam heating system was completely demolished and the boiler room was relocated and replaced with two high-efficiency hydronic hot water boilers. One chiller was installed to serve four air-handling units, two for the office areas, one for the computer classrooms, and one for the community and school use fitness area.
The building sections allow enhanced performance of the daylighting control and ventilation systems. The selected mechanical systems reduced the amount of ductwork, which enhances the building section design. Exterior materials, such as brick and precast walls, and precast and sloped roofs contribute to the long service life expected for this project.
The project was phased and classes were in session from September through May during the project time line from 2007 to 2009. A new boiler room was constructed during the first phase and the building was converted to hydronic hot water heating, along with natural gas-fired semi-instantaneous domestic water heating. The existing boiler room was demolished during Phase Two of construction. Wisconsin Code requires the building to be designed for winter conditions of –25°F (–32°C). and summer design conditions of DB 86°F and WB 70°F (30°C and 21°C). 2009 ASHRAE Handbook—Fundamentals lists the closest Wisconsin location as Hayward, Wis., which is about 50 miles (80 km) south of Maple, Wis. ASHRAE winter design conditions at 99.6% are indicated as –17.2°F (–27°C), and summer design conditions at 1% of DB 84.3°F and WB 69.8°F (29°C and 21°C). Heating degree days (HDD base 65) are 8610 and the cooling degree days (CDD base 65) are 336.
Reuse Before New
Although many of the high school's component buildings were old, a number of the sections were thought to have value in a renewed facility. The oldest portions were removed. The former small gym that had been converted to classrooms was repurposed again, this time as a sloped floor auditorium. The challenge was to keep it all going while the construction progressed over two years.
The basics of energy efficiency start with the site and building orientation. Changes in either of these areas were limited by the existing site, and the desire to improve the outdoor recreational facilities. The gently rolling site was leveraged through location of new entrances and optimizing cut and fill around the complex. Even the location of the rainwater retention and fire protection tanks considered the impact on the amount of fill required (or not required) at the final location under the building.
As per standard office practice, the envelope insulation system exceeded code minimum; the spray-applied air barrier, and new high performance windows, created the proper conditions to allow the use of low-temperature perimeter heating systems. The generous use of precast concrete panels overlaid with a brick cladding also provided a high performance, high mass, low life-cycle cost envelope system.
Two 3.3 MMBH (967 MW), 95%+ efficient condensing boilers were designed and installed to handle the entire facility. The hydronic heating system distributes hot water out to the heating system at 140°F (60°C) and returns it to the boilers at 100°F (38°C). The air-handling unit heating coils are designed for a 40°F (22°C) temperature drop. This helps to reduce pump sizing and reduce operational costs by 40% and reduce distribution piping sizing by 50%. The water temperature was a good match with the design floor temperature of 80°F (27°C) and allowed the stainless steel condensing boilers to achieve high combustion efficiencies. Air-handling coils, unit heaters and perimeter radiation in the existing remodeled areas did require larger heat transfer surface areas to compensate for the low supply water temperatures. A combustion air control system increases the boiler plant efficiency further by preheating the combustion air.
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