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Chisago Lakes Middle School reduced its natural gas consumption by nearly 20 percent after replacing an inefficient steam boiler within a 93 percent efficient condensing boiler.

Case Study: Chisago Lakes Area Schools

New efficient condensing boiler saves energy at Middle School

Aims and objectives. With an inefficient steam boiler and natural gas prices remaining high, in 2008 the Chisago Lakes Area Schools sought improvements to save energy and reduce its utility bills. It decided to suspend reliance on a 62 percent efficient steam boiler at the Middle School that was required to maintain an inefficient operating temperature of 228 degrees F. The steam boiler was kept to supplement heating during the coldest periods and is shut down the rest of the heating season.

Implementation. Chisago Lakes Schools installed a 93 percent efficient condensing boiler at its 140,000 square-foot Middle School. “The new boiler system has worked great for us,” said Tim Burton, director of Buildings & Grounds. “Since the installation, we have not run our backup 6 million BTU existing steam boiler. The new boiler has handled the full building load year round. We have seen a huge reduction in maintenance costs. We no longer have to completely tear down the boiler each year and clean tubes. The new system also reduced the radiant heat issue we were having in surrounding rooms. The heat generated by the old plant would overheat the office above the boiler room and rooms to each side.”

Key to efficient boiler operation—low return water temperatures. Condensing boilers require a low return water temperature to operate at their highest efficiency, according to several sources, including information from the Center for Energy and Environment. In general, heating systems must be able to perform adequately at return water temperatures below about 130 degrees F in order to obtain boiler efficiencies above 87 percent (see Figure 1). Thus, the more hours during the heating season that the temperature of water returning to the boiler can be kept below that value, the higher will be the average seasonal efficiency. High-efficiency boilers are especially well suited to applications such as snow melting and in-floor radiant heating, because those applications typically have a large temperature drop and low return water temperatures.

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Figure 1. This graph shows the effect of inlet water temperature on boiler efficiency.

Outcomes--energy saved, operations improved. The Middle School’s annual natural gas consumption has decreased by 19 percent from 2010 to 2012 (see Figure 2). The boiler installation was completed at the end of the 2010-11 heating season, so the benefits to gas consumption occurred during only half of calendar year 2011. The B3 Benchmarking ratio for the Middle School is currently 51 percent, meaning it uses only about half as much energy than if it were a new building built to current energy codes.

Annual natural gas consumption

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Figure 2. This Minnesota B3 Benchmarking chart shows 19 percent natural gas savings in 2012 for the Middle School.

Documenting results. Chisago Lakes Area Schools is able to quantify its savings via the Minnesota B3 Benchmarking tool. The B3 tool is available for all state, local government, and public school buildings. To add your building to this database, go to the B3 website and click on “Contact Us.”

“All the tracking I do is through the B3 program,” said Burton. “The B3 program is a very useful tool for me.  The graphs are great for my own understanding as well as for presentations.  The recent B3 updates have been helpful and I feel they have made the program stronger.” 

Funding. Chisago Lakes Area Schools received a $111,000 Energy Efficiency and Conservation Block Grant from the Minnesota Department of Commerce. The grant was provided by the American Recovery and Reinvestment Act of 2009, the federal stimulus program designed to save energy and create jobs. Financing was aided by an energy conservation rebate from Xcel Energy, the school’s natural gas supplier. The boiler upgrade cost was $222,000.

Resources for public building projects. Public institutions seeking energy efficiency improvements are encouraged to consult with their local gas or electric utility. All utilities in the state are mandated to achieve an annual energy savings of 1.5 percent of annual retail energy sales. Utility representatives can help assess opportunities for efficiency and identify what rebates may help finance projects.

The Minnesota Department of Commerce, Division of Energy Resources (DER) offers technical support to local government units, state agencies, school districts, and institutions of higher learning that are seeking energy efficiency and renewable energy measures. Contact DER to discuss which of its efficiency programs may be of help. 

The U.S. Department of Energy’s Advanced Energy Retrofit Guide for K-12 Schools provides guidance for building retrofits (you’ll need to register with DOE to be able to download your free copy). For more on energy efficiency, including information on B3 Benchmarking, contact the DER Information Center at energy.info@state.mn.us or 800-657-3710.