After numerous water leaks, compressor failures, and damper control problems, it was time for South High Community School in Worcester, Massachusetts to replace their 30 year old HVAC rooftop units.
Not only had the units outlasted their expected mechanical life, but the ability to deliver fresh outdoor air and maintain comfortable heating and cooling conditions became increasingly difficult and expensive. To specify a state-of-the-art replacement HVAC roof-top system, the energy and environmentally conscious school district enlisted the services of a local engineering firm.
Energy recovery ventilation
Director of Facilities, Jeff Lassey met with engineer Mike Lescar-beau of Lindgren & Sharples, P.C. to establish new system design requirements. Electric heat would need to be replaced with a lower cost energy source. To avoid altering the recently installed rubber roof, the new replacement units would need to utilize existing roof curbs and ductwork.
Lassey wanted to include energy recovery ventilation (ERV) technology to minimize this energy load
Because the replacement units would also be supplying outdoor air to meet the schools IAQ requirements, Lassey wanted to include energy recovery ventilation (ERV) technology to minimize this energy load. Lassey had utilized the technology on a neighboring high school three years earlier to successfully lower operating costs. To meet the new design challenge, Lescarbeau specified a total of (19) gas/electric rooftop units with integrated state-of-the-art Airxchange energy recovery wheels to replace the all-electric units.
Building automation system
Without energy recovery ventilation, HVAC systems waste energy contained in building exhaust air while consuming new energy to precondition code-driven fresh air requirements. By contrast, systems utilizing Airxchange wheels reuse (recycle) up to 80% of the energy in exhaust air to heat, cool, dehumidify or humidify a like amount of incoming fresh air.
This recycling of energy can reduce HVAC design loads and annual energy use by as much as 50%. As a result, Lescarbeau was able to specify smaller, lower cost heating and cooling units to easily fit within the existing roof curb profile. Adaptor curbs were designed and fabricated by a local curb supplier to fit the new rooftop units to the existing curbs. Direct digital controls and modulating dampers were added for individual zone control by an existing building automation system.
Outdoor air energy savings
The wheels would have provided an additional $100,000 annually in outdoor air energy savings
Airxchange energy recovery wheels were calculated to save South Community High School approximately $60,000 annually or $1.2 million in dollars over the average life of the new roof-top units. The initial added investment in wheel based rooftop units is expected to be recouped during the first year of operation based on savings from the Airxchange wheels alone.
Had the school not converted their heating energy source from electric to gas, the wheels would have provided an additional $100,000 annually in outdoor air energy savings. “For roughly the same price as a larger more energy intensive rooftop unit, we can provide the same amount of work with a smaller, wheel based unit that performs with up to 40% greater efficiency. This translates to significant energy savings over the life of the equipment,” said Craig Campbell, Trumbull and Campbell Associates.
Improving indoor comfort
The replacement HVAC rooftop system earned South High Community School a Central Mass Green Award for energy efficiency. The award was created by the Worcester Business Journal (WBJ) to recognize organizations ‘going the extra mile’ to make green choices. According to the WBJ, “We all know being in business is about making money.”
“But thanks to technological advancement, and a realization that our global resources are finite, making sustainable choices can both ease the conscience and help the bottom line.” South High’s energy recovery wheels are estimated to reduce CO2 emissions by 265 tons annually which translates to approximately 353 lbs per student. “With the wheels integrated into the DX rooftop units, the installation went very well. We’ve also received positive comments about the improved indoor comfort and air quality,” said Mike Lescarbeau, Lindgren and Sharples.
