Red Wing High School's mechanical systems were designed and built around Trane EarthWiseâ„¢ centrifugal chillers, high-efficiency boilers from PVI Industries, and 26 Trane Modular Climate Changerâ„¢ air handlers equipped with variable-frequency drives and Traqâ„¢ dampers. The air-distribution system includes six 25,000-cfm energy recovery heat wheels manufactured by SEMCO.

Benefits Of Indoor Comfort

Kevin Johnson, director of buildings and grounds for the Red Wing Independent School District, continues to find and implement energy-saving strategies while enhancing indoor comfort.

"It is well documented that quality indoor environments are much more conducive to learning," he said. "Here at Red Wing High School, thanks in part to the air-handling units and round ductwork, our classrooms have very little noise from the ventilation system. Couple that with HEPA filtration and an energy-recovery system, and I feel we provide a quality environment in which to work and learn."

Today, as in the 1990s, balancing IAQ with energy efficiency is a major concern. Air quality requirements mandated by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standards 62-89, Ventilation For Acceptable Air Quality largely reversed the impact of the late 1970s post-oil embargo reduction in proper air ventilation to occupied spaces. The challenge has been and continues to be to provide outside-air ventilation without driving energy costs through the roof.

During the school's design, it was clear that heating or cooling more outside air to meet IAQ requirements would dramatically increase the school's energy costs. However, numerous innovations ensured good IAQ without excessive energy consumption.

The Chiller

At the heart of the HVAC system are a 200-ton and a 400-ton Trane CenTraVacâ„¢ centrifugal chiller. The chillers consume 0.59 kW per ton and 0.56 kW per ton of electricity at full load, respectively - very efficient even by 2004 standards.

Johnson reported one problem with the school's chiller, which was covered by the Trane warranty, during the nearly 10 years of operation. "These chillers are bulletproof," he said.

A Tracer Summitâ„¢ chiller plant management system monitors and controls chiller operation, and uses a BACnetâ„¢ interface to communicate with the school's overall control system, provided by Egan Automation of Brooklyn Park, Minn.

Cooling towers for the chiller plant's condensing water system are equipped with variable-frequency drives. Air distribution is provided by 26 Modular Climate Changer air handlers equipped with axial Q fans and variable-frequency drives. The project also includes 300 variable air volume boxes with reheat coils.

Measurement and control of outside air traditionally has been achieved only through a combination of airflow-measuring stations and outside air-damper control. The Trane Traqâ„¢ dampers, introduced to the market in 1995 as the school was being built, use factory- mounted direct digital control to integrate these functions to measure and control outside air. Red Wing High School was one of the first buildings to use the dampers, which are now widely used on the air-handling units' mixing box modules.

Also important to the school's IAQ and energy efficiency is the use of six 25,000-cfm energy-recovery heat wheel units installed in the intake and exhaust air plenums.

Manufactured by SEMCO, the units transfer sensible and latent heat from the building's exhaust airstream to precondition the incoming outdoor air.

The efficient sensible and latent heat transfer characteristics make the enthalpy wheels effective in both heating and cooling applications. The wheels (also equipped with variable-frequency drives to control their capacity) installed at Red Wing High School are 80 percent effective - on a design heating day, -20 degrees outside air is preheated to 52 degrees by the energy-recovery units.

Reducing Energy Costs

Preconditioning outside air during the heating season reduced the original design boiler capacity requirements by 400 bhp and produced first-cost savings of more than $500,000. In recent years, Johnson has been able to meet the school's heating needs for most of the heating season by converting and using one otherwise unused potable hot-water heater, without using the boilers.

On a design cooling day, 150,000 cfm of outside air at 92 degrees dry bulb/75 degrees wet bulb is reduced to 79 degrees dry bulb/65 degrees wet bulb. This preconditioning capacity allowed the system designers to reduce the chiller-plant capacity by 440 tons, resulting in significant first-cost savings. Eliminating the need to install heating coils in the air-handling units resulted in an additional first-cost avoidance of $100,000.

The HVAC system design also allowed the use of smaller chilled- and hot-water piping sizes for more first-cost savings. The high school also qualified for numerous rebates from Xcel Energy, the power utility serving the area.

Planned IAQ

Johnson noted that to get the best IAQ possible, Red Wing Schools has a board-approved Indoor Air Quality Management Plan. "Our IAQ plan is based on the Environmental Protection Agency (EPA) ‘Tools for Schools' IAQ program designed specifically for schools. The plan's goal is to provide IAQ that contributes to a favorable learning environment for students and productivity for teachers and staff, as well as an environment that is as healthy as possible."

Johnson also serves as the school district's overall IAQ coordinator. The plan was developed with help from the U.S. EPA; Institute for Environmental Assessment; Minnesota Department of Health; Department of Children, Families and Learning; and Red Wing Health and Safety Committee, plus extensive input from staff.

The IAQ plan was approved by the school board and went into effect in July 2001.

As a result, HVAC systems maintenance became much more aggressive and frequent. All air filters in the district were changed from low to medium efficiency, or to high-efficiency HEPA filters (none of which use fiberglass) based on equipment capabilities. Prefilters are used to extend the life of the HEPA filters.

The IAQ plan also encompasses regulations pertaining to cleaning supplies and air fresheners (not allowed, unless supplied by custodial staff) and plants and animals in the classroom (requires permission).

Red Wing High School principal Larry Sonju said of the school's comfort and air quality, "The folks who really know are in the classrooms. They speak very highly of the HVAC systems here.

"Our head custodian, Al Harteneck, and his staff do a great job operating the systems for best efficiency. Kevin Johnson, director of buildings and grounds, came to us from another school system in Minnesota a couple years ago. He has done a terrific job in setting the right direction. We very much respect his judgment."

Sonju added, "I think that it is important that people understand that there is a real advantage to having good indoor air quality."

Publication date: 02/07/2005