The source of these savings is a customized Semco (Columbia, MO) total energy heat recovery wheel built into 10 of the school's 13 air-handling units (AHUs) manufactured by Johnson MarCraft (St. Louis). These and other features enabled the Fanning/Howey design team to downsize the school's boilers and chillers substantially, reducing energy costs while providing a comfortable year-round learning environment for 1,800 students.
Contemporary ConstructionAlthough the concept of heat recovery is not new, the process has been updated and made more efficient through technology. The wheels are made of aluminum core or plastic, and coated with a desiccant (typically silica gel for plastic and molecular sieve for aluminum core wheels) that transfers water molecules from one air stream to the other, thereby using sensible and latent heat already in the interior space to preheat or precondition the outdoor air required for ventilation.
"Heat recovery wheels have not been used often in recent years because of the high initial equipment and installation costs," says Terrance R. Liette, P.E., executive director Fanning/Howey. "Older systems also had an unusually high number of maintenance problems. We know of many older systems that have been abandoned due to maintenance issues."
Liette adds, however, that technology and expertise have made them more attractive. "Better-quality materials and construction have resulted in longer equipment life and lower maintenance costs," says Liette.
Tim Lehman, Fanning/Howey's mechanical department coordinator says several factors are making the systems increasingly attractive. "Although systems with heat recovery wheels remain somewhat more expensive than systems without heat recovery, costs can be offset by smaller boilers and chillers. The result is little or no first-cost penalty, immediate payback through lower energy costs, and operating savings for the life of the building," Lehman says.
Smaller Components, Easier MaintenanceFor the Northville High School project, which was completed in September 2000, Fanning/Howey specified Semco aluminum core, molecular sieve, total energy heat recovery wheels, which were built into Marcraft custom AHUs. These units range in size from 1,800 cfm to 80,000 cfm. "We considered having separate components for the heat recovery system and air handlers, but elected to use custom units with built-in wheels," Liette says.
"While more expensive, this approach vastly reduces the number of parts, making the system easier to maintain." In fact, Lehman says the only additional maintenance concerns are "having an extra set of filters to replace, and the occasional need to grease bearings," which are typical maintenance operations performed on most mechanical equipment. No unusual maintenance operations are required for this equipment.
In winter conditions, Northville High School's heat recovery wheels can preheat the incoming outdoor air from 0 degrees F to as high as 58 degrees while adding moisture to the dry air. The system also is capable of lowering the outdoor air temperature and removing moisture in the summer. The reduction of the ventilation air load enabled the Fanning/Howey design team to offset the higher up-front cost of the heat recovery equipment with downsized boiler and chiller plants.
The building uses two Bryan Steam (Lancaster, PA) 4,000-MBtuh flexible water-tube heating water boilers, and two 340-ton water-cooled, indoor, rotary screw chillers from Trane (La Crosse, WI). Respectively, these systems are 50% and 20% smaller than those required for a conventional hvac system in a comparably sized school building.
An "A" in EfficiencyNorthville High School is also expected to achieve long-term operational savings through the use of high-efficiency vav fans and a building automation system with zone control, enabling school officials to shut down areas of the building when not in use. Only the school's 12,400-sq-ft natatorium uses a different type of ventilation system. Along with the unique temperature and humidity control needs of an indoor pool, the space features a curved translucent panel wall for diffused natural light. Here, Fanning/Howey designed a separate constant-volume ventilation system including a self-contained heat recovery and dehumidifying unit to maintain space conditions.
"Heat recovery holds a great deal of promise for school systems seeking to optimize the cost effectiveness of new or existing hvac systems," Lehman says. "In the coming years, we're confident that Northville Public Schools will find the operational savings resulting from the smaller boiler and chiller to be well worth the investment." ES