By Mike Grant, Regional Drives Specialist, Yaskawa Electric America

For years, Gene Trull fielded complaints from students, teachers and administrators about the humidity and poor indoor air quality at Timber Ridge Elementary School. As maintenance supervisor for the Cobb County schools in suburban Atlanta, Trull oversees the heating, cooling and ventilation needs for students in one of the country’s hottest, muggiest regions.

In the past, Timber Ridge had relied on a conventional packaged variable air volume (VAV) system to handle its HVAC needs. Due to their inadequate cooling capacity, HVAC units at the school could not accommodate the required level of outdoor air without delivering leaving-coil temperatures of approximately 62° F. While this left rooms cool enough, humidity levels routinely soared to between 70 and 80 percent. In addition to the comfort problems caused by excessive humidity, the school experienced frequent and persistent mold problems in its classrooms.

Attempts to combat the problems by closing outdoor air dampers and installing commercial dehumidifiers failed. These solutions decreased humidity and eradicated mold, but they also resulted in air-quality complaints and inflated energy costs. In addition, the solutions failed to meet the required levels of outdoor air mandated by the ASHRAE 62 Ventilation Standard.

Cobb County schools are not alone: More than 95 percent of all commercial buildings in the United States rely on rooftop units for their air conditioning requirements. These units are compact, reliable and cost effective. Just like at Timber Ridge, this type of air conditioning equipment is often unable to provide the increased amounts of outdoor air mandated by ASHRAE 62. Since all the major building codes across the country recognize that standard, building owners and managers are increasingly finding themselves in non-compliance.

Attempts to deliver the required outdoor air quantities by over-sizing traditional units are not the answer. This approach tends to result in short cycling situations in compressors during off-peak conditions. This in turn causes extended periods of high space humidity, which drives building managers to lower thermostat settings to achieve the desired temperature. In addition to creating cold and clammy environments, lowering thermostats significantly increases energy costs.

View HPAC Engineering article - January 2006 (page 68 and 69)