The computer lab and the gymnasium require vastly different loads, as do the lunchroom, band room, and administrative offices. It was clear that Chisolm Trail Mid-dle School needed a more sophisticated comfort system in order to meet all these requirements.

Its primary goal was to increase the fresh air to conform with the 15 cfm per student required by ASHRAE 62.99. The old system consisted of water-source heat pumps that couldn’t address the ASHRAE guidelines. Corroded drain pans were causing water leak problems. In addition, there were noise issues with ceiling-hung blowers in the classrooms, and the school district needed a more energy-efficient system to service the school’s 102,000 sq ft. In an effort to improve the indoor air quality (IAQ), humidity control became a major consideration.

“The old system was nearly 25 years old, and it simply wasn’t up to the task,” says Les Reddin, maintenance director for the Round Rock Independent School District.

Poor IAQ in educational facilities can lead to short- and long-term health problems, such as headaches, allergies, and asthma. It also promotes the spread of infectious diseases, increases absenteeism, creates an unfavorable learning environment, and deteriorates the physical building and equipment, leading to costly renovations.

According to the Environ-mental Protection Agency (EPA), good IAQ contributes to a favorable environment for students, teachers, and staff, which increases productivity and promotes comfort, health, and well-being. These elements combine to assist a school in its core mission of educating children.

The Solution

The new system features 3-, 4-, and 5-ton Lennox L Series® units with the Humiditrol™ option to handle the classroom loads and two 15-ton units to handle the cafeteria and band hall. The Humiditrol option controls humidity within the space, especially when internal loads are not sufficient to create a cooling demand, but humidity levels are high.

This option also will go into the reheat mode, removing moisture from the space, based on a call from the humidity sensor independent of the room thermostat. To further enhance humidity control and balance energy costs, the units were equipped with enthalpy economizers.

In an effort to further improve IAQ, ventilation and CO2 ratings were also examined. Because occupancy in a space can be directly correlated to the CO2 level, and the occupancy of various rooms change dramatically throughout the school day, the decision was made to install demand control ventilation. CO2 sensors were installed in the ceiling of each classroom with a thermostat and a humidity sensor on the wall. Modulating ventilation based on actual occupancy rather than design occupancy allows a reduction in energy costs without compromising IAQ.

Reddin explains, “The idea was to bring in outside air and condition it, then deliver it to each classroom. The advantage of package units versus a chilled water system centers not only on control, but also service.

Understanding that service is required and inevitable during the life of the system, it is more convenient and practical to have multiple units so that it is possible to have partial cooling in most service situations.”

In order to provide greater temperature control, a controller was installed in each unit, which provides limited control (±3ºF) of the space temperature setpoint.

The equipment arrived on time and installation went according to schedule. The project was started on May 25, 1999, right after school let out, and finished in time for the August 16 start date. After six months of equipment operation, Reddin reports the school has not experienced any humidity problems since the Humiditrol units had been installed.

Publication date: 04/16/2001