Davidsen Middle School

Davidsen Middle School is a new 130,000 square foot middle school with 1,511 student stations on a 26 acre tract for general education of sixth, seventh, and eighth grade students. Special purpose instructional spaces include an art studio, music suite and physical education facility with exterior play courts, play fields and a running track. Exceptional students are also accommodated within the facility. Vocational education includes spaces for Business Technology, Family Consumer Sciences, and Communications Technology. The core support spaces of the program include a Media Center, an Administration and Student Services Suite, a Multi-Purpose Room, a Gymnasium, and a Food Service facility.

A Construction Manager was selected by the Owner and was involved in pre-construction services throughout the design process. With a project budget of $89 per square foot and a fourteen-month construction period, it was essential that building systems, scheduling and delivery methodologies became an integral component of the design process. The design team acknowledged from the beginning that many design decisions would be budget and schedule driven.

The School District challenged the engineering team to find a system that not only met the School District's own design guidelines, but was both cost effective and energy efficient. After a Life Cycle Cost Analysis that compared six different types of air conditioning systems, the team decided to use a chilled water system with fan coil units to serve the classrooms. VAV air systems serve the administration and media buildings, and constant volume systems were used in the cafeteria and gymnasium. But real savings were realized with the use of fan coil units serving the majority of the school along with decoupled fresh air systems providing dehumidified fresh air.

Fan coil units have been used in schools for many years, but in Florida, they have fallen into disfavor. The reason? Fan coil units are simply not very good at moisture removal, especially during part load operation, and the ambient air in Florida is generally very humid. To handle the moisture load from the fresh air, decoupled fresh air units distribute fresh air through a separate duct system directly into each room. This strategy makes humidity control easy, efficient, and inexpensive. The fresh air units provide dehumidified air at 53 degrees F, while the fan coil units provide comfort cooling. The fan coil units are inexpensive, easy to operate and maintain, and provide zone temperature control without the use of expensive VAV boxes.

Several other innovative energy conservation techniques were applied that had the effect of reducing the construction cost for the HVAC system as well.

The two 180 ton air-cooled screw chillers are piped in a series arrangement to achieve a 15 degree F temperature drop. The benefit is smaller pumps and piping, and less pumping energy than when a 10 or 12 degree F temperature drop is used. The chillers do not require separate primary loops, so only two chilled water pumps are necessary - a primary pump and a standby pump. Three-way control valves are used at each of the coils throughout the school. The chilled water distribution system is very simple with no VFD, no secondary loop, and a simplified DDC control system.

Dual path units are used for the VAV and constant volume air systems. The dual path unit has a separate fresh air path so that the fresh air (the air stream with the most moisture) is cooled and dehumidified as the first stage of cooling. This strategy saves both cooling and reheat energy by avoiding the overcooling of spaces to achieve humidity control during periods of part load.

The gym air handler utilizes two-speed operation to save energy. During the event mode (basketball games and assemblies), the fan runs at full speed to deliver the maximum airflow. During non-event periods (gym class), the fan runs at a slower speed to deliver the minimum expected airflow, and the outside air damper closes to deliver minimum fresh air. Since the gym air handler is normally run in the non-event mode, large energy savings are realized.

In Florida, providing good indoor air quality means not only compliance with the ventilation rates recommended by ASHRAE 62, but it is vitally important to control humidity and reduce the opportunity for mold growth. The systems at Davidsen Middle School provide positive humidity control through the use of decoupled fresh air and dual-path systems. Decoupled fresh air systems deliver cool dry fresh air directly to the classrooms. The dual path units mix cool dry fresh air into the supply air stream, which, in turn, is delivered to the spaces through the supply air ductwork.

Unlike the fan coil units we all remember from the past, the fan coil units at Davidsen Middle School are double-wall units with positively sloped stainless steel drain pans. The ductwork is galvanized steel with external insulation wrap. These surfaces do not support the growth of mold and mildew like duct liner and duct board, do not collect as much dust, and are much easier to clean when dirty.

The controls sequence includes the capability for electric heat in the fan coil units to be energized to reheat should the space become subcooled by the fresh air unit. This approach gives the capability of performing zone reheat. Providing continuous reheat in the decoupled fresh air systems to deliver air to the space at a continuous 74 degrees F was considered, but would have added approximately 65 tons of load.

A thermostat controls each fan coil unit giving teachers the freedom to find a comfortable setting within reason. The classrooms were designed by a sound consultant to maintain a noise level of NC 25. Accordingly, the ductwork systems for the classroom areas incorporate a fully ducted return making each fan coil unit so quiet that it is difficult to discern whether it is running in an empty classroom. The result is an enhanced learning environment. This is a very comfortable, livable system.

Maintenance for the fan coil concept has been made relatively simple. Each fan coil unit is installed in a closet, and is accessible from the classroom it serves. Filter changes and other maintenance procedures are easily performed through the closet door. On the airside, there are no VAV boxes to troubleshoot and maintain, and no variable speed drives to fail. The 100% fresh air unit is a constant volume air handler, eliminating the need for an airflow sensor and damper control to maintain constant fresh airflow rate as would be required in VAV systems. On the waterside, there are fewer pumps to maintain because of the series chiller arrangement. There are no variable speed drives and greatly simplified controls, because the staging of chillers in a series chiller arrangement does not require starting and stopping of pumps, and opening and closing of valves. Quite simply, there are fewer moving parts to fail.

Test and balance operations become more streamlined (and less expensive) with fan coil units. On the one hand, VAV boxes must be calibrated through the energy management system and then set for both maximum and minimum flows. On the other hand, each fan coil unit requires only a single airflow setting.

Probably the greatest benefit from the use of fan coil units is their low cost. The HVAC system at Davidsen Middle School was installed for $10.05 per square foot in the year 2000, while HVAC systems in architecturally similar schools cost between $15 to $20 per square foot with other conventional systems. Importantly, these savings are realized while the energy usage is very competitive with that from other efficient, but more expensive, VAV systems. For school districts that are perennially strapped for capital, this is a tremendous benefit - $5 to $10 per square foot that can be spent on teachers, computers, and textbooks. The result? More cost-effective schools.

All of the technologies employed at Davidsen Middle School are familiar to most of the readers of this paper. The old fan coil concept with a new design has met the needs of the School District. The innovative approach of using an old technology, improved through the use of new technologies, has benefited the School District by combining economy, simplicity, maintainability, comfort, and efficiency.