Do your schools still use classroom unit ventilators? If so, consider replacing them with displacement ventilation (DV) units, which use the natural buoyancy of warm air to improve ventilation and comfort. Although relatively new to the United States, DV has been used in Scandinavian countries since the 1970s.

Two types of DV can be used in a classroom:

  • Conventional displacement ventilation (CDV) units are situated on an interior wall and reuse the existing fin-tube radiation on the exterior wall.

  • Induction displacement ventilation (IDV) is situated on the exterior wall of the classroom with a heating/cooling coil integral with the IDV.

IDV is an excellent choice when renovating an existing school. An IDV unit is situated in each classroom on the exterior wall with ducted minimum outside air to the IDV unit and exhaust high from each classroom as shown in the IDV diagram (p. 35).

Displacement ventilation systems have occupancy load sensors, which automatically reduce temperature and fresh-air intake when the room is unoccupied, resulting in energy savings. The ductwork is more compact than conventional ventilation systems and other types of displacement ventilation systems, which is important when retrofitting low-profile buildings with limited space above the ceilings.

Comfort Control

Advantages of displacement ventilation:

  • Cost-effectiveness

    The U.S. Department of Energy states that displacement ventilation systems typically reduce energy costs by 15 percent compared with conventional mixing systems.

  • Higher ventilation effectiveness

    DV has higher ventilation effectiveness because the air is introduced at the occupied zone rather than the unoccupied ceiling zone. The amount of required outside air is 15 to 20 percent less, resulting in less energy because less air needs conditioning.

    In each IDV unit, outside air is mixed with room air, which is conditioned if required, and directed into the room at floor level and at low velocity. Individual comfort is greatly improved as the upward air flow pattern toward the ceiling exhaust promotes removal of heat-borne contaminants and provides improved IAQ in the occupied zone.

    Carbon-dioxide measurements are an indicator of the effectiveness of ventilation in diluting airborne contaminants. The cool supply of air near the floor displaces the contaminants and carries them toward the ceiling by convective thermal plumes. As a result, the CO2 concentration in the occupied zone is lower.

  • Acoustics

    Schools accommodate a range of activities in a variety of spaces. Research has shown that acoustics are less than acceptable in many learning environments. Poor acoustics hinder the learning process, inhibit productivity and contribute to lower test scores. Good acoustics, just like indoor air quality, lighting and physical condition, are crucial in creating a healthful learning environment.

"Classroom Acoustics" by the Acoustical Society of America states that in speech-intelligibility tests, listeners with normal hearing can understand only 75 percent of the words read from a list in many classrooms. Students with learning disabilities, auditory-processing problems, and English as a second language are mainstreamed in these classrooms. Young children are vulnerable because if they miss a few words, they are less able than older students to "fill in" the missing thoughts.

DV helps to meet demanding acoustic requirements for classrooms. The ANSI S12.60 Acoustics Standard and the Collaborative for High Performance Schools (CHPS) both recommend a background noise level of 35 decibels for core learning spaces. DV systems are quieter because of the low air velocities at the diffusers.

Additional Considerations

DV is most effective when the space has either a cooling load or neutral air requirement. CDV is not well-designed for heating. The warm supply air, if supplied from the diffuser at a very low velocity, will tend to rise toward the ceiling exhaust before it can effectively heat the space. When significant heating is required during occupied periods, a supplemental heating system is the preferred method of heating. IDV systems usually contain three induction-displacement terminals on the exterior wall, so during conditions of heating, the center induction displacement terminal will provide heating, and the two induction-displacement terminals will provide neutral air to the space.

The primary architectural design requirement unique to DV is the need for higher ceilings for thermal stratification. A minimum 9-foot ceiling is recommended; ceilings of 10 feet or higher will enhance the benefits. A well-insulated building envelope with high-performance fenestration and exterior shading for non-north-facing windows will moderate peak cooling loads. DV relies on a vertical, buoyancy-driven air movement from the floor supply towards the ceiling exhaust.

A well-insulated building envelope is important for proper system operation during winter. Downdrafts from cold exterior walls and windows will oppose the displacement airflow. With a well-insulated envelope and high-performance windows, DV will work well in the winter when the space has a cooling load.

The energy savings from DV systems are mostly from cooling. More energy savings can be achieved in warm climates, but DV does reduce energy costs in cold climates, too. Efficiency is site-specific: Climate, humidity, building design and other factors will have an impact.

Rydeen, FAIA, is an architect/facility planning specialist and former president of ATS&R Planners/Architects/Engineers, Minneapolis. Stofferahn, PE, is partner and director of mechanical engineering with ATS&R. Lange, PE, is partner and director of mechanical engineering with ATS&R. jrydeen@atsr.com, tstofferahn@atsr.com, jlange@atsr.com

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Replacing Aging Units

The Anoka-Hennepin (Minn.) School District needed to replace aging ventilation systems in several buildings because of costly maintenance, non-availability of repair parts and equipment failures.

Full IDV systems were installed in four elementary schools. Partial DV systems were installed at six other elementary schools and two middle schools. The partial systems are less expensive and were used where there were budgetary restrictions and where at least some part of the existing systems was adequate.

"To find a replacement for the existing classroom univent system, we conducted an exhaustive investigation of ventilation systems and testing. The positive displacement system selected offered the best solutions to a wide variety of obstacles and indoor air quality concerns unmatched by any systems that I have seen yet today," says Louis Klingelhoets, director of building and grounds.

"I have also learned that not all positive displacement systems are the same; some require secondary fans, others require increased fan capacities, thus requiring larger ductwork. The system is energy-efficient and cost-effective. It provides a high degree of comfort and it is less disruptive to classroom activities."