For most of the 20th century, the design of heating, ventilating, and air-conditioning (HVAC) and lighting systems for educational classrooms changed very little.
For part of the century, cast-iron steam radiators generally were installed in classrooms. These radiators were oversized so classroom windows could be opened to provide ventilation. Typical design provided for an exhaust shaft adjacent to each classroom entry door, often with a steam-induction coil mounted within it to create a “draft” and induce the flow of fresh air across the classroom from the open windows. Because there were no controls, and the temperature of the low-pressure steam in the radiators could not be varied, classrooms were overheated a majority of the time.
In the 1920s and 1930s, schools tried to gain control of classroom air temperatures and ventilation airflow quantities. They replaced one of the cast-iron radiators in each classroom with a floor-mounted “unit ventilator”(UV). A UV was a cast-iron steam radiator connected to an outside air intake opening and fitted with a fan to draw a fixed quantity of ventilation air across the radiator and discharge it into the classroom. During occupied hours, the UV delivered pre-heated ventilation air to the classroom, and during unoccupied hours it functioned as a radiator. Again, exhaust or “relief” shafts were typically situated adjacent to each classroom entry door.
In the following decades, enhancements to the basic UV system were made.
Cast-iron radiators were replaced with fin-tube radiation.
Exhaust shafts in the classrooms were connected to exhaust fans, which were interlocked with the UVs to operate whenever the UVs were operating in the occupied mode.
Steam coils were replaced with hot-water coils, so that coil temperatures could be controlled more easily (via valve modulation at the coil or reset of hot-water temperature at the heating plant).
Controls packages were added to the UVs to allow for operation of the UVs in “free cooling” modes.
The use of UVs for classroom HVAC has endured through these many decades for a variety of reasons. When the primary requirement of the HVAC system is to heat the classroom, the most effective place to locate the equipment is directly below the location of the greatest heat loss — beneath the windows.
The relatively small slab-to-slab dimensions associated with schools in the last half of the century discouraged the use of ducted systems, especially in classroom wings. UVs allowed for room-by-room zoning in classrooms, which experience large swings in occupancies. UVs were simple to operate and maintain, and they could be operated even without power and without moving parts, as radiators, to prevent property damage.
The changes in school design criteria in the last 10 years have caused HVAC designers to re-examine the choice of UVs for classrooms. Some of the potential problems with UVs:
A dramatic reduction in fenestration and improved insulation has reduced perimeter wall heat loss and has made perimeter heating less critical.
The larger outside air openings resulting from modern ventilation airflow requirements enhance the possibility of coil freeze-ups and of the entry of windblown contaminants (especially through intakes located at or near grade)
The increased concern over indoor air quality (IAQ) in classrooms makes the very low level of filtration (10 percent in UVs, versus 30 percent or greater in typical air-handling units) and the poor air-distribution efficiency associated with UVs less acceptable.
The increased use of air conditioning in schools means that additional piping (such as chilled water and condensate drain-pan piping) must be run to exterior walls and coordinated with casework.
Updated energy codes generally require that energy recovery be incorporated into the HVAC design in most applications; the large quantities of pre-heated (and, in air conditioned schools, pre-cooled) outside air introduced into school classrooms by floor-mounted UVs are exhausted directly to the outdoors.
A properly designed HVAC system for modern classrooms will incorporate features that respond to these issues. Depending on the climate and architecture, the system may be a central station variable-air-volume air-handling unit (AHU), with each classroom provided with its own thermostat; a water-source heat pump (WSHP) system, with one or more heat pumps per classroom; a two- or four-pipe fan coil unit (FCU) system, with one or more FCUs per classroom; or a system of package rooftop VAV units, with each classroom provided with a thermostat.
If room terminal equipment such as WSHPs or FCUs, is used, these units should be ducted if the budget allows. This will enable these systems to provide for the same level of air distribution efficiency as the AHUs. The minimum level of filtration efficiency should be 30 percent, and 65 percent is preferred.
Finally, some form of energy recovery should be employed in any classroom HVAC system, so that the large quantities of outside air required for classroom ventilation can be precooled or preheated by the equally large quantities of classroom exhaust air. Energy wheels, fixed-plate heat exchangers, and runaround coils are among the available options today; these options may be provided in stand-alone units or as components of AHUs.
In the earlier part of the 20th century, schools depended primarily on daylight for the majority of classroom and common area illumination. Daylight was provided to the interior of most schools via skylights, light wells or windows. The only light fixtures provided were incandescent, and they were used only on cloudy or rainy days with reduced daylight.
As lighting technology developed, schools increasingly were provided with fluorescent lighting. Typically, the lighting consisted of direct/indirect, pendant-mounted fixtures; these fixtures were fitted with magnetic ballasts and T-12 lamps.
Maintenance always has been an issue for schools, and lighting is no exception. Often, classroom lighting would end up with a variety of lamp colors (cool white, warm white, etc.) and wornout, noisy ballasts. The lighting fixtures often were no match for the demanding environment experienced in a school. Bent louvers, cracked lenses and missing components were evidence of abuse and neglect.
Today, the selection of a classroom lighting system depends upon a list of variables: ceiling heights, ceiling types, classroom type and use, interior or exterior location. Despite the many variables, common goals exist when lighting a classroom: even distribution of light, durable and safe fixtures, appropriate lamp color, energy efficiency, flexible control and maintainability.
Direct/indirect lighting has become a popular option in classrooms with adequate ceiling heights. The benefit of direct/indirect lighting is realized in a computerized classroom, where the glare from direct overhead lighting on computer screens can cause serious eye strain. A lighting system with direct/indirect fixtures may come with multi-level switching so that an instructor can have direct lighting only, indirect lighting only or both. Dimmable fluorescent fixtures can provide another level of flexibility so that an instructor could control lighting levels, as well as types.
It is clear that modern classrooms are more dependent than ever on a flexible and sophisticated infrastructure. The need for solid infrastructure and thoughtful design reaches across all disciplines involved in the design and construction of schools. The need for increased access for physically handicapped individuals, increased security, flexible lighting systems, better indoor air quality, and higher operating efficiencies must be incorporated into modern educational infrastructure. These requirements will result in more flexible and sophisticated HVAC and lighting systems.
Strickland, PE, is chief mechanical engineer of Fitzemeyer & Tocci Associates, Inc., Woburn, Mass., a mechanical/electrical engineering firm that specializes in educational engineering, design and construction administration services as they apply to HVAC, plumbing, fire-protection and electrical systems.