One main difference is in the unit cabinetry. Unit
ventilators typically have heavier sheet metal cabinets which hold up better
in the school environment. Classroom unit ventilators also have
architectural advantages. School room units can have shelving or cabinetry
built alongside the unit to help hide the comfort source.
Another difference is that unit ventilators are usually designed to
deliver large amounts of outside air through exterior wall openings. Indoor
air quality has become a major issue, especially in schools. ASHRAE
Standards recommend ever larger percentages of outside air per student (i.e.
15 cfm per student). This results in larger ventilation loads. A classroom
unit ventilator is specifically designed to handle these loads. In addition,
utilizing chilled water and hot water coils allows for more stable control
of coil discharge air temperature than with a direct expansion coil.
Heat gains can be as large as 7°F when a classroom suddenly fills with
students. By taking in the outside air, unit ventilators can provide "free
cooling" during cooler weather. A discharge air thermostat maintains a
minimum leaving air temperature from the unit to prevent "dumping" of cold
air onto students seated near the unit.
Unit ventilators are generally available in larger sizes than fan coils,
and both unit types often have their controls hidden inside the cabinet or
remotely mounted to prevent students or other occupants from tampering with
them.
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