Outdoor Air Quantity
Classrooms and other school spaces must be ventilated to remove odors and other pollutants. The national consensus standard for outside air ventilation is ASHRAE Standard 62.1-2001, available at http://www.ashrae.org/
If outside air is provided through a mechanical system, then at least 15 cubic feet per minute (cfm) of outside air must be provided for each occupant. A typical classroom with 30 people requires a minimum of 15 x 30 or 450 cfm of outside air.
In spaces where the number of occupants is highly variable such as gyms, auditoriums and multipurpose spaces, demand controlled ventilation (DCV) systems can be used to vary the quantity of outside air ventilation in these spaces in response to the number of occupants. One technique for doing this is to install carbon dioxide (CO2) sensors that measure concentrations and vary the volume of outside air accordingly. If an auditorium fills up for school assembly, then CO2 concentrations will increase, a signal will be provided to the HVAC system and outside air volumes will be increased accordingly. When the spaces served by an air handler have highly variable occupancy, this type of control can both save energy and help control moisture (and mold) by reducing the quantity of humid outside air when it is not needed for ventilation. CO2 and other sensors must be periodically calibrated and maintained.
In addition to "atmospheric dust," airborne particulates can include pollen, mold (fungal) spores, animal dander, insect proteins, pesticides, lead, and infectious bacteria and viruses. Designers can integrate features into the ventilation system that will provide benefits for the school occupants as well as the efficiency and longevity of the HVAC system. In addition, these features can reduce the need for expensive cleaning of the duct work and air handling units.