The air volume per diffuser or grille is that which is necessary for the cooling, heating, or ventilation requirements of the area served by the unit. The air volume required, when related to throw, sound, or pressuredesign limitations, determines the proper diffuser or grille size.
Generally, air volumes for internal zones of building spaces vary from 1 to 3 cfm/ft2 of floor area. Exterior zones will require higher air volumes of 2.5 to 4 cfm/ft2 of floor area. In some cases, only the heating or cooling load of the exterior wall panel or glass surface is to be carried by the distribution center. Then, the air volume per linear foot of diffuser or grille will vary from 20 to 200 cfm, depending on heat transfer coefficient, wall height, and infiltration rate.