VARIABLE AIR VOLUME SYSTEMS.

High-velocity variable air volume (VAV) systems control space conditions by altering air quantity instead of temperature. Constant-volume systems, such as terminal reheat, multizone, and double duct, maintain a constant air flow and control space temperature by using reheat coils or mixing hot and cold air streams. VAV installations are more difficult to design and control than constant volume systems because widely fluctuating air flows and static pressures are encountered; however, VAV is usually more energy-efficient. The fan system that moves the air and overcomes resistance is the key ingredient in a VAV installation. This first article in a two-part series discusses types, modulation, and selection of VAV system fans.