A VAV system, as shown in Figure 10, controls temperature in a space by varying the quantity of supply air rather than varying the supply air temperature. A VAV terminal unit at the zone varies the quantity of supply air to the space. The supply air temperature is held relatively constant: while supply air temperature can be moderately reset depending on the season, it must always be low enough to meet the cooling load in the most demanding zone, and to maintain appropriate humidity. Variable air volume systems can be applied to interior or perimeter zones, with common or separate fans, with common or separate air temperature control, and with or without auxiliary heating devices. The greatest energy saving associated with VAV occurs at the perimeter zones, where variations in solar load and outside temperature allow the supply air quantity to be reduced. If the peak room load is not determined accurately, an oversized VAV system will partially throttle at full load and may become excessively noisy throttling at part loads.
Humidity control is a potential problem with VAV systems. If humidity is critical, as in certain laboratories, process work, etc., systems may have to be limited to constant volume airflow. Particular care should be taken in areas where the sensible heat ratio (ratio of sensible heat to sensible plus latent heat to be removed) is low, such as in conference rooms. In these situations, the minimum set point of the VAV terminal unit can be set at about 50% and reheat added as necessary to keep humidity low during reduced load conditions.
Other measures may also be used to maintain enough air circulation through the room to absorb sufficient moisture to achieve acceptable humidity levels. The human body is more sensitive to elevated air temperatures when there is little air movement. Minimum air circulation can be maintained during reduced load by (1) raising the supply air temperature of the entire system, which increases space humidity, or supplying reheat on a zone-by-zone basis; (2) providing auxiliary heat in each room independent of the air system; (3) using individual zone recirculation and blending varying amounts of supply and room air or supply and ceiling plenum air with fan-powered VAV terminal units, or, if the design permits, at the air-handling unit; (4) recirculating air with a VAV induction unit, or (5) providing a dedicated recirculation fan to increase airflow.
Variable Diffuser. The discharge aperture of this diffuser is educed to keep the discharge velocity relatively constant while reducing the conditioned supply airflow. Under these conditions, the induction effect of the diffuser is kept high, cold air mixes in the space, and the room air distribution pattern is more nearly maintained at reduced loads. These devices are of two basic types—one has a flexible bladder that expands to reduce the aperture, and the other has a diffuser plate that physically moves. Both devices are pressure-dependent, which must be considered in the design of the duct -distribution system. They are either powered by the system or pneumatically or electrically driven.