These systems are usually larger than draw-through units.
Because fan-discharge air is turbulent, the coil must be far enough downstream from the fan to ensure sufficiently laminar air flow for effective heat transfer. The coils are located on the high-pressure or outlet side of the fan. The fan then discharges the air either directly into a duct, or into a plenum with several supply ducts attached.īlow-through units are the opposite of the draw-through. This minimizes the distance between the coils and the fan inlet while still providing uniform flow through the coils. The air passes through the dampers and filter bank before entering the coils. This system uses less fan energy than an overhead VAV system, and heating and cooling energy are reduced because the supply air enters the conditioned space at the floor level near the occupants, not mixing with the entire air volume in the conditioned space as with overhead VAV systems.Īir-handling units are characterized by how the air is driven through the tempering coils.ĭraw-through units have the tempering coil(s) on the low-pressure or inlet side of the fan. The supply air then moves directly though the diffusers into the conditioned space or into terminal units that may add heat to the air before entering the conditioned space. Low-pressure supply air moves from the air handler (or supply-air shaft) into an underfloor air plenum. If necessary, air is heated at the terminal unit.Ī variation on VAV design is the underfloor air-distribution system. Variable-volume systems provide ventilation and cooling, usually through a single-duct system. In response to a static pressure sensor in the duct, supply-air volume is regulated in one of three ways: 1) bypass dampers, 2) inlet vanes either upstream from or inside the supply fan casing, or 3) a variable-speed drive (VSD) on the fan motor regulating the fan speed. The supply-air fan is controlled to maintain a static pressure setpoint in the ductwork.
The volume of supply air from a VAV air handler varies in response to terminal units' intake of supply air to maintain zone temperature setpoints. The tempered air is then distributed to the zones via single ducts. Individual zone requirements are met by mixing cold and warm air through dampers in the air handler. The multizone unit supplies air to several zones from a centrally located air-handling unit. Depending on the zone requirements, the air streams may either blend at a terminal mixing box or provide just cooling.Īnother constant-volume system is the multizone unit. Zones requiring heating may have a heating component in the terminal unit while some rely on a completely separate system for heating.Ī dual-duct system uses a single fan to move air through both cooling and heating coils in the air handler, distributing the air through separate hot and cold ducts. Constant-volume units can be used in single-zone or multizone applications.Ī single-duct system provides ventilation and cooling to the conditioned space. The following descriptions provide an overview of generic system types commonly found in larger commercial and institutional buildings.Ĭonstant-volume systems operate at a constant airflow rate only temperature varies to maintain the zone setpoint. Terminal units, the devices located near the conditioned space that regulate the temperature and/or volume of supply air to the space are described in Operation and Maintenance of Terminal Units.Īir-distribution systems fall into two broad categories, constant-volume (CV) and variable-air-volume (VAV). Sheet Metal and Air Conditioning Contractors' National Association, Inc. (SMACNA) publications are the recognized standard for fan-casing, plenum and duct construction, and installation.Air Movement and Control Association (ACMA) publications are the recognized standard for all products related to air movement such as fans and dampers.Two major organizations oversee the certification process for these systems: They use relatively large amounts of energy so applying smart operational strategies and good maintenance practice can significantly reduce energy consumption. Their many components need to operate in unison to properly maintain desired conditions. They provide fresh air to maintain adequate indoor-air quality while providing conditioned air to offset heating or cooling loads. Air-distribution systems include air handlers, ductwork, and associated components for heating, ventilating, and air-conditioning buildings.
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