This article uses the American Society of Heating, Refrigerating and Air-Conditioning Engineers “single-duct” variable-air-volume (VAV) system as an example. This is the most frequently encountered form of VAV. It consists of a single air-handling unit (AHU) with a variable-speed fan (or, in older systems, variable inlet guide vanes on fans) conveying air to a number of cooling-only VAV terminal units (“boxes”), each of which serves a zone. Each zone consists of one or more spaces with similar load characteristics. The AHU fan reduces speed and air volume as the spaces it serves require less air, thus saving fan and cooling energy. In some cases, supply-air temperature is reset to deliver the warmest air that can provide the required cooling for the worst-case zone. The box controlling the zone is controlled through the building-management system (BMS) with a wall-mounted thermostat (temperature sensor); it additionally can be controlled with a carbon-dioxide sensor or an occupancy sensor in conjunction with the thermostat. This form of VAV is popular in high-rise buildings because interior spaces frequently are in cooling mode during winter as well as summer.
A “bypass” VAV system also provides variable airflow to spaces. Instead of a variable-speed fan, a bypass duct is used to return air to a constant-speed fan, “short-circuiting” the air before it gets to spaces. Such a system offers limited savings, as fan speed and volume stay the same. The system, however, does save some energy, as bypass air increases and fan-pressure decreases. This design is used only in the smallest of systems and has been almost entirely replaced by single-duct VAV.
VAV systems can be equipped with either passive (damper-only) boxes or “fan-powered” boxes. A fan-powered box includes its own small fan to ensure local air movement and enhanced heat transfer across a heating/cooling coil. First, operating and maintenance costs are factors often precluding the use of fan-powered boxes. However, their use in perimeter applications, especially the glass-curtain walls common in high-rise buildings, can eliminate the need for baseboard or cabinet perimeter heat. This allows neat, flush floor space right up to windows and makes energy-wasting simultaneous heating and cooling easy to lock out.
When a single-duct VAV system is duplicated with both a warm-air duct and a cool-air duct, a “dual-duct” VAV system results. Dual-duct systems can be used to provide warmer air to perimeters of glass-curtain-wall high-rise structures during winter while cool winter air is used to cool interior spaces. The system has the potential to be a great energy saver, but the added cost of a duplicate AHU air path, ductwork, terminal units, etc. usually precludes its selection.