In the 1970s, variable-air-volume (VAV) systems gained prominence mainly because they addressed the period's energy concerns. But while they have delivered results for many years, their position may soon be usurped by a new technology that has none of their drawbacks. Poised to push VAV into obsolescence, a new HVAC strategy bringing together a dedicated outside air source, radiant cooling and heat recovery may soon transform the industry.

While VAV systems' advantages over other all-air systems include lower fan energy consumption and reduced bucking, their list of shortcomings is long and has defied resolution for the past 30 years. For starters, these systems perform poorly in terms of ventilation, air distribution and humidity control. Also, they use air as their energy transport media, which compromises energy efficiency because air is not ideal for heat transfer. Moreover, they leave buildings susceptible to biological and chemical warfare because they mix large quantities of recirculated air, often more than 80%, with outside air (OA) at the air-handling unit.

In sharp contrast, a new HVAC system called DOAS—dedicated outside air system—conveys 100% outside air to each conditioned space via a constant-volume outside air unit, using total energy recovery. And aside from delivering ventilation air straight to each space, DOAS excels at humidity control and thus nearly eradicates microbial problems and related sick-building issues.

Its outside air unit uses a total energy recovery device—an enthalpy wheel—to cool and dehumidify outside air during summertime. This eases the outside air load on the cooling coil by up to 80% and also substantially lessens the required size of the building refrigeration plant, often by over 40%. The enthalpy wheel also works well during winter. It allows the use of smaller winter heating and humidification plants and significantly lowers energy consumption in many geographic areas.

Combined with ceiling radiant cooling panels, DOAS can diminish the threat of biological and chemical contamination. Since a DOAS/radiant system does not distribute any recirculated air, it will not deliver biological or chemical agents to other parts of the facility. Instead, it dilutes or exhausts them from each individual space. Indeed, this system is expected to one day supplant VAV.

In fact, this technology has already received a favorable government evaluation. In July 2002, the U.S. Department of Energy released a report that ranked 55 of the most promising technologies for commercial building HVAC systems and analyzed the top 15 energy-savings technologies in detail. Radiant ceiling cooling claimed the number one spot because it uses water, which excels at heat transfer, as the medium instead of air. The second spot went to total energy recovery, while third place was claimed by DOAS. Indeed, the integration of these three technologies represents an ideal combination, capable of a total energy savings of 1.6 quads—almost equaling the overall savings of the remaining top 12 technologies.

The DOE report also pointed to another attractive aspect of the technology. Based on simple payback, both DOAS and radiant cooling boast zero payback periods.

Penn State University has already put the technology to the test, installing it in a 40-ft. by 80-ft. architecture studio. It has been operated in both hot, humid summer conditions and cold winter periods. During summer, it took in outside air of 85ºF and 80% relative humidity (148 grains/lb. humidity ratio) and lowered the humidity ratio to 84 grains. During winter, it maintained an operative temperature of 68ºF, with outside air temperatures dipping to zero.

Such results support the contention that DOAS is the future of HVAC. Its advantages over VAV systems are numerous, from superior humidity control to thermal comfort without the latter's noise and draft. And very importantly, while its first cost is comparable or lower than that of VAV systems, it yields 30-40% lower energy operating costs.

Source: HVAC: The Next Generation
Stanley A. Mumma
Consulting-Specifying Engineer, Jan. 1, 2003
http://www.csemag.com