Dedicated Outdoor Air Systems

In most buildings, HVAC systems combine fresh outdoor air with recirculated air in the main air handler for conditioning and distribution into the interior space. Some new buildings are using a different configuration called a dedicated outdoor air system (DOAS). In this design, the outdoor air is conditioned separately from the return air before it enters the building (see Figure 1). Dedicated outdoor air systems are a useful tool for improving humidity control and delivering precise amounts of ventilation air. Also, compared with conventional HVAC systems, they eliminate restrictions on the different types of HVAC components that designers can specify, and they often use energy more efficiently (see Table 1). Although the operating cost savings vary widely in different applications, the first cost of a DOAS application may not be more than that of a conventional system.

Figure 1: Configuration of a DOAS versus a conventional system
A conventional variable-air-volume (VAV) HVAC system has a single, allpurpose unit for conditioning both return air and outdoor air (A). In a dedicated outdoor-air system (DOAS), the outdoor air and return air are conditioned in separate units (B). This configuration gives a DOAS the ability to improve humidity control, provide more accurate delivery of ventilation air quantities, allow designers to use a wider variety of HVAC components, and increase energy efficiency.
Table 1: Energy savings of a DOAS versus a conventional VAV system
When researchers compared a DOAS with a traditional VAV system, they found that less energy was necessary for space heating and cooling with a DOAS, but there were no overall savings in air-moving power. The researchers based their calculations on the following assumptions: the DOAS reduced the necessary outdoor air volume for ventilation by 20 percent, outdoor air constituted 50 percent of the heating load and 25 percent of the cooling load, and the COP of the compressor for the return-air unit was increased by 20 percent due to an 11°F increase in evaporator temperature.

Some of the buildings that are currently incorporating this strategy include all new U.S. federal government buildings designed in 2004 or later.

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