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 (Figure 1). DOASs are a useful tool for improving humidity control and delivering precise amounts of ventilation air. 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 (Figure 2). Although the operating cost savings vary in different applications, the first cost of a DOAS application may reflect 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, all-purpose 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.
Figure 2: Energy savings of a DOAS versus a conventional VAV system
When researchers compared a dedicated outdoor air system (DOAS) with a traditional variable air volume (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%, outdoor air constituted 50% of the heating load and 25% of the cooling load, and the coefficient of performance of the compressor for the return air unit was increased by 20% due to an 11° Fahrenheit increase in evaporator temperature.
Table showing 8 to 12% energy saved in space heating, 15 to 20% saved in space cooling and none saved in ventilation power. Data from the US Department of Energy

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

A DOAS doesn’t rely on new technology; it uses conventional HVAC equipment configured to condition outdoor ventilation air separately from return air. It’s this technique that differentiates it from conventional systems. A DOAS requires two sets of equipment—one for outdoor air and one for return air—whereas a conventional variable air volume (VAV) or constant air volume system requires just one.

Whether a building conditions air with a DOAS or with a conventional system, there are two different types of cooling loads that the HVAC system must control:

  • Sensible cooling load. The energy required to cool air to the desired temperature.
  • Latent cooling load. The energy required to remove the moisture in air to reduce humidity to a target level.

By conditioning the outdoor air and return air in two separate HVAC systems, a DOAS effectively separates the two cooling loads. The outdoor air HVAC unit removes the latent load to control humidity, and the return air unit removes the sensible load to produce a comfortable temperature. It’s possible to decouple the two loads because the primary source of building humidity in most climate areas is fresh outdoor ventilation air. The outdoor air unit can also handle the smaller amount of latent load from the building interior by providing air that’s slightly drier than the target humidity level.

The outdoor air unit typically cools and dehumidifies air in the summer, and it humidifies and heats or cools air in the winter. Therefore, the simplest unit consists of a preheating coil, a cooling coil, a reheating coil, and a humidifier. ASHRAE standards also require that a DOAS use energy recovery, which can be accomplished with a device called an energy-recovery wheel. Latent and sensible energy wheels transfer heat and moisture between building exhaust air and incoming air, thereby recovering energy that would have been lost to the outdoors and providing humidification or dehumidification (Figure 3).

Figure 3: Typical DOAS outdoor air unit
In this dedicated outdoor air system (DOAS), the outdoor air first passes through a preheat coil—a necessary step during winter operation in cool climates to avoid frosting of the energy wheel. The wheel then brings the outdoor air closer to the temperature and humidity of the conditioned exhaust air, after which the cooling coil cools and dehumidifies the air. A second energy wheel raises the temperature of the air to match that of the exhaust air, thereby preventing any overcooling before the conditioned outdoor air is fed into the building.
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