|// HVAC: Dedicated Outdoor Air Systems
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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). However, the amount of operating cost savings varies widely in different applications and isn’t always necessary to make a DOAS application cost-effective, because its first cost 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. In applying this strategy, a government engineer cited the dramatic improvement that a DOAS can make in controlling outdoor air, mold, humidity, and water penetration.
How Does It Work?
A DOAS doesn’t rely on new technology. It uses conventional HVAC equipment configured to condition outdoor ventilation air separately from return air. It is 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 (CAV) system requires just one.
Whether a building conditions air with a DOAS or a conventional system, there are two different types of cooling loads that the HVAC system must control:
- Sensible cooling load. This is the energy required to cool air to the desired temperature.
- Latent cooling load. This is 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 is 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 is slightly drier than the target humidity level.
The outdoor-air unit typically cools and dehumidifies air in the summer and humidifies and heats or cools it in the winter. Therefore, the simplest unit consists of a preheating coil, a cooling coil, a reheating coil, and a humidifier. ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) 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 (see Figure 2).
Figure 2: Typical DOAS outdoor air unit
In this dedicated outdoor air system (DOAS), the outdoor air first passes through a preheat coil, which is necessary for winter operation in many cool climates to avoid frosting of the energy wheel. Next, an energy wheel brings the outdoor air closer to the temperature and humidity of the conditioned exhaust air, and then 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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What Are the Options?
There are three different configurations for DOASs. For delivery to the conditioned space, outdoor and supply airstreams can follow one of the paths described below (see Figure 3):
In a separately ducted system, outdoor air is conveyed to the zoned, conditioned space separately from supply air. After it exits the outdoor-air HVAC unit, the air enters the conditioned space through diffusers independent from any other mechanical system that may be thermally conditioning the space. Alternatively, the outdoor air may combine with return air in a mixing box or terminal unit that serves just one zone. In a zonal HVAC control system, individual zones of a building are controlled separately; the DOAS will deliver the proper amount of outdoor air directly to each zone. In a conventional system, the zone that requires the highest ratio of ventilation to total supply air dictates the fraction of fresh ventilation air that must be in the supply air leaving the air-handling unit. A DOAS can vary the fraction of ventilation to supply air, which can reduce the outdoor airflow rate by 40 percent. Energy savings result from conditioning only the amount of air necessary for each zone.
In a dual-path system, outdoor air joins the supply airstream in a mixing box before it enters multiple zones. When air leaves the outdoor-air HVAC unit, it may enter a mixing box or terminal unit that conditions air for more than one zone, or it can be added just downstream of the main return-air handler. The dual-path approach requires less ducting because it isn’t necessary to construct separate distribution systems for the two airstreams. However, this approach sacrifices the outdoor-air savings possible with a DOAS, because all zones receive the same ratio of outdoor air to return air.
Figure 3: Three DOAS configurations
The three common approaches to DOASs range from having completely separate systems (A) to configurations in which the two systems deliver conditioned air through one set of ducts (B) or the outdoor air unit is simply an extra conditioning step for the outdoor air before it’s conditioned together with return air (C).
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How to Make the Best Choice
When evaluating whether your building is a good candidate for DOAS, it is useful to know that savings are most likely to occur for facilities found in humid climates or those that need tight humidity control, such as libraries and museums or buildings that require a large volume of outdoor air. Because there is no comprehensive set of case study data, however, these guidelines are really the only ones that exist, so it is not simple to pick a good candidate for cost-effective application.
Cost-effective applications do exist, however, even though there is a general perception that a DOAS costs more than conventional systems because it entails replacing one all-purpose system with two parallel systems. Several case studies and a recent economic analysis show that in many cases the two can cost the same as, or less than, one all-purpose system. Supporting this point, the U.S. Department of Energy published a report in July 2002 on the energy savings potential of technologies for commercial-building HVAC systems. It listed the DOAS as one of the most promising technologies for energy savings, partly because of its low first cost.
The reason two parallel systems are not necessarily more expensive than one, with regard to new construction and major renovations, is that the DOAS can reduce the costs of other mechanical systems in a building. It may be possible to reduce the first costs of the following components:
- Chiller or direct-expansion system
- Condenser water pump
- Air-distribution plenums and terminal boxes
- Air handler
- Electrical service for chillers, blowers, and pumps
- Wasted “rentable” space that would have been consumed by mechanical equipment
For instance, by using a DOAS design instead of a conventional system, a medical clinic and office building in Missouri reduced the first cost of its HVAC system by 31 percent. This nine-story building was originally designed with a conventional HVAC system consisting of 16 air-handling units to bring in outdoor air for ventilation and to handle the space-conditioning needs. However, when the construction manager estimated the first cost of the conventional HVAC system, it came to $7,700,000 with the entire building exceeding the budget by about 10 percent. The designers went back to the drawing board and revised the design to incorporate a dedicated outdoor air system instead.
Using a DOAS reduced the cost of the HVAC system to $5,300,000—a reduction of 31 percent. This cost reduction was mainly due to simplifications of theHVAC system from the elimination of outdoor air ducting, louvers, and associated parts as well as simplified temperature controls for each of the 16 air-handling units.
This building reaped financial rewards from a DOAS, but the major benefit of conditioning outdoor air in a separate unit is being able to provide superior humidity control and precise delivery of ventilation air. In addition, compared to conventional HVAC systems, DOASs can use energy more efficiently and remove restrictions on the different types of HVAC components that designers can use.
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What’s on the Horizon?
We expect that in the future there will be more new buildings incorporating DOASs. It’s likely that this will allow researchers to continue to investigate this area by collecting first-cost and energy-consumption data from buildings that use DOASs. This experience will help designers and building owners become better informed about how to choose the most suitable systems for their buildings.
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Copyright 2006 – Platts, a Division of The McGraw-Hill Companies, Inc.