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.
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).
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 (Figure 4).
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%. 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.
In a preconditioned system, outdoor air is conditioned and then fed directly into the VAV system, along with recirculated air, before being distributed to different zones Like a dual-path system, this approach minimizes ductwork by maintaining a single distribution system, while also sacrificing potential outdoor air savings. Because the two air streams are fed directly into the VAV system, there’s no need for a mixing box.
How to make the best choice
When evaluating whether your building is a good candidate for DOAS, it’s 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, museums, or buildings that require a large volume of outdoor air. Because there’s no comprehensive set of case study data, these guidelines are really the only ones that exist, so it’s not simple to pick a good candidate for cost-effective application.
Even though there’s a general perception that a DOAS costs more than a conventional system because it entails replacing one all-purpose system with two parallel systems, cost-effective applications do exist. Supporting this point, the US Department of Energy published a report titled Energy Savings Potential and RD&D Opportunities for Commercial Building HVAC Systems (PDF) on the energy-savings potential of technologies for commercial building HVAC systems. Authors conservatively estimate savings of 35%.
DOASs are an ideal choice in new construction and major renovations. A DOAS can reduce the costs of other mechanical systems in a building, making its two parallel systems a cost-effective choice. 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
What’s on the horizon?
We expect building planners to incorporate DOASs more frequently in new construction. 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.
Who are the manufacturers?
Designing a DOAS requires applications knowledge so that the system is properly configured and the ventilation air subsystems are properly sized. A few engineering design firms specialize in the application of DOASs:
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