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.