The concept is straightforward: Condition and deliver fresh ventilation with a dedicated outdoor-air system (DOAS) to treat outdoor air and space latent loads efficiently. The engineering of dedicated outdoor-air equipment has been well-developed since the turn of the century. Yet, a Web search for "DOAS" still is more likely to point to the classic play "Death of a Salesman" or "differential optical absorption spectroscopy" than "dedicated outdoor air systems."

This article will explain engineering aspects of two DOAS-related innovations. Both technologies are mature, with more than a decade of successful application, use readily available components, and can be cost-efficient in a well-designed system. Engineers can achieve equal or better performance without the complexities and cost of a reheat coil and controls. Little or no excess cooling capacity and avoidance of reheat translates into significant energy savings. How can designers benefit from DOAS and not end up buried in a costly, complex design?

The advantages of DOAS are compelling. First, with DOAS the proper amount of fresh air can be sent to each room at all times, rather than rely on ventilation-effectiveness estimates, variable-air-volume- (VAV-) box minimums, and probability to meet ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality. Standard 62.1 requires that every occupied space be served with sufficient outdoor air to meet a space's ventilation requirements under all load conditions. Second, DOAS detach latent from sensible cooling. Therefore, it is a relatively simple matter to provide ample dehumidification without excessive reheat in highly insulated, heat-reflective, cool-roofed buildings. The dehumidification capacity needed to meet Standard 62.1 is calculated at the outdoor design dew point with solar loads at zero.

After ventilation and dehumidification requirements have been fulfilled, terminal units operating in parallel with DOAS need only handle the remaining sensible load. Terminal units could include fan-coil units, radiant-cooling panels, water-source heat pumps, or any other cooling equipment. The terminal units used with DOAS are much smaller and simpler than the types of equipment utilized in systems without DOAS.

DOAS are being installed on a number of projects because of persuasive advantages, so why are we not witnessing the full paradigm shift to large numbers of DOAS that was predicted in 2001?1 In the HVAC industry, change is measured by decades. Even though DOAS have been tested and applied for more than 10 years, they still are deemed "exotic technology" by some mechanical engineers. DOAS also may not be considered on some projects because of perceived higher first and energy costs and specialized controls and components with foreboding names, such as "hot-gas bypass" and "enthalpy recovery wheel."

Concepts
As its name indicates, a dedicated outdoor-air unit is designed to condition 100-percent outdoor air. Unlike return air from a space, which has a relatively constant temperature and humidity, outdoor air can run wild from a chilly, dry spring morning to the hottest, most humid summer afternoon. An outdoor-air unit must be able to cool and dehumidify a broad range of entering-air conditions to meet design leaving-air conditions. In contrast, the cooling-coil entering-air conditions of a mixed-air unit are buffered by an airflow that is partly outdoor air mixed with mostly return air. The dehumidification demand on an outdoor-air unit is more challenging than that on a mixed-air unit simply because entering-air conditions vary so much.