A Dedicated Outdoor Air System (DOAS) is an HVAC configuration designed exclusively to condition and deliver 100% fresh outdoor air for ventilation purposes, treating it separately from the building’s sensible heating and cooling loads. By decoupling ventilation from thermal conditioning, a DOAS provides precise control over indoor air quality (IAQ) and humidity levels. This approach stands in contrast to conventional mixed-air systems that blend recirculated and outdoor air within a single air handler.
How a DOAS Works
A DOAS unit draws in outdoor air, filters it, and conditions it before delivering it to occupied spaces. The system typically incorporates components for latent load management, such as desiccant wheels, chilled water coils, or direct expansion cooling coils, to dehumidify incoming air before it enters the building. Many DOAS units also include energy recovery ventilators (ERVs) or heat recovery wheels that precondition incoming air by transferring energy from the exhaust airstream, reducing the total energy required for conditioning by as much as 50 to 80 percent.
Supply air temperature from a DOAS typically falls in the range of 65 to 75°F, delivered at a neutral or slightly cooled state. Because the DOAS handles only ventilation air, the volume of air it processes is significantly smaller than that of a conventional all-air system. Outdoor air ventilation rates are calculated according to ASHRAE Standard 62.1, which generally specifies airflow requirements of 5 to 20 CFM per person depending on the occupancy type and intended use of the space.
Common Applications
DOAS is particularly well suited for buildings with high occupancy densities or strict IAQ requirements. Common installations include:
- Schools and universities, where consistent fresh air supply supports student health and cognitive performance
- Hospitals and healthcare facilities requiring rigorous air quality and humidity control
- Laboratories with specific ventilation rate mandates
- Office buildings targeting enhanced energy performance and occupant comfort
DOAS is frequently paired with parallel sensible cooling systems such as chilled beams, fan coil units, variable refrigerant flow (VRF) systems, or underfloor air distribution (UFAD) to handle the remaining thermal loads in the space.
Relevant Standards and Codes
ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) is the primary standard governing outdoor air ventilation rates that a DOAS must satisfy. ASHRAE Standard 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) provides guidance on energy recovery requirements, often mandating exhaust air energy recovery when outdoor airflow exceeds certain thresholds. Additionally, ASHRAE Standard 55 addresses thermal comfort conditions that inform DOAS supply air temperature targets.
Practical Significance
By isolating the ventilation function, a DOAS allows building engineers to optimize both air quality and energy consumption independently. Conventional systems often over-ventilate or under-ventilate depending on thermal demand, leading to wasted energy or poor IAQ. A DOAS eliminates this compromise. The energy recovery components common in DOAS units further reduce operating costs, and the reduced airflow volumes allow for smaller ductwork and lower fan energy consumption. For building owners pursuing LEED certification or meeting increasingly stringent energy codes, a DOAS represents a proven strategy for achieving both indoor environmental quality credits and energy performance targets.