Raise Ceiling Heights with a DOAS System

Dedicated Outdoor Air Systems (DOAS) have been gaining popularity as an HVAC solution, largely due to the significant energy savings they make possible. Another benefit of these systems is that they typically use only one-third of the space compared to a traditional forced air HVAC system. This space savings can make a huge difference for projects in Washington DC, where building height regulations have resulted in extremely tight slab-to-slab heights in order to maximize possible floor space. This added premium on space, particularly ceiling heights, can make a DOAS system a great fit for new construction or major renovation/retrofit projects.

Differences Between DOAS and Traditional HVAC Systems
The majority of systems serving modern commercial office buildings use forced air (typically moved by large fans) to distribute cooling through a building. A portion of the delivered supply is outdoor air, which improves the quality of indoor air distributed throughout the building.

By contrast, a DOAS system transports cooling via water (or refrigerant) and the air itself is cooled and distributed as needed. From a space comparison standpoint, a DOAS system only needs a one inch pipe to deliver the same amount of cooling that a forced air system transports in a one square foot duct. The only ductwork required in a DOAS is that needed to serve the necessary outdoor air to the space, which provides ventilation and humidity control.

The reduction in ductwork and mechanical shafts translates into additional floor and ceiling space. Whether using chilled beams or VAV terminal units to handle the final distribution of air to the space, mechanical space requirements in ceiling plenums are drastically reduced. GHT helped develop low profile DOAS VAV boxes that require as little as 8 5/8 inches of space in the ceiling plenum.

I don’t think I could find a designer who has spent time designing high-end office space in Washington, DC who wouldn’t like to squeeze a couple more inches of ceiling height. DOAS solutions offer this opportunity, while providing notable energy savings.

Have you achieved a higher ceiling height with a DOAS system? Let us know.


As President and Senior Principal of GHT, Paul O’Brien, PE, LEED AP ID+C oversees GHT’s delivery of high quality services from start to finish. Through constant communication with clients and staff, he validates project goals and ensures the design of sustainable MEP engineering solutions that balance comfort, efficiency, and purpose.