What Are Dedicated Outdoor Air Systems (DOAS) and Why Use Them?

A dedicated outdoor air system is an HVAC architecture that separates ventilation from space conditioning: a dedicated unit supplies 100% of the code-required outdoor air — dehumidified, filtered, and often energy-recovered — directly to spaces or to terminal devices, while separate sensible systems (chilled beams, radiant, VRF) handle room-by-room temperature control. The power of the concept is that each system is optimized for its actual task. The DOAS is a latent and ventilation machine — deep dehumidification, high-efficiency filtration, energy recovery. The sensible systems are temperature machines — quiet, efficient, responsive. Together they do what neither does well alone.

The DOAS unit itself is the heart: sized for the total ventilation airflow, with cooling deep enough to dehumidify (often with reheat or wraparound heat pipes to avoid overcooling the supply air), energy recovery (enthalpy wheels or other recovery matched to the climate), and filtration appropriate to the occupancy. Supply air dew point is the critical control parameter — it must stay low enough that the sensible systems never see condensation conditions. Distribution can go directly to spaces or to the terminal devices (active chilled beams use the DOAS air as their primary air). Controls coordinate the two systems: the DOAS maintains ventilation and humidity, the sensible systems maintain temperature, and neither fights the other. I write the coordination sequence explicitly — including what happens on DOAS failure, when the sensible systems must protect themselves from moisture — because the failure modes of decoupled systems need as much design as the normal operation.

A complete DOAS design owns the latent load, recovers energy, and coordinates with the sensible systems. Here's what I verify. Decouple deliberately — and design the handoff.

• Latent ownership explicit: DOAS sized and controlled to handle the full moisture load, supply dew point specified
• Sensible pairing selected: chilled beams, radiant, or VRF matched to the building and coordinated with the DOAS
• Energy recovery included: recovery technology matched to the climate and the ventilation load
• Condensation protection: sensible systems interlocked to stay above dew point, failure modes designed
• Coordination sequence: DOAS and sensible controls written as one integrated strategy, not two separate ones