What Is a Dedicated Outside Air System (DOAS) Design?
DOAS separates ventilation from heating/cooling: a dedicated unit delivers the code-required outside air (dehumidified, filtered, tempered) while parallel systems (chilled beams, VRF, radiant) handle the sensible loads. The decoupling lets each system do its job efficiently — the ventilation sized for people, the cooling sized for loads. The high-performance building's HVAC architecture: better humidity control, better efficiency, better indoor air quality.
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How DOAS works
The DOAS unit (100% outside air — no recirculation) conditions the ventilation air: dehumidification (the critical function — delivering air dry enough that the parallel sensible systems never condense), filtration (the IAQ function — MERV 13+ standard), and tempering (neutral or slightly cool supply). The parallel systems (chilled beams, radiant panels, VRF indoor units) handle only the room's sensible loads — smaller, simpler, and never dealing with moisture. Energy recovery (the enthalpy wheel between exhaust and supply — recovering 70%+ of the ventilation energy) makes the thermodynamics work.
Advantages
Humidity control (the DOAS delivers dry air regardless of the sensible load — the parallel systems cannot dehumidify, so the DOAS must; the design guarantees it), energy efficiency (the decoupled systems each run at their optimal conditions — the chiller plant's efficiency rises when it serves only sensible loads at higher chilled-water temperatures), and indoor air quality (the guaranteed ventilation rates, the superior filtration — the pandemic elevated this priority). Smaller ductwork (ventilation-only airflows are fractions of conventional VAV flows).
Design considerations
The dehumidification sizing (the latent load calculation — the DOAS capacity set by the worst-case humidity, not the temperature), the supply-air condition (neutral 65-70°F typical — avoiding overcooling the spaces), the parallel system's condensation prevention (the chilled-water temperatures and controls ensuring surfaces stay above dewpoint), and the controls integration (the two systems coordinated — the DOAS and the sensible systems must not fight). ASHRAE's DOAS design guides inform the practice. First cost runs higher; operating cost runs lower.
The bottom line
DOAS decouples ventilation (dehumidified, filtered outside air) from sensible cooling (beams/VRF/radiant) — superior humidity control, efficiency, and IAQ. Size for the latent load; integrate the controls.
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