How Is a Dedicated Outdoor Air System (DOAS) Properly Designed?

The decoupling concept reorganizes the entire airside design. Ventilation air quantities are set by code and occupancy — typically a small fraction of the air a conventional system moves — so the DOAS ductwork, fans, and unit are all sized to that smaller quantity. Zone-level sensible loads are handled by water-side or refrigerant-side terminal equipment that needs no ductwork at all in the case of chilled beams or radiant, or small local ductwork for fan coils. Shaft space shrinks, ceiling plenums open up, and fan energy drops because the building moves far less air. This separation also fixes the part-load humidity problem that plagues conventional systems. On a mild but humid day, a VAV system satisfies thermostats quickly and its cooling coil barely runs — so moisture stays in the air and the building feels clammy. A DOAS doesn't care about the sensible load: it keeps delivering dried ventilation air at its design dew point regardless, holding space humidity steady through every part-load condition. For owners in humid climates, that alone justifies the architecture.

The energy recovery device is the economic engine of a DOAS. Conditioning 100 percent outdoor air is expensive — every cubic foot must be heated, cooled, and dehumidified from ambient to supply conditions — and recovery cuts that load dramatically by transferring heat and moisture between the exhaust and supply airstreams. I size and select the recovery device for the climate's dominant penalty: moisture recovery matters most where summers are humid, sensible effectiveness where winters are brutal, and frost control where wheels can ice. Downstream of recovery, the cooling coil does the deep dehumidification work, selected for a low apparatus dew point with face-and-bypass or variable geometry control so it can hold dew point across part-load conditions. Reheat brings the air back to neutral — I prefer recovered heat or heat-pump-based reheat over new energy wherever practical. The complete sequence reads as a chain: recover, cool deep, reheat to neutral, deliver. I trend supply dew point continuously, because a DOAS that drifts off its dew point is silently failing its one essential job.

DOAS controls coordinate three things: how much air, how dry, and when. Demand-controlled ventilation modulates outdoor air with occupancy — CO2 sensors in densely occupied spaces, occupancy sensors or schedules elsewhere — while never dropping below the code minimum. Dew-point control on the supply holds the latent line regardless of outdoor conditions. And integration with the terminal systems prevents fighting: the DOAS maintains ventilation and humidity baselines while zone equipment answers thermostats, with the BAS arbitrating any overlap. Commissioning a DOAS means proving the decoupling works. I verify ventilation rates at design and turndown, measure supply dew point across outdoor conditions, confirm the terminal equipment holds sensible setpoints without the DOAS's help, and test the failure modes — what happens to humidity if the DOAS trips, and how fast the BAS alarms. The owner gets trend logs proving humidity control through a humid season, which is the performance record conventional systems can never produce. • Size the DOAS from ventilation plus latent load — never from the sensible load • Deliver supply air dehumidified to a controlled dew point, reheated to neutral • Make energy recovery integral, selected for the climate's dominant penalty • Control on dew point and demand-controlled ventilation, not just dry-bulb temperature • Pair with chilled beams, VRF, fan coils, or radiant — and commission the pair as one system