What Is a Dedicated Outdoor Air System (DOAS)?
A DOAS is an HVAC system that handles 100% of the building's ventilation air separately from the heating/cooling system — conditioning outdoor air to neutral and delivering it directly to zones or to local equipment. It decouples ventilation from thermal conditioning, which is exactly what modern energy codes and systems like VRF need. The MEP engineer sizes the DOAS from the ASHRAE 62.1 ventilation calculations.
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Why decouple ventilation
Traditional systems condition mixed air centrally, which wastes energy over-ventilating or under-ventilates zones. DOAS delivers exactly the required outdoor air per ASHRAE 62.1, with energy recovery (enthalpy wheels recovering 70%+ of exhaust energy) making it efficient. The local systems — VRF fan coils, chilled beams, radiant panels — then handle only the zone's heating/cooling load, which they do far more efficiently without the ventilation burden.
Design components
The DOAS unit includes outdoor-air intake, filtration (MERV-13+ is standard post-COVID), energy recovery, heating/cooling coils to bring air to neutral temperature and control humidity, and supply ductwork to each zone. In humid climates, the DOAS handles the entire latent (moisture) load — which means it is doing critical dehumidification work the local units cannot. Sizing starts from the ventilation rate procedure, not from rules of thumb.
Coordination and cost
DOAS adds ductwork (a second air distribution system alongside the local units), ceiling space coordination, and controls integration — the two systems must sequence together. Installed cost premium over a conventional VAV approach is typically 10-20% on the HVAC budget, with energy savings and superior indoor air quality as the return. In schools, offices, and healthcare, DOAS is increasingly the default design, not the upgrade.
The bottom line
DOAS separates ventilation from heating/cooling per ASHRAE 62.1, with energy recovery and humidity control. It pairs with VRF, chilled beams, and radiant systems — design them as one integrated system.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.