In a conventional variable air volume (VAV) system, one air handler handles everything: it mixes return air with outdoor air, cools and dehumidifies the mixture, and distributes it to multiple zones. This coupling creates fundamental inefficiencies — the system must over-ventilate to ensure every zone gets enough fresh air, and it cannot independently control humidity at part-load conditions. The Dedicated Outdoor Air System (DOAS) solves this by splitting the job into two parallel systems: a dedicated unit that conditions 100% outdoor air for ventilation and humidity control, and a separate system that handles only sensible heating and cooling in each zone (Penn State DOAS Research, Wikipedia).
The VAV problem DOAS solves
When the multiple spaces equation of ASHRAE Standard 62.1 is applied to an all-air VAV system, 20-70% more outdoor air is typically required to ensure proper ventilation air distribution to every zone compared to a DOAS delivering air directly to each space. This excess outdoor air increases the cooling, heating, and dehumidification load. At part-load conditions, VAV systems also struggle with humidity — as the cooling coil modulates down to match reduced sensible loads, it may not dehumidify adequately, leaving indoor relative humidity too high (Penn State).
A properly designed DOAS handles 100% of the space latent loads and up to 30% of the space sensible loads. The parallel sensible-only system (fan coils, VRF, chilled beams, or radiant panels) handles the remaining sensible load — which is only about 40% of the design chiller load. This decoupling means each system operates at its optimal efficiency point rather than compromising to handle both latent and sensible loads simultaneously (Penn State).
Energy savings: the numbers
The savings are substantial. NIST researchers Emmerich and McDowell evaluated DOAS potential in U.S. commercial buildings meeting ASHRAE 90.1 requirements and found annual HVAC energy cost savings of 21-38%. A DOAS/radiant ceiling panel system simulation predicted 42% less annual electrical energy than a conventional VAV system with economizer control (Wikipedia/NIST).
ACEEE's 2024 analysis found even greater savings with high-efficiency DOAS configurations. DOAS paired with heat recovery ventilators (HRVs) reduced HVAC energy use intensity (EUI) by an average of 61% — ranging from 71% in cooler climates to 37% in warmer ones. DOAS with energy recovery ventilators (ERV) achieved 59% average EUI reduction. In some climates, total HVAC energy use was cut by up to 70%. Annual carbon emissions reductions of 40% or more were found when comparing DOAS to existing gas-fired systems (ACEEE).
| Metric | Conventional VAV | DOAS + Parallel |
|---|---|---|
| Outdoor air handling | Mixed with return air | 100% OA, dedicated unit |
| Latent/sensible control | Coupled (compromised) | Decoupled (optimized) |
| Excess OA for multi-zone | 20-70% more required | Exact amount per zone |
| Humidity at part load | Often too high | Precisely controlled |
| Annual HVAC cost savings | — | 21-38% (NIST) |
| HVAC EUI reduction (HRV) | — | 61% avg (up to 71%) |
| HVAC EUI reduction (ERV) | — | 59% avg (up to 71%) |
| Carbon emissions reduction | — | 40%+ vs gas-fired |
Deliver cold, not neutral.Trane's ASHRAE-published guidance recommends delivering DOAS air cold (not reheated to neutral temperature) directly to occupied spaces. Cold supply air offsets part of the sensible cooling load, allowing local HVAC units to be downsized. This avoids the energy cost of reheating dehumidified air for much of the year — a common energy waste in conventional systems. Use recovered energy for reheat only when needed (Trane/ASHRAE).
Energy recovery: required by code
ASHRAE Standard 90.1 requires energy recovery ventilators (ERVs) in many commercial applications based on climate zone and outdoor air percentage, with a minimum 50% energy recovery effectiveness. ASHRAE Standard 189.1 (High-Performance Green Buildings) raises the bar to 60%. Since DOAS units process 100% outdoor air, an enthalpy wheel (total energy recovery) is essential to pre-condition incoming air using exhaust air — cutting ventilation loads by up to 70% and ventilation energy costs by up to 65% (ACHR News/ASHRAE 189.1).
ACEEE defines high-efficiency DOAS configurations with specific performance targets: HRV-based systems need a Sensible Recovery Effectiveness (SRE) of 82% or greater with fan power of 0.77 W/cfm or less. ERV-based systems need SRE/Latent Recovery of 77%/60% respectively with fan power of 1.0 W/cfm or less. These systems are typically paired with VRF or high-efficiency heat pumps for the parallel sensible system (ACEEE).
When DOAS makes sense
- Commercial buildings: Offices, schools, healthcare — anywhere with multiple zones and significant ventilation requirements.
- Humid climates: DOAS excels at independent dehumidification, critical in ASHRAE climate zones 1-3 where latent loads dominate.
- High-performance new construction: ASHRAE 189.1, LEED, and Zero Energy Ready programs all reward DOAS with energy credits.
- Electrification retrofits: DOAS + heat pump replaces gas-fired heating + conventional VAV with 40%+ carbon reductions.
- Radiant cooling applications: DOAS handles ventilation and dehumidification while radiant panels handle sensible cooling — the highest-efficiency combination.
- Codes requiring it: 2021 IECC and 2022 ASHRAE 90.1 include energy credits for high-efficiency DOAS configurations.
Naming a product or manufacturer in this article is for identification purposes only and does not constitute an endorsement by HVAC Zone Inc. We are a multi-brand dealer and remain neutral on brand preference.
Evaluating a DOAS for your commercial building? Request a consultation — we assess your ventilation loads, climate zone, and existing equipment to determine if a DOAS configuration delivers the right energy savings and humidity control for your facility.