A dedicated outdoor air system, or DOAS, is a unit that brings in, filters and conditions 100% outdoor air and delivers it directly to the spaces, separately from the equipment that heats and cools those spaces. Instead of one air handler doing everything, the ventilation job and the space temperature job are split between two systems.
DOAS has become a common approach in offices, schools, multi-unit residential buildings and healthcare, especially where the space conditioning is done by VRF, fan coils, chilled beams, radiant systems or water-source heat pumps. This guide explains how it works, where it fits, and the design choices that matter most in Canada.
How a DOAS differs from a conventional system
In a conventional mixed-air VAV system, outdoor air is mixed with return air in one air handler, and the combined airflow is sized for the cooling load. The amount of outdoor air reaching each zone depends on how much total air that zone receives, which varies with load. ASHRAE Standard 62.1 accounts for this with the multiple-zone system ventilation efficiency calculation, which often requires more outdoor air at the unit than the sum of zone requirements.
A DOAS delivers a known quantity of outdoor air to each zone, regardless of the zone's thermal load. That gives:
- Verified ventilation to each space, which is easier to measure and balance.
- Smaller ventilation airflow, because the DOAS moves only outdoor air, typically a fraction of what an all-air system moves.
- Decoupled latent control, since the DOAS can be designed to dehumidify outdoor air and handle most of the building's latent load.
- Smaller ducts for the ventilation system, often freeing ceiling space.
Typical DOAS components
A typical DOAS unit includes:
- Outdoor air intake with damper and filtration (often MERV 8 prefilter and MERV 13 final filter).
- Energy recovery device: an enthalpy wheel, sensible wheel, fixed-plate exchanger, or run-around glycol loop.
- Cooling coil (chilled water or DX) for dehumidification.
- Heating: hot water, electric, gas, or heat pump.
- Optional reheat, which may use hot gas reheat or a recovered heat source.
- Supply fan and, in most cases, an exhaust or relief fan to recover energy from building exhaust.
Pairing DOAS with terminal systems
| Terminal system | Why it pairs well with DOAS | Points to watch |
|---|---|---|
| VRF | VRF handles sensible loads efficiently but provides no ventilation | Ducting DOAS air to indoor unit returns can upset VRF coil control; direct-to-space delivery is often cleaner |
| Active chilled beams | DOAS air is the primary air driving induction | Primary air must be dry enough to keep beams above dew point |
| Passive chilled beams and radiant | Need separate ventilation and latent control | DOAS must control space dew point carefully |
| Fan coils | Fan coils handle sensible loads; DOAS handles ventilation | Fan coil condensate is reduced if DOAS removes latent load |
| Water-source heat pumps | Heat pumps in each zone; DOAS for ventilation | Neutral or slightly cool supply air reduces heat pump cycling |
See chilled beams: active and passive systems and variable refrigerant flow systems for more on those pairings.
Supply air temperature: neutral or cold
A key design decision is the DOAS supply air condition.
- Neutral air is delivered near room temperature, around 20 to 22 C (68 to 72 F), after dehumidification and reheat. It does not upset space temperature, which keeps the terminal equipment simple, but the reheat costs energy unless it comes from recovered heat.
- Cold, dry air is delivered at the cooling coil leaving condition, such as 13 C (55 F) or colder, without reheat. It provides some sensible cooling to the space, reducing terminal equipment load and avoiding reheat energy, but can overcool lightly loaded zones in shoulder seasons.
Many designers use a reset: cold air when the building needs cooling, warming toward neutral when zones are in heating. The right answer depends on the terminal system and the climate.
Energy recovery in Canadian climates
In most of Canada, the heating energy to temper outdoor air is the largest single ventilation cost. NECB 2020 requires heat recovery from exhaust for many systems, with requirements based on climate zone, airflow and operating hours. Requirements differ between NECB editions and provincial adoptions, so confirm what applies with the authority having jurisdiction.
Points to get right:
- Frost control: at very cold outdoor temperatures, exhaust air moisture can freeze on the recovery device. Strategies include preheat, wheel speed modulation, bypass dampers or defrost cycles. Each reduces recovery during the coldest hours, so the method affects heating capacity and energy.
- Recovery effectiveness: higher effectiveness lowers peak heating capacity and energy. Rely on AHRI 1060-certified performance where applicable.
- Cross-contamination: wheels can carry some exhaust to supply. For most offices this is acceptable when the unit is configured correctly with purge sections and correct fan placement. For hospitals, labs and some kitchens, plate exchangers or run-around loops are often preferred or required.
- Humidity: enthalpy devices transfer moisture, which helps keep indoor humidity up in winter and reduces humidification load.
The guide on energy recovery ventilators and heat recovery ventilators covers device selection in more depth.
Controlling outdoor airflow
A DOAS can run at constant airflow, but many systems now use demand controlled ventilation. Zone CO2 sensors or occupancy sensors modulate zone dampers, and the DOAS fan follows to maintain duct pressure. This must be designed within the limits of Standard 62.1 and local code, keeping area-based ventilation even when the zone is unoccupied during occupied hours.
Airflow measuring stations at the DOAS intake, and at zone terminals where DCV is used, let the system prove that it is actually delivering the required outdoor air. These are worth specifying.
Design checklist
- Calculate zone outdoor air requirements using the Standard 62.1 Ventilation Rate Procedure or the local code method.
- Choose the terminal system and decide how DOAS air will be delivered: direct to space, to terminal unit inlets or to returns.
- Set supply air dew point to handle the building latent load, based on internal moisture sources and outdoor design dew point.
- Select energy recovery and frost control for the local winter design temperature.
- Decide supply air temperature strategy and reset logic.
- Plan relief or exhaust so building pressure stays slightly positive.
- Specify airflow measurement and commissioning of the outdoor air quantities.
Frequently asked questions
Is a DOAS always more efficient than a mixed-air system?
Not always. A DOAS with a good terminal system and effective energy recovery usually performs well, but a mixed-air system with airside economizer can provide significant free cooling in Canadian shoulder seasons. An energy model comparing options for the specific building is the reliable way to decide.
Can a DOAS provide free cooling?
Only to the extent of its outdoor airflow, which is usually small. Some designs oversize the DOAS for an economizer mode, but this needs to be weighed against ductwork and fan costs.
Key takeaways
- A DOAS separates ventilation and latent control from space heating and cooling.
- It pairs naturally with VRF, chilled beams, radiant systems, fan coils and heat pumps.
- Energy recovery with reliable frost control is essential in Canadian climates and often required by NECB.
- Supply air temperature strategy and dew point control drive both comfort and energy.
- Measure outdoor airflow so ventilation can be verified during commissioning and operation.
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