💧 Latent Share of Cooling
🔄 Energy Recovery Savings
Sizing Breakdown
Full Calculation Table
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How to Use the DOAS Calculator
Choose the Canadian city closest to your project for summer design temperature and humidity, or select Custom to enter project-specific design conditions.
Enter the total ventilation airflow the DOAS needs to deliver, from the commercial ventilation calculator.
Enter room design temperature, how far below room temperature the neutral supply should sit, and the cold coil dew point target for dehumidification.
Click Calculate DOAS Sizing to see cooling coil load, latent versus sensible split, heating coil load, and remaining sensible load left for parallel terminal units.
Understanding DOAS Design
A dedicated outdoor air system handles only ventilation air, typically neutralizing it close to room temperature and stripping out humidity before delivering it to the space or to a parallel sensible cooling system. This decoupled approach has become increasingly common in Canadian commercial buildings partly because it lets each piece of equipment specialize rather than asking one AHU to handle ventilation, sensible cooling, and dehumidification all at once, and partly because ASHRAE Standard 90.1 now pushes many outdoor-air-heavy systems toward energy recovery that a DOAS is well suited to deliver.
The Case for Decoupling Ventilation from Sensible Cooling
A conventional mixed-air AHU sized to handle both ventilation and the bulk of the zone sensible load ends up oversized for cooling during shoulder seasons when ventilation air alone, sized under ASHRAE Standard 62.1, would satisfy outdoor air requirements. A DOAS paired with fan coils, VRF terminals, or chilled beams lets the sensible system respond quickly to zone-by-zone load changes while the DOAS handles a steadier job: conditioning and dehumidifying a fixed outdoor air volume regardless of zone-by-zone variation.
Neutral Supply: Why Not Just Cool the Air Fully?
If a DOAS delivered outdoor air significantly colder than room temperature, that cold air would add uncontrolled sensible cooling to whichever zone it enters, on top of whatever the parallel fan coil or VRF terminal is independently trying to deliver. Neutral supply, typically within 1 to 3 degrees Celsius of room temperature, avoids this overlap and keeps the sensible cooling responsibility cleanly with the parallel system. This calculator uses your specified neutral offset to determine how much reheat the DOAS needs to add after dehumidification.
Dehumidification Comes First, Then Reheat
To control humidity, the DOAS typically cools outdoor air well below its final delivery temperature, down to a cold coil dew point that wrings out moisture, a target worth checking against the apparatus dew point calculator, then reheats the air back up to the neutral supply temperature. This means the cooling coil often does more total work than the final neutral supply temperature alone would suggest, since it has to first overcool for dehumidification and then add reheat energy back in. Some systems use the reheat energy productively through a heat pipe or similar heat recovery within the unit itself, reducing the net energy penalty of this reheat step.
Energy Recovery Is Almost Always Worth It
Because a DOAS conditions 100 percent outdoor air continuously, it has access to a steady stream of exhaust air that can recover both sensible heat and moisture before that air leaves the building. In Canadian climates, an energy recovery wheel or similar device on a DOAS typically pays back quickly given how much heating and cooling energy is otherwise required to condition unrecovered ventilation air across a full season of heating degree days at Canadian winter design extremes. See the energy recovery wheel calculator for a more detailed sizing of that specific component.
Sizing the Parallel Sensible System
Once the DOAS handles ventilation and dehumidification, the remaining sensible cooling load for each zone still needs a delivery mechanism, typically fan coils, VRF terminals, or chilled beams. This calculator reports the DOAS neutral supply capacity but does not size that parallel system; feed the zone's total sensible load, minus whatever small sensible contribution the neutral DOAS air provides, into the appropriate terminal unit sizing tool for that equipment type.
Frequently Asked Questions
DOAS stands for dedicated outdoor air system. Unlike a conventional AHU that mixes return air with outdoor air and handles both ventilation and the bulk of the zone sensible cooling load, a DOAS handles 100 percent outdoor air only. It typically neutralizes that air close to room temperature and handles the latent dehumidification load, leaving sensible cooling and heating to a separate parallel system like fan coils or VRF terminals. See the AHU sizing calculator for the conventional mixed-air approach instead.
Decoupling lets each system do the job it does best. A DOAS can be optimized purely for dehumidification and outdoor air conditioning, often with energy recovery, while the parallel sensible system, such as fan coils or a VRF system, handles the variable sensible load more responsively and efficiently than a single mixed-air system trying to do both jobs at once.
Neutral supply means the DOAS delivers air close to room temperature, typically within a couple degrees Celsius of the space setpoint, after handling latent dehumidification. Because the air enters near room temperature, it does not create noticeable drafts or add unplanned sensible cooling that would need to be accounted for separately in the parallel terminal unit sizing.
Not strictly, but energy recovery is extremely common on DOAS units because the system handles 100 percent outdoor air continuously, which means significant heating and cooling energy is available for recovery from the exhaust air stream. In Canadian climates with substantial heating degree days, an energy recovery wheel or heat pipe can meaningfully reduce the DOAS heating coil size and annual energy use. Size that component with the energy recovery wheel calculator.
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