Ventilation rates - calculations and inputs for mechanical ventilation

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Ventilation calculations require a two-step process where, first, ventilation requirements are determined for each zone in the building, and second, the HVAC systems are analyzed to define how they will effectively provide the ventilation to each zone.

Gather Project Information

Start by gathering the following project information:

  • Floor plans that indicate space types and floor area of each space, and furniture layout (if available - to determine more precise occupancy)
  • How air will be supplied to the space (e.g. overhead through diffusers vs. underfloor)
  • HVAC system types (e.g. multizone vs. single zone; mixed-air/recirculating vs. 100% outside air system or DOAS), and understanding of which zones they will be serving

Space/Zone-Level Inputs

Ventilation rates are determined based on a required volume of outside air per person, outside air per area, or a combination of both of these factors. The required values are dependent on the space type.

  • The outside air per floor area component of the input is intended to address air quality based on the space type itself (small source of contaminants like VOC, or to address any odors associated with normal use of the space).
  • The outside air per occupant component of the input is intended to ensure adequate outside air for people in the space. The value for each space type also accounts for the expected activity level of the occupants (e.g higher ventilation rates in a fitness center where occupants are excercising and lower ventilation rates in offices where occupants are sitting).
    • Note that if actual room occupancies are unknown, default values from the ventilation code may be used.

Additionally, ventilation rates determined in accordance with ASHRAE Standard 62.1 are dependent on the method of HVAC air distribution, and how effective it is at providing ventilation to the occupants' breathing zone. For example, warm air supplied by overhead diffusers is subject to stratification and may not mix fully throughout the whole space so it is considered less effective than underfloor or displacement distribution which delivers the air directly to the breathing zone. Therefore, the overhead system requires more outside air than the underfloor or displacement system.

Some BEM software tools have ventilation calculation capabilities and allow all of this to be set up easily by assigning an appropriate space type category to each zone in the model. If not, spreadsheet calculations are typically performed to calculate the appropriate ventilation requirement in each zone.

Refer to the Additional Resources section for documentation of how to perform the calculations. The 62.1 User's Manual and CSE Magazine article provide examples of the calculations. The User's Manual also includes a spreadsheet calculator with the proper formulas for zone and system level calculations.

System Level Inputs

While many calculations are performed at the zone level to determine ventilation rates, ventilation is ultimately brought into the building by an HVAC system's outdoor air intake. The zone level calculations are next used to determine how to size the outdoor air flow rate into the HVAC system (by another round of calculations) and ensure that enough ventilation is distributed to each zone. The calculation is dependent on the system type.

Some BEM software tools have ventilation calculation capabilities and allow all of this to be set up easily and automatically by assigning an appropriate space type category to each zone, and selecting the system type serving the zone.

Single-zone systems

For single zone systems, the zone level calculation is used directly to size the outdoor air quantity for the system. The outdoor air intake rate is equal to the zone's ventilation requirement. Note that the total supply airflow rate to the zone may be larger than the ventilation rate because the total supply accounts for both ventilation and heating/cooling of the zone.

100% outside air (OA) systems

100% OA systems may serve a single zone or multiple zones.

  • For single-zone systems, the outdoor air intake rate must be at least equal to the zone's ventilation requirement. Most often, the size of the 100% OA system will exceed the minimum ventilation rate because it also must be sized to meet heating and cooling loads.
  • For multiple-zone systems, the outdoor air intake rate for the system is the sum of the individual zone ventilation rates. Again, the overall size of the system is likely to exceed the minimum ventilation rate requirements.

Dedicated outside air systems (DOAS)

DOAS systems are generally sized to only meet the ventilation flow rates of the zones served, so the size will be the sum of the individual zone ventilation rates. Heating and cooling loads are generally met by a separate HVAC system (often single zone systems) that will not need additional outside air intakes for ventilation.

Multi-zone recirculating systems

Multi-zone recirculating systems have a much more complex calculation approach for sizing the outdoor air intake. It involves calculating the percentage of outside air to total airflow at each zone during design conditions, determining the ventilation efficiency for each zone (based on the OA/total airflow ratio), determining occupancy diversity for all zones served by the system, and identifying a "critical zone" which acts as a main driver for calculating the ratio of outdoor air to recirculated air. Refer to the CSE Magazine example calculation for a good discussion on how a single zone may unnecessarily increase the OA intake size and some strategies for avoiding this.[1]

Refer to the ASHRAE 62.1 User's Manual for some good example calculations, and its accompanying spreadsheet calculator to perform your project's calculations.

Additional Resources

Links to external websites are provided as a convenience for further research, but do not imply any endorsement of the content or the operator of the external site, as detailed in BEMcyclopedia's general disclaimers.


  1. Laue, Julianne. "Case study: Small-office HVAC calculations". CSE Magazine.
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