ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality, is the most widely used method for setting outdoor air rates in commercial and institutional buildings in North America. In Canada, the National Building Code and most provincial building codes require ventilation systems in buildings outside the scope of Part 9 housing to be designed to ASHRAE 62.1, usually a specific edition referenced in the code. Healthcare spaces are typically governed by CSA Z317.2 instead.
The standard offers several procedures. The ventilation rate procedure (VRP) is the prescriptive one and the most commonly used. It sets minimum outdoor air for each zone based on occupancy and floor area, then calculates how much outdoor air the air handling system must bring in to deliver those amounts.
Which edition applies?
Because codes reference a specific edition, the rates and equations you must use depend on the code in force for your project, not on the newest version of the standard. Editions differ in details such as default occupant densities, zone air distribution effectiveness values and the method for calculating multiple-zone system efficiency. Confirm the referenced edition with the applicable code and the authority having jurisdiction before starting.
The procedure below follows the general structure common to recent editions.
Step 1: Breathing zone outdoor airflow
For each zone, the outdoor air required in the breathing zone combines a people component and an area component:
Vbz = Rp x Pz + Ra x Az
Where:
- Vbz is breathing zone outdoor airflow, in L/s (cfm)
- Rp is the outdoor air rate per person, from the standard's table for the occupancy category
- Pz is the zone population, the largest number of people expected in the zone during typical use
- Ra is the outdoor air rate per unit area, from the same table
- Az is the zone floor area
The two components exist because some contaminants come from people (bioeffluents and their activities) and others from the building and its furnishings.
Typical rates
These values appear in recent editions of the standard for common occupancies. Always check the table in the edition you are designing to.
| Occupancy | Rp, L/s per person (cfm/person) | Ra, L/s per m2 (cfm/ft2) | Default occupancy, people per 100 m2 |
|---|---|---|---|
| Office space | 2.5 (5) | 0.3 (0.06) | 5 |
| Conference room | 2.5 (5) | 0.3 (0.06) | 50 |
| Classroom (age 9 and older) | 5 (10) | 0.6 (0.12) | 35 |
| Retail sales | 3.8 (7.5) | 0.6 (0.12) | 15 |
| Restaurant dining | 3.8 (7.5) | 0.9 (0.18) | 70 |
Use actual design occupancy where it is known. Default occupant densities are for use only when the actual number is not known.
Step 2: Zone air distribution effectiveness
Not all outdoor air delivered to a zone reaches the breathing zone. Zone air distribution effectiveness, Ez, accounts for this. Typical values from the standard include:
| Air distribution configuration | Ez |
|---|---|
| Ceiling supply of cool air, ceiling return | 1.0 |
| Ceiling supply of warm air, ceiling return, supply more than 8 K (15°F) above space temperature | 0.8 |
| Ceiling supply of warm air, less than 8 K (15°F) above space, with adequate throw to the floor | 1.0 |
| Floor supply of cool air, ceiling return (underfloor or displacement) | 1.0 to 1.2 |
Overhead heating with warm air at high temperature is a common reason for an Ez of 0.8 in Canadian perimeter zones, which raises the required outdoor air in winter.
Step 3: Zone outdoor airflow
The outdoor airflow that must be supplied to the zone is:
Voz = Vbz / Ez
Step 4: System outdoor air intake
How Voz values are added up depends on the system type.
Single-zone systems
When one air handler serves one zone, the outdoor air intake flow is simply Vot = Voz.
100 percent outdoor air systems
When an air handler supplies only outdoor air to several zones, such as a dedicated outdoor air system, the intake flow is the sum of the zone outdoor airflows: Vot = sum of Voz.
Multiple-zone recirculating systems
When one air handler supplies a mix of outdoor and return air to several zones, such as a typical VAV system, the calculation must account for the fact that the outdoor air fraction in the supply is the same for every zone, while each zone's need differs. The steps are:
- Calculate uncorrected outdoor air intake, Vou. This combines the people and area components across all zones, with an occupant diversity factor D applied to the people component: Vou = D x sum of (Rp x Pz) + sum of (Ra x Az). D is the ratio of system peak population to the sum of zone peak populations.
- Determine system ventilation efficiency, Ev. Ev reflects how well the system delivers outdoor air to the critical zone, the one needing the highest fraction of outdoor air in its supply. The standard provides either a simplified table method or a more detailed calculation, depending on the edition.
- Calculate outdoor air intake: Vot = Vou / Ev.
The critical zone is often an interior conference room or classroom with high occupancy and low supply airflow at part load. In VAV systems, low minimum airflow settings can push Ev down and drive outdoor air intake up. Our guide to constant volume vs VAV systems discusses the effect.
A worked example (single zone)
An open office of 400 m2 (4,300 ft2) with 25 people, served by a ceiling supply system that heats with air more than 8 K above space temperature:
- People component: 2.5 x 25 = 62.5 L/s
- Area component: 0.3 x 400 = 120 L/s
- Vbz = 62.5 + 120 = 182.5 L/s
- Ez in heating = 0.8, so Voz = 182.5 / 0.8 = about 228 L/s (about 483 cfm)
- In cooling, Ez = 1.0, so Voz = 182.5 L/s (about 387 cfm)
The design must deliver the higher value whenever the heating condition applies, or the air distribution must be changed to improve Ez.
Dynamic reset and demand control
The standard allows outdoor air to be reset as conditions change, for example as occupancy varies, using demand-controlled ventilation with CO2 sensors, occupancy sensors or schedules. The area component must still be maintained when the zone is occupied. Energy codes such as NECB and ASHRAE 90.1 require demand control in some high-occupancy spaces; check the edition in force.
Other requirements in 62.1
The VRP sets airflow rates, but 62.1 also includes other requirements, such as:
- Outdoor air intake location and separation from exhaust and contaminant sources
- Filtration upstream of cooling coils and other wet surfaces
- Drain pan design and condensate management
- Exhaust rates for washrooms, kitchens and other spaces
- Air classification and restrictions on recirculating air from certain spaces
- Building pressurization and humidity limits in some editions
Verifying ventilation in operation
Calculated rates only matter if the system delivers them. Outdoor air dampers drift, minimum positions are set incorrectly, and VAV systems at part load can starve zones. Measuring outdoor air at the air handler and zone airflows during testing and balancing confirms that the design is being delivered.
Frequently asked questions
Does 62.1 apply to houses?
No. Low-rise residential buildings are covered by ASHRAE 62.2 in the US, and in Canada by Part 9 of the National Building Code and CSA F326, depending on the province.
Why does my multiple-zone system need so much outdoor air?
Usually because of a critical zone with high occupancy and low supply airflow. Increasing that zone's minimum airflow, or serving it differently, can reduce total system outdoor air.
Key takeaways
- The ventilation rate procedure sets zone outdoor air from a people component and an area component: Vbz = Rp x Pz + Ra x Az.
- Zone air distribution effectiveness, often 0.8 for overhead warm air heating, increases required outdoor air.
- Single-zone and 100 percent outdoor air systems add zone requirements directly; multiple-zone systems must account for system ventilation efficiency.
- Canadian codes reference specific editions of 62.1, so confirm which one applies.
- Verify delivered outdoor air through air balancing, not just design calculations.
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