How to Use the Ventilation Rate Procedure (VRP) in ASHRAE 62.1

A Step-by-Step Guide for HVAC Designers

If you’re designing ventilation systems under ASHRAE 62.1, you’ve likely used the Ventilation Rate Procedure (VRP) before. But even experienced engineers sometimes struggle with properly sequencing the calculations — especially once system ventilation efficiency and multi-zone adjustments enter the picture.

This guide walks step-by-step through how to apply the Ventilation Rate Procedure from ASHRAE Standard 62.1 in a clean, structured way so you can avoid common mistakes and confidently document your work. This information is based on the 2022 version of the standard, so make sure to use the applicable version for your project.

Overview of the Ventilation Rate Procedure

The VRP determines:

1. The minimum outdoor airflow required at each breathing zone
2. The zone outdoor airflow
3. The system outdoor air intake
4. Minimum zone primary airflow

Step 1 — Determine Breathing Zone Outdoor Airflow (V_bz)

For each zone, use Equation 6-1 to calculate the breathing zone ventilation rate:

${V_{bz}=R_p \times P_z +R_a \times A_z}$

Where:

• ${R_p}$= Outdoor airflow rate per person (cfm/person or L/s-person)
• ${R_a}$= Outdoor airflow rate per area (cfm/ft² or L/s-m²)
• ${P_z}$ = Zone population
• ${A_z}$= Zone floor area (ft²)

How to do this properly:

1. Identify the occupancy category from Table 6-1.
2. Extract R_p and R_a from the table.
3. Determine the design population (if unknown, use default occupant density x floor area).
4. Multiply and sum.

Note: Always verify whether default occupant density is permitted for your project jurisdiction.

Step 2 — Calculate Zone Outdoor Airflow (V_oz)

Next, correct for air distribution effectiveness using Equation 6-2:

${V_{oz}=\frac{V_{bz}}{E_z}}$

Where:

• E_z = Zone air distribution effectiveness (from Table 6-4)

E_z depends on:

1. Heating vs cooling mode
2. Supply air temperature difference
3. Air distribution configuration

If E_z is different for heating and cooling modes, make sure to use the lower value for each zone. If E_z < 1.0, your required outdoor airflow increases.

Step 3 — Determine System Type

Before proceeding, classify your system:

• Single-zone system
• 100% Outdoor air system
• Multiple-zone recirculating system

This determines whether system ventilation efficiency must be calculated.

Note: For Single Zone systems and 100% Outdoor Air systems, skip to step 7

Step 4 — Diversity Factor

This is where many calculations go wrong. First, calculate occupant diversity (D) of your multi-zone system using Equation 6-6:

${D=\frac{P_s}{\sum P_z}}$​

​Where:

• P_s is the total system population
• P_z is the population for each zone

Occupant diversity takes into account the potential double counting of occupants when counting up the population of each zone. For example, and office space may have offices and a lunch room. Each office and the lunch room would all have a zone population (P_z), however the occupants in the offices would be the same people in the lunch room - they would occupy different spaces at different times in the day. It would be inaccurate to add the occupancy values of the lunch room and offices as unique occupants.

Step 5 - Calculate Uncorrected Outdoor Air

The uncorrected outdoor air intake (V_ou) is calculated using Equation 6-5:

${V_{ou}=D \times \sum (R_p \times P_z) + \sum (R_a \times A_z)}$​

Step 6 - Calculate System Ventilation Efficiency

The next step is to calculate System Ventilation Efficiency (E_v) using Equations 6-7 and 6-8:

${E_v=0.88 \times D+0.22}$ for ${D<0.60}$​

${E_v=0.75}$ for ${D \ge 0.60}$​

The system efficiency takes into account any potential loss of efficiency in distributing outdoor air when the diversity factor has been applied.

Step 7 - Calculate System Outdoor Air Intake value

For a Multi-zone Recirculating System,the outdoor air intake value (V_ot) can then be calculated with Equation 6-10:

${V_{ot}=\frac{V_{ou}}{E_v}}$

This is the ultimate value for the outdoor air that your air handling unit will need to take in.

For a Single-Zone System, use Equation 6-3:

${V_{ot}=V_{oz}}$​

You’re done. No system ventilation efficiency correction is needed - your system ventilation intake (V_ot) is equal to V_oz.

For a 100% Outdoor Air System, use Equation 6-4:

${V_{ot}=\sum V_{oz}}$​

This is similar to a single zone system, except that you must now take the sum of all zone outdoor airflows.

Step 8 - Minimum Zone Primary Airflow Rates

For multi-zone recirculating systems, since the relative values of outdoor air to primary (conditioning) air may change for each zone, there is one last step to ensure each zone gets the outdoor air it needs. For each zone, calculate the minimum primary airflow rate (V_pz-min) using Equation 6-9:

${V_{pz-min}=V_{oz}\times1.5}$

For single zone and 100% outdoor air systems this step is not necessary.

Step 9 — Verify Minimum Exhaust and Transfer Air Requirements

The VRP determines minimum outdoor intake, but you must also:

• Check exhaust airflow requirements (restrooms, kitchens, labs) from Table 6-2
• Verify relief air paths
• Confirm building pressurization strategy

Outdoor intake may need to exceed VRP minimum to balance exhaust.

Practical Workflow Summary

1. Identify occupancy category
2. Calculate V_bz
3. Apply E_z to obtain V_oz
4. Determine system type
5. For multi-zone systems:
   1. Determine P_s
   2. Calculate D
   3. Calculate V_ou
   4. Calculate E_v
6. Calculate V_ot
7. Calculate V_pz-min

When the Standard Doesn’t Apply

Make sure you’re aware of the ventilation requirements for your specific project and in your specific jurisdiction. While ASHRAE 62.1 is used across most of the world, some authorities may enforce alternative ventilation standards or different versions of 62.1.

Some projects such as healthcare and laboratories have alternative requirements for fresh air, air change rates, or pressurization. Make sure you have all the information you need for your specific project prior to starting your ventilation calculations.

Final Thoughts

The Ventilation Rate Procedure in ASHRAE 62.1–2022 is methodical — but only if you follow the sequence correctly. The most important mindset shift is this: You are not calculating outdoor air once — you are calculating it at the zone level, correcting it at the system level, and validating it against system behaviour.

When done properly, the VRP ensures:

1. Adequate dilution of indoor contaminants
2. Code compliance (required in most jurisdictions)
3. Balanced system design
4. Energy-efficient outdoor air control

If you approach it zone-by-zone and stay disciplined about the order of operations, the process becomes straightforward — even for large, complex multi-zone systems.