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Ventilation for Acceptable Indoor Air Quality

This article is the third installment of our Review of ANSI/ASHRAE Standard 62.1-2004. The new ANSI/ASHRAE Standard 62.1-2004 Ventilation for Acceptable Indoor Air Quality incorporates a number of significant changes to the previous standard – 62-2001.

In Part 2 of this article, we covered Sections 1-5. This article covers changes found in Section 6.

Section 6

Like Section 5, Section 6 – Procedures, has undergone major revision. A new paragraph 6.1 – General, is inserted to provide an overview of the Ventilation Rate and Indoor Air Quality Procedures. This shifts the paragraph numbering for the Ventilation Rate and Indoor Air Quality Procedures to 6.2 and 6.3 respectively. The two procedures have undergone a number of revisions. (Sections 7 and 8 have only minor changes.)

As noted previously, the paragraphs from Paragraph 6.2 – Ventilation Rate Procedure, that deal with evaluating the quality of the outdoor air have been moved to Section 4. Paragraph 6.2.1 – Outdoor Air Treatment, contains beefed-up requirements for treating outdoor air that is found to be unacceptable in accordance with Section 4. It contains specific requirements for Particulate Matter and Ozone.

Ventilation Rate Procedure

The Ventilation Rate Procedure from the 2001 Standard was relatively straightforward in comparison with the procedure contained in the 2004 Standard. The 2004 Standard requires that the outdoor air required be calculated for each zone. Depending upon whether there is one air handler per zone or one air handler for multiple zones, different correction factors are then applied to produce the total outdoor air required. These correction factors are obtained from the new Tables 6-2 and 6-3. There is also a subparagraph 6.2.6 – Design for Varying Operating Conditions, which provides for short-term conditions such as the peak occupancy being of short duration.

The calculation for the amount of outdoor air required per zone is based upon a formula and variables whose values are obtained from Table 6.1 in the 2004 Standard. Table 6.1 replaces Table 2 from the previous standard. Table 2 from the previous standard had outdoor air requirements based on the number of occupants in a space, the square footage of the space, or the type of space. For example, the outdoor air requirement for an office was 20 cubic feet per minute (cfm) per person, for a corridor in a public space it was 0.05 cfm per square foot (ft2), and for a hotel bedroom, it was 30 cfm regardless of the square footage or occupancy. The new standard bases the outdoor air requirements on both the occupancy and the square footage of the space for most spaces, but retains just a square footage component for spaces such as corridors.

Now, the outdoor air requirement for an office is 5 cfm per person plus 0.06 cfm/ft2, for a corridor is 0.06 cfm/ft2, and for a hotel bedroom is 5 cfm/ft2 plus 0.06 cfm/ft2. It would appear that the requirements for the office and the hotel bedroom are now the same. This is true except in cases where the occupant density (occupants per 1000 ft2) varies, or is unknown at the time of the ventilation system design. In the case of unknown occupancy, the new Table 6.1 has default values to be used for occupant density and provides a calculated combined outdoor air rate for each type of space. The table below compares the outdoor rates from the previous standard with the new standard based upon the default values in Table 6.1:

Space Type

Outdoor Air

Occupant Density
#/1000 ft2

Outdoor Air Requirement


20 cfm/person


17 cfm/person


0.05 cfm/ft2

0.06 cfm/ft2

Hotel Bedroom

30 cfm/room


11 cfm/person

The reason for using both occupancy and square footage to determine the outside air requirement is that the occupancy contribution is intended to dilute human bio-effluents and the square footage portion is intended to dilute contaminants off-gassing from building materials. Another reason for using for the change to using both occupancy and square footage to calculate the total outdoor airflow is that designs based on the previous standard, when based solely on occupancy, would overestimate the required outdoor airflow when occupant densities were high. It should also be noted that the values contained in Table 6-1 are all for non-smoking areas; smoking areas are no longer addressed by the standard.

Paragraph 6.2 – Ventilation Rate Procedure, also has new subparagraphs for Dynamic Reset (6.2.7) that discusses designing automated changes to the outdoor air intake rate, and Exhaust Ventilation (6.2.8), which provides exhaust ventilation requirements in Table 6-4 for specific types of spaces such as toilets, copy/printing rooms, locker rooms, etc. Subparagraph 6.2.9 discusses Ventilation in Smoking Areas. It states that smoking areas are to have more ventilation and/or air cleaning than non-smoking spaces and that air from smoking areas cannot be re-circulated to non-smoking areas. It also states, “Specific ventilation rate requirements cannot be determined until cognizant authorities determine the concentration of smoke that achieves an acceptable level of risk.”

Indoor Air Quality Procedure

Paragraph 6.3 – Indoor Air Quality Procedure, has also undergone significant revision. The revised procedure stresses that it is a “performance-based” approach that sets concentration limits for specific contaminants and goals for perceived indoor air quality. It goes on to state, “For the purposes of this procedure, acceptable perceived indoor air quality excludes dissatisfaction related to thermal comfort, noise and vibration, lighting, and psychological stressors.”

The designer is responsible for determining contaminant sources, target concentrations or limits, and the perceived indoor air quality goal in terms of the percentage of building occupants or visitors that express satisfaction with the indoor air quality.

The designer is then required to select one or a combination of design approaches to determine the minimum outdoor airflow rates that will achieve the limits and goals established. These approaches are as follows:

  • A mass balance analysis that calculates the amount of outdoor air necessary to keep contaminant concentrations below a certain level. Air cleaning systems can be used in the analysis to reduce the amount of outdoor air required.
  • “Design approaches that have proved successful in similar buildings.” This requires that the designer be able to document the basis for determining that the previous design was successful and that the previous design is relevant to the new design.
  • A design approach that involves post-construction contaminant monitoring and occupant evaluations to validate the design. The designer is required to have contaminant monitoring and subjective evaluation plans documented. A subjective evaluation approach is contained in Appendix B of the standard.
  • Using one of the above approaches for specific contaminants and the Ventilation Rate Procedure for general outdoor air requirements. This allows for the Ventilation Rate Procedure to address general aspects of indoor air quality and the IAQ Procedure to control specific contaminants.

Subparagraph 6.3.2 – Documentation, requires that information concerning the contaminants of concern and the design approach be documented. It also requires the documentation on the basis for successful past designs and monitoring and evaluation plans. Subchapter 6.4 – Design Documentation Procedures also contains requirements for documenting assumptions and design criteria.

As can be seen from the above discussion, the ASHRAE 62.1 Standard has undergone extensive revisions since 2001. The use of carbon dioxide as an indicator of indoor air quality, the structure and wording of the standard, and technical requirements have all changed. As of yet, there have been no addenda or interpretations issued on the standard. Based on the significant number of changes to the standard, there will be. These may be viewed on the ASHRAE Web site at http://www.ashrae.org/ when they are issued, free of charge.