DPWES Letters to Industry

Land Development Services
12055 Government Center Parkway
Fairfax, Virginia 22035-5503
703-324-1780, TTY 711

February 28, 2003

Ventilation Air Policy, Intermittent and Variable
Occupancies, Occupant Load Calculations

Code Reference: International Mechanical Code, 1996 Edition

Background: The Virginia Uniform Statewide Building Code (USBC) incorporates by reference the International Mechanical Code (IMC), 1996 edition. This code in turn prescribes the minimum amount of outdoor air required for the ventilation of various indoor spaces-based on the occupancy of each such space and its estimated maximum occupant load or other parameters. The IMC's tabular values for Required Outdoor Ventilation Air are derived from ASHRAE 62-1989, which itself is not incorporated within the VUSBC. In occupancies where the primary sources of air contamination are the occupants themselves, the prescribed ventilation rates (mathematically) limit the rise of carbon dioxide within such spaces to about 700 parts per million (ppm) above ambient outdoor air levels. Thus, if the outdoor air has a carbon dioxide level of 300 ppm (as presumed in ASHRAE 62-1989), a steady state condition of about 1,000 ppm carbon dioxide will be reached within a space with IMC prescribed occupant loading and outdoor ventilation rates. ASHRAE 62-1989 states: "Comfort (odor) criteria are likely to be satisfied if the ventilation rate is set so that 1,000 ppm carbon dioxide is not exceeded." ASHRAE 62-1989 also implies that short-term exposure to contaminant levels as high as 1.25 times the steady-state value (e.g., 1.25 times 1,000 ppm carbon dioxide) should be safe, provided the required air quality (e.g. 1,000 ppm carbon dioxide, maximum) is maintained on average over the entire occupancy period. In cases of "intermittent or variable occupancy," it is sometimes possible to provide ventilation rates lower than those prescribed by the IMC and still maintain indoor air quality comparable to that described above.

Policy: Effective immediately, in cases of "intermittent or variable occupancy" where the primary sources of air contamination are the occupants themselves, and outdoor ventilation air is the principal means of maintaining indoor air quality, equivalence to the IMC prescribed ventilation rates may be demonstrated through mathematical modeling of carbon dioxide concentrations within the space. Carbon dioxide concentrations so modeled must be shown to be no higher than 750 ppm above outdoor ambient conditions (which may be assumed at 400 ppm for this locality), with exceedance of this threshold being limited to 15 minutes and 1,000 ppm above outdoor ambient conditions during a single occupied period.

"Average occupancy" calculations, such as described in ASHRAE 62-1989 Section, may not be used as an alternative for demonstrating equivalence (Ref. IMC 403.2 Exception). Occupant loads utilized for design of ventilation systems shall be as prescribed in Table 403.3 of the IMC. Furniture layout designs utilized for occupant loads shall only be considered for permanently installed furniture (Ref. IMC 403.3).

In order to demonstrate equivalence to the provisions of the IMC, any of the following approaches may be used:

  1. 1. Use of the common ventilation formula as specified in Section 403.3.2 of the IMC.
  2. Rooms provided with exhaust or supply air fan systems may utilize air supplied through adjacent occupiable space(s), where there is excess ventilation air, to compensate for an inadequate supply of required ventilation air into the room.
  3. Use of mathematical modeling to demonstrate that the carbon dioxide concentrations in the space would not exceed the limits outlined above, during the time of occupancy.

A computer program has been developed for modeling indoor air quality in "intermittent and variable occupancy" situations that will be used to verify design and supporting calculations. Copies of the computer program are available from Land Development Services, upon request to:

Guy Tomberlin, Chief Plumbing Inspector
Commercial Inspections Division
12055 Government Center Parkway, Suite 630
Fairfax, Virginia 22035-5504
Phone: 703-324-1910, TTY 711
Fax: 703-324-1846, TTY 711

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