In the May issue of Automated buildings Leonard A. Damiano, in his article entitled ICC's International Mechanical Code vs. NFPA 5000 he erroneously suggested that Section 403.3.2 of the IMC "disallows the use of CO₂ Demand Controlled Ventilation".

In fact this section was specifically included in the code to allow for innovative technologies like CO₂ based ventilation control. Irrefutable proof of this is provided in the commentary to the IMC code. In the commentary to the IMC code also published by The International Code Council (available at: http://www.bocai.org/book_imc2.asp) it provides the following background to section 403.3.2 :

" The intent of this section is to allow the rate of ventilation to modulate in proportion to the occupants. This can result in significant energy savings. Current technology can permit the design of ventilation systems that are capable of detecting the occupant load in the space and automatically adjusting the ventilation rate accordingly. For example carbon dioxide (CO₂) detectors can be used to sense the level of CO₂ concentrations, which are indicative of the number of occupants. People emit predictable quantities of CO₂ for any given activity and this knowledge can be used to estimate the occupant load of the space."

For anybody who really wants to understand what the International Mechanical Code really means I highly recommend the Commentary. To quote ICC in its description of this informative document: "The 2000 International Mechanical Code Commentary is a reliable, comprehensive, easy-to-use, reference. It demonstrates the code, tells when and how to apply the 2000 International Mechanical Code requirements, and includes the complete code text along with historical and technical information. "

I highly recommend a copy to Mr Damiano.

Also to clarify how CO₂ is used in a space. It is a direct measure of the outside air dilution rate of the space. As a result it can be used to effectively measure the fresh air ventilation effectiveness of the space and can be used to measure and control cfm/person ventilation rates to ensure target ventilation rates are being met in the zone at all times, based on actual occupancy. This provides a great advantage over airflow monitoring that can tell you how much air is entering a building but can give no indication if ventilation is being delivered in proper quantities to where people are located.

With regard to ASHRAE standard 62, interpretation IC62-99-33 first accepted by ASHRAE in 1988, CO₂ demand control ventilation is applied under the ventilation rate procedure under provisions for variable and intermittent occupancy. The next version of this standard currently entitled 62 n makes provision for CO₂ control under the operational mode of "Dynamic Reset" and has made CO₂ control much simpler by dividing outside air ventilation rates into a building and people component. CO₂ can be used to control the people component while the building component ensures a minimum ventilation rate to control other non-people related sources.

As a backup to this note this I have put together an Adobe® PDF document that can be downloaded from the web that provides an annotated background on CO₂ based ventilation control. It provides description and links to 6 peer reviewed articles that have appeared in ASHRAE Journal and HPAC Magazine and Engineered Systems. It also includes some other application notes I have written with regard specialized applications, ASHRAE 62 and other codes. It can be downloaded at http://www.airtest.ca/docs/co2reference.pdf 

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