The data calculated above is the measurement error for a single instrument. The compounding effect of multiple instrument error rates allows much greater uncertainty in the outside air control application.

Effect of Variable Respiration Rates

The final variable that we will examine is “N”, the respiration rate, CO2 generation and their effects on ventilation. Is it realistic to assume that everyone in a building will be seated, is of the same size, sex, health, and consuming the same diet? Can all the other factors that influence respiration rates be held constant?

When we examine the range of activities and their impact on respiration, we find that “N” can easily vary to 0.50 for walking, 0.60 for Light machine work and 0.90 for the upper threshold of “light activity”. All of these are cited in the Appendix to ASHRAE Standard 62-1999. These figures assume a target ventilation rate of 15 CFM. When the target is 20 CFM, then the range of “N” varies from 0.40 for office work, to 0.50 for Walking. When these are used to calculate the amount of ventilation per person, we come up with the following range of rates.

The point is that any change from the minimum respiration and intake ventilation target has dramatic impacts on the amount of ventilation required, if 700 ppm above a constant outside CO2 rate is used.

Building Codes

With regard to the ventilation code issues, specifically the International Building Code, we find no mention of CO2 in Chapter 4 of the International Mechanical Code – 2000 (sections 401 – 402 – 403) on Ventilation. Nor could we find any mention of CO2 in Section 1002 of the 2000 Building Performance Code version of the IBC. All references to ventilation are related to dilution “rates” using acceptable outside air.

Our search could find no code authorities that use CO2 concentration or mass balance control as a substitute for ventilation rate requirements. The most significant misinterpretation is the California Energy Commission Title 24. The California Energy Code allows CO2 as an input to estimate occupancy level in variable occupancy spaces only and thereby the ability to reset the ventilation rate accordingly. It does not indicate nor endorse CO2 for use as a direct input for control of intake rates.

Conclusions

The preceding citations, calculations, analysis and comments have attempted to provide authoritative sources and their reasons for dissuading people from using “CO2 demand-controlled ventilation”. The positions represented have been supported by many that recognize the difficulties of providing “rate” control, without a “rate” input.

When stated very simply: ASHRAE 62-1999 is “the” ventilation standard for Indoor Air Quality in the United States. Both the Ventilation Rate Procedure and the Indoor Air Quality Procedure

<Prev 1 2 3 4 5 6 7 8 Next>