Ashrae Journal - December 2008 - 20

The general worldwide average CO2 concentration is around 380 ppm,16 which varies by season and location, but the trend is upward. The concentration is lower over marine surface sites and higher over land, especially in towns and cities. Therefore, a simple assumption for CO2 would be 400 ppm, which is conservative (higher than actual) in most building locations. Alternatively, the actual ambient concentration surrounding the building could be measured. Determining the Ambient CO2 Concentration This is particularly recommended for sites located in larger urban settings or near major roadways, where local CO2 concentrations can easily be several hundred ppm higher than 400 ppm. If two sensors are used to determine the CO2 concentration difference, sensor accuracy becomes important, especially when the indoor concentration approaches the outdoor (or supply) concentration at low occupancy levels. lation airflow and the resulting steady-state CO2 concentrations for single-zone systems. While those articles provide a good derivation of the equations involved, the following is a brief summary of how steady-state room CO2 concentration can be calculated from an overall mass balance of CO2 into and out from the space. (2) concentration was less than 108 ppm by volume (194 mg/m³ for CO2) over a one-hour period. In practice, most spaces do not remain steady in occupancy and airflow for long enough time periods to reach full steadystate equilibrium of CO2 concentration. However, for simple and indirect checking of outdoor airflow per person, it is not necessary for a space to be in complete equilibrium for the values measured to give at least a good indication of the current apparent rate of outdoor airflow with respect to occupancy. Related to that, Lawrence and Braun showed that a steady-state analysis was sufficient for evaluating a building for potential CO2-based DCV retrofits.12 When considering decisions that are broader in nature, such as “is this concentration of CO2 too high,” which implies that the rate of outdoor airflow reaching the breathing zone is too low for the occupancy, then simplifying assumptions can be made. We must keep in mind that the actual purpose of CO2 monitoring is to provide an indication of the level of bioeffluents in the space, which is what we are trying to control using ventilation. Taylor13 indicated, in the sidebar titled “Assumptions of Steady-State Conditions,” that the generation of bioeffluents should closely mimic the rate of generation of CO2 since both are generated at a rate related to the number of people in the space and their activity level. Determining Expected CO2 Concentrations The rate of CO2 generation and removal can be expressed on a ‘per person’ basis. The rate of CO2 generation is based on the activity level (m, or metabolic rate).13,15 (3) The rate of CO2 removal is a function of the outdoor air ventilation rate being supplied to the breathing zone. Standard 62.1-2007 specifies that the outdoor ventilation air required is computed based on a component for the occupants, as well as for the building area, or: (4) Where Rp Pz Ra Az = = = = ventilation rate per person number of people in the zone ventilation rate per building unit area building area Standard 62.1-2007 specifies minimum outdoor air ventilation rates in the breathing zone based on the zone occupancy category. Table 6-1 in this standard provides the required values per person and per floor area for the different occupancy types, and also includes a default value for the net combined ventilation rate per person using default occupancy values. Expressed as the flow rate per person, the default combined values can range widely, from 5 cfm (2.7 L/s) per person to 25 or more cfm (12.4 L/s). Thus, the “normal” expected CO2 concentration at the design outdoor airflow rate and occupancy could vary widely as well, even at steady-state conditions, and a simple single CO2 concentration above ambient conditions would not apply universally to all occupancy space categories. The ASHRAE Journal article by Taylor13 and the 62.1 User’s Manual, Appendix A14 provide a good technical review of the calculations involved with determining required outdoor venti20 ASHRAE Journal The combined outdoor ventilation air rate per person to the breathing zone can be found by dividing Equation 4 by the number of occupants. The amount of ventilation air to the breathing zone (Vbz) is determined based on the outdoor air rate supplied to the zone (Voz) and the zone air distribution effectiveness (Ez), as shown by Equation 5. (5) Using the CO2 generation rate per person and the combined outdoor ventilation air rate per person, the steady-state room CO2 concentration (in ppm) that will occur in the breathing zone at design occupancy and assuming the minimum outdoor ashrae.org December 2008
Ashrae Journal - December 2008

Table of Contents for the Digital Edition of Ashrae Journal - December 2008
