Ashrae Journal - December 2008 - 26

mum. A second, and higher, concentration would be the level used as an alarm point signifying a situation where something was wrong. The alarm point would be expected to be used as an upper limit as part of a CO2 monitoring program, while the caution point might be used as an upper setpoint limit used in a CO2-based DCV system. Overall, four sets of values are derived and presented here, with each being useful for different situations. 1. Actual expected maximum steady-state CO2 concentration. This is the value computed based on the combined outdoor air ventilation rate per person (as defined in Table 6-1 of Standard 62.1-2007) and the expected metabolic rate for the occupants in that space, as defined in either Appendix A of the 62.1 User’s Manual or estimated using Table 4 in Chapter 8 of the 2005 ASHRAE Handbook— Fundamentals as a guide. 2. Upper limit for a CO2 monitoring program (alarm point). This value is the critical “action level” for CO2 concentration in the space that would trigger action or at least be recorded as an incident in the monitoring program. The values proposed here are based on the expected maximum steady-state CO2 concentrations (Situation 1) rounded upward to allow for sensor error and to give concentrations in “hundreds of ppm” for simplicity. 3. Upper limit for CO2-based DCV (caution point). This is the upper limit used in the control system algorithm that adjusts outdoor ventilation airflow based on measured CO2 concentrations. The values proposed here are based on the expected maximum steady-state CO2 concentrations (Situation 1) minus 10% to allow for control system response times, sensor inaccuracy, etc. Finding the right minimum CO2 concentration to begin damper position movement can be tricky, but the damper is opened gradually as CO2 values rise toward the maximum concentration setpoint, at which the outdoor air damper would be at the maximum open position. 4. Upper limit for a building undergoing LEED-EB monitoring. This is the maximum value allowable in a space if a building were trying to obtain the LEED-EB program IEQ Credit 1. The values proposed here are based on the expected maximum steady-state CO2 concentrations (Situation 1) +15%, as specified in the LEED-EB credit description. (The discussion below compares the results of the LEED-EB values and the alarm point concentrations.) These four different sets of values are summarized in Table 1, and are based on the assumption that the ambient concentration is 400 ppm, there is a zone air-distribution effectiveness of 1.0, and the zone is at design occupancy and is provided with the minimum outdoor air ventilation rate. The table is organized using the various occupancy categories listed in Table 6-1 of Standard 62.1-2007. Discussion Researching Multizones This article focuses on the development for single-zone systems. The situation is more complex when considering multizone systems, as each zone contributes to the overall CO2 balance and concentrations in the supply and return airstreams. For example, one method for calculation of the zone ventilation effectiveness for multizone systems is given in Appendix A of Standard 62.1-2007. The equations needed for the concentration equations can become complex since the concentration in one zone depends on what is going on in every other zone. A research project is being initiated by ASHRAE Technical Committee 4.3, which should help shed some light in this area. A monitoring program for a multizone system still would be checking for the amount of outdoor air entering the breathing zone for any individual space. The rough mass balance of the amount of outdoor air entering the breathing zone carrying away CO2 generated in the space still exists. The recommended concentrations in Table 1 might be the basis for application to multizone systems, but this analysis and discussion is reserved for other articles and is awaiting the results from ASHRAE research programs. primary purpose is to present proposed room CO2 concentrations for use in a CO2 monitoring program. These values are presented as an aid to a building operator, manager or systems designer responsible for developing a CO2 monitoring program, or, alternatively, in determining control limits for a CO2-based DCV system. The concentration criteria allow for a potential sensor error in the range of 50 ppm to 75 ppm. When conducting CO2 monitoring for the space, if the room concentration exceeds the amount recommended in Table 1 for that particular occupancy type, then this would indicate a potential problem with the system that must be investigated. It is interesting to compare the recommend CO2 monitoring program concentration limits (the action level) presented to the concentrations that are a set 15% above the steady-state values (the LEED-EB criteria for IEQ Credit 1). These concentrations are the same, with differences due to rounding to the nearest hundreds of ppm concentration for the recommended CO2 monitoring program action levels. The results presented here also can be applied to CO2-based DCV systems. The upper limit of the control algorithm is recommended to be the steady-state room concentrations minus 10%, and these values are given in the second from right column in Table 1. The DCV control should have as an alarm point the concentrations listed as the CO2 monitoring program action levels. Potential Responses if Recommended Criteria Are Exceeded The three columns on the right side of Table 1 summarize the key recommendations and purpose of this article. The 26 ASHRAE Journal The actual response taken if the CO2 concentration difference is exceed in a space as part of an outdoor air monitoring program can vary. The criteria given in Table 1 would be used ashrae.org December 2008

Ashrae Journal - December 2008

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