ASHRAE Journal - February 2009 - (Page 14) Demand control ventilation (DCV) is a means of counting people and using that information to implement the ventilation rate procedure in real time. DCV cannot be used to implement the indoor air quality procedure since it ignores all the critical pollutants that the procedure is required to limit. I feel Standard 62.1 has addressed the problem of off-gassing, unlike December’s article by Thomas M. Lawrence, Ph.D., P.E., “Selecting CO2 Criteria for Outdoor Air Monitoring.” When Standard 62.1 changed its ventilation rate requirement, from just cfm/person to the combination of cfm/person and cfm/ft2, I believe it was an attempt to address the problem of off-gassing. In the article, these values are used in a combined rate as just cfm/person. The problem with this is that it produces a single CO2 generation rate to control to, when in actuality, to comply with the code, the rate must vary as the occupancy decreases. A simple example of this is as the occupancy decreases to zero, the combined cfm/person goes up until it reaches infinity. My interpretation of the article is that one should use a fixed CO2 level and that the combined cfm/person never changes for a given room. I feel this interpretation does not comply with the standard. In an earlier ASHRAE standard a 600 ppm difference was inappropriately used by many engineers, for any and all types of spaces. Now I fear we will have engineers doing the same, but with a new group of numbers, which may or may not represent the occupancy and areas of their project buildings. Thomas Baumgardner P.E., Member ASHRAE, Slippery Rock, Pa. The Author Responds First, I thank Mr. Baumgardner for his comments concerning the article. He has valid points from an overall indoor air quality and outdoor air ventilation perspective, but I feel he misses the main intent of this article. The purpose of the article and the CO2 values listed in the table are not to provide general guidelines for CO2 levels at all times during occupancy, but merely to provide guidance on what would be the highest level of CO2 expected in a space. The article does not attempt to specify “a single number that does not change with occupancy” or “that one should use a fixed CO2 level and that the cfm/person never changes for a given room,” as his letter states, but rather simply the upper limits of CO2 to reasonably expect for a given space if designed according to the ventilation rate procedures of Standard 62.1 and at default design occupancy levels. This misinterpretation seems to be the basis for his comments, and this response helps clarify those misinterpretations. ASHRAE Standard 62.1-2007 does indeed have two pro14 ASHRAE Journal CO2 Criteria for Monitoring OA cedures for compliance, ventilation rate or indoor air quality, but to comply with this standard one needs to only use one or the other. Standard 62.1 also allows for dynamic reset of the outdoor air intake flow, and one purpose of this article, with regard to control implementation, was to help identify the maximum upper limit to expect (such as with a demand controlled ventilation system). There is no implication in this article of how the CO2 values might change at part-load occupancy, as that was not its stated purpose. Monitoring of CO2 levels in a space is allowed by the LEED® programs (EQ Credit Point 1 for mechanically ventilated, densely occupied spaces or all naturally ventilated spaces), or is required as part of the proposed Standard 189.1P for densely occupied and naturally ventilated spaces. The problem is that there has been limited guidance provided regarding what the upper limit of CO2 might be. Mr. Baumgardner notes earlier references to arbitrarily assuming a 600 ppm differential, and some programs still use this value (for example, the California Title 24 requirements for demand controlled ventilation), and he helps point out the need for guidance as intended in the article. Mr. Baumgardner also correctly points out the problems with trying to use CO2 differential at low occupancy levels, and that relates to the reason why LEED programs and the proposed Standard 189.1P specify direct outdoor airflow monitoring for non-densely occupied spaces. Thomas M. Lawrence, Ph.D., P.E., Member ASHRAE, Athens, Ga. Building America Having participated in the 1970s and 1980s energy consciousness, I have this deep fear that we may repeat our previous mistakes. In December’s “Building America,” Joseph Lstiburek does recall meaningful ideas and construction explorations that were directed to how we might reduce energy consumption in the U.S. The ideas were good as far as they went, but they failed to recognize the critical role that our society and culture play in addressing important goals. It would be valuable to do a cost analysis, similar to the analysis done for photovoltaics, on superinsulation projects. The logical process that gets the author from “green building” to “a building first” stops too soon. To have a building, we need a client and users of the building and a program for the use of the building. It is there that we discover what the building is to be and do. Dismissing this will return us to the failed lessons of the past. Jack Kremers, P.E., AIA, Life Member ASHRAE, Carbondale, Ill. ashrae.org February 2009 http://www.ashrae.org
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