Rating System

LEED BD+C: New Construction

Rating System Version

v2 - LEED 2.2

Inquiry

We believe there is conflict between what the City of Chicago Building Code will allow for what is known as Demand Control Ventilation and what the USGBC has interpreted as the way carbon dioxide monitoring should be installed for Indoor Environmental Quality Credit 1 (IEQc1). We feel that only installing indoor CO2 sensors to control outdoor air intake CFM meets the intent of IEQc1 and installation of an outdoor air CO2 sensor is unnecessary. Could you please interpret our request to not have to install an outdoor CO2 sensor in order to achieve credit IEQc1 under LEED Version 2.1? Please consider the information that follows in support of our request. Thanks. According to the Chicago mechanical code, section 18-28-403.1.2 Demand ventilation: "The amount of outside air delivered by a mechanical supply system may be reduced during operation below the quantities listed in Table 18-28-403.3 if the system is capable of measuring and maintaining CO2 levels in occupied spaces no greater than 1000 ppm. The system capacity shall be greater than or equal to the ordinance requirements." The ventilation rates listed in the Chicago mechanical code were chosen to provide more than adequate fresh air, and this provision allows the level to be adjusted as long as the space ventilation, as measured by the CO2 sensors, remains that way. The credit (Indoor Environmental Quality credit 1) has the following intent: "Provide capacity for indoor air quality (IAQ) monitoring to help sustain long-term occupant comfort and well-being." The IEQc1 requires "a permanent monitoring system that provides feedback on space performance in a form that affords operational adjustments." We feel a ventilation system as designed with indoor CO2 sensors only (no outdoor CO2 sensor) meets the intent and the requirements of this LEED credit as far as the Chicago code will allow. Our understanding it that it has been the policy of the USGBC to allow local code to dictate when it is more stringent. In this case, it is also more practical. Carbon Dioxide sensors have an accuracy of anywhere from 30 ppm for the best sensors to 100 ppm for average technology. If the comparison is made between two readings, taken at sensors with such accuracy, then the result would have an accuracy of +/- the sum of the two accuracies; in this case, that would be +/- 60 to 200 ppm. If the decision is to be made, in the case of Chicago, as to whether the indoor CO2 level is a certain value greater than the outdoor CO2 level, we do not have accurate enough technology to be certain to within 20%. The decision for the Chicago code to have an indoor CO2 level of 1000 ppm was arrived at based upon a consensus of those knowledgeable in the subject, and it serves to improve the ability of automatic control systems to provide adequate ventilation. The Chicago code committee decided that 1000 ppm is the level considered adequate for inside, and that to use one level for demand control and another for additional ventilation would be cumbersome. The system designed should meet the intent and requirements of the credit without adding additional costs that make LEED buildings less attractive to development. In the City of Chicago, monitoring only the space levels of CO2 (i.e., no outdoor CO2 measurement) will not only be the most effective way to accomplish correct ventilation, it is the only way presently offered by code. In addition, the draft LEED Version 2.2 for IEQc1 allows the outdoor CO2 concentration to be assumed to be 400 ppm when no direct measurement is provided. Using the Chicago code allowed maximum indoor CO2 level of 1000 ppm, the maximum CO2 differential would be 600 ppm thereby meeting the intent of ASHRAE Standard 62.1 without requiring the installation of an outdoor CO2 sensor.