Ashrae Journal - December 2008 - (Page 19) or DCV) using CO2. These include discussions on demandcontrol ventilation design guidelines,1 applying supply air CO2 control,2 retrofitting CO2 control in existing buildings,3 the potential sustainability benefits for CO2 DCV,4 and the risk versus rewards involved.5 Various options beyond occupancy sensing using CO2 are possible as well for outdoor airflow rate control and dynamic reset.6 The existence of differing criteria, or the lack of guidance on the exact criteria to use for determining the upper limit of CO2 in a monitoring program or for a DCV system can lead to confusion. It is generally thought of as the responsibility of the design engineer to define the upper allowed limit. However, this may not always be done. In an existing building, the building operator may be forced to make a decision on what is the highest acceptable concentration of CO2. This article is intended to provide overall guidance and direction on what should be considered a too high concentration of CO2 in a given space undergoing a CO2-based outdoor air monitoring program. The results also can be used to help determine the upper control limit for a system using DCV The guidance is . built upon the fundamental concepts described in key ASHRAE documents, such as Standard 62.1, the corresponding User’s Manual and the ASHRAE Handbooks, with added engineering judgment to focus on the key aspects needed for a CO2-based outdoor air monitoring program. Indoor Air Quality Monitoring Options The two most common options used for monitoring indoor air quality of a space are the direct measurement of the total outdoor airflow and the monitoring of CO2 concentrations. The direct measurement of outdoor airflow may appear to be conceptually straightforward, but its application in practice can be technically difficult with issues such as low intake airflow velocities, flow through exhaust dampers, or measuring low differentials in velocity pressures.7,8 CO2 monitoring is commonly used as an indirect indication of indoor air quality problems due to insufficient outdoor air ventilation rate, but it certainly is not without the potential for error and misinterpretation. One reason is that it neglects pollutants not associated with occupants, such as off-gassing from building materials and furnishings, making it only an indirect method of determining adequate ventilation. Key Issues With CO2 Monitoring, Equipment and Systems CO ASHRAE Journal zones at the required locations and uses a central monitoring sampler to determine CO2 levels. While locating the CO2 sensor directly in the zone usually works sufficiently for the purposes of CO2 monitoring, using a centralized sampling system may be more accurate, especially if the monitoring is based on comparing CO2 concentrations, such as breathing zone levels compared to the outdoor air. Another consideration is the CO2 sensor accuracy level and if it is sufficient for use in zone monitoring. Typical cut sheet accuracies range between 50 and 100 ppm at 2,000 ppm. At the lower range of 50 ppm, consider the concentration error for two sensors (zone and outside air) in a space designed for 15 cfm (7 L/s) per person (with a corresponding 700 ppm differential between zone and outside air). This net 100 ppm potential error is roughly equivalent to an error in outdoor airflow monitoring of ±2 cfm (1 L/s) of outdoor air per person. If sensors with 75 ppm accuracy were used in the same situation, the equivalent to error in outdoor airflow monitoring would be ±3 cfm (1.5 L/s). Advances in CO2 sensor technology over the past decade now allow allow for less frequent recalibration, e.g., once every five years. Some Some devices also are advertised to conduct self-calibration, but but this technique is still relatively new. Questions Questions can be legitimately raised as to whether a space can can ever be assumed to be fully mixed, and at steady-state with respect respect to CO2 concentration. Whether or not the space can be considered fully mixed is related to the zone air-distribution effectiveness. Values for this can be found in Standard 62.12007, Table 6-2. Under constant occupancy conditions, the time needed for a building or room to reach equilibrium is a function of the air change rate (assuming the occupancy, ventilation rate and outdoor concentration remain steady as well). A time period with stable occupancy and ventilation rate equal to three times the room time constant is required for room CO2 concentrations to reach 95% of their steady-state value.9 In a room with a low air change rate, it may take up to 12 hours of constant conditions for true equilibrium conditions to be reached.10 Persily11 specified the following criterion for a building to be considered in equilibrium: (1) Where The conversion constant (324×106) is based on SI units with: ∆C = change in indoor-outdoor concentration in one hour, mg/m³ · h G = CO2 generation rate in the zone, L/s V = Volume of the zone, L 2 When developing a CO2-based outdoor air monitoring program, a number of issues need to be considered. These include how and where to measure the CO2 concentrations; what type of sensing system to use; the required accuracy level of the sensor(s); calibration requirements for whatever devices are chosen; and whether to base the evaluation on absolute concentrations of CO2 or on the differential with respect to the ambient air. The type of ventilation system being used can influence these choices. A CO2 sensor need not be located within the zone itself; rather a system could be chosen that draws samples of air from the December 2008 2 For example, consider a typical K – 12 school classroom with assumed default occupancy of 35 persons per 1,000 ft² (100 m²) and a total CO2 generation rate of 0.18 L/s (0.38 cfm) based on the data shown in Figure C-2 of Standard 62.1-2007. Assuming a room ceiling height of 9 ft (3 m), the resulting typical room volume is 30,000 ft³ (300 000 L). Using this equation, the room could be considered in equilibrium if the change in CO2 19
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