CO2 Monitoring for Demand Controlled Ventilation in Commercial Buildings
Carbon dioxide (CO2) sensors are often deployed in commercial buildings to obtain CO2 data that are used, in a process called demand-controlled ventilation, to automatically modulate rates of outdoor air ventilation. The objective is to keep ventilation rates at or above design specifications and code requirements and also to save energy by avoiding excessive ventilation rates. Demand controlled ventilation is most often used in spaces with highly variable and sometime dense occupancy. Reasonably accurate CO2 measurements are needed for successful demand controlled ventilation; however, prior research has suggested substantial measurement errors. Accordingly, this study evaluated: (a) the accuracy of 208 CO2 single-location sensors located in 34 commercial buildings, b) the accuracy of four multi-location CO2 measurement systems that utilize tubing, valves, and pumps to measure at multiple locations with single CO2 sensors, and c) the spatial variability of CO2 concentrations within meeting rooms.
The field studies of the accuracy of single-location CO2 sensors included multi-concentration calibration checks of 90 sensors in which sensor accuracy was checked at multiple CO2 concentrations using primary standard calibration gases. From these evaluations, average errors were small, –26 ppm and –9 ppm at 760 and 1010 ppm, respectively; however, the averages of the absolute values of error were 118 ppm (16%) and 138 ppm (14%), at concentrations of 760 and 1010 ppm, respectively. The calibration data are generally well fit by a straight line as indicated by high values of R2. The Title 24 standard specifies that sensor error must be certified as no greater than 75 ppm for a period of five years after sensor installation. At 1010 ppm, 40% of sensors had errors greater than ±75 ppm and 31% of sensors has errors greater than ±100 ppm. At 760 ppm, 47% of sensors had errors greater than ±75 ppm and 37% of sensors had errors greater than ±100 ppm. A significant fraction of sensors had errors substantially larger than 100 ppm. For example, at 1010 ppm, 19% of sensors had an error greater than 200 ppm and 13% of sensors had errors greater than 300 ppm.
The field studies also included single-concentration calibration checks of 118 sensors at the concentrations encountered in the buildings, which were normally less than 500 ppm during the testing. For analyses, these data were combined with data from the calibration challenges at 510 ppm obtained during the multi-concentration calibration checks. For the resulting data set, the average error was 60 ppm and the average of the absolute value of error was 154 ppm.
Statistical analyses indicated that there were statistically significant differences between the average accuracies of sensors from different manufacturers. Sensors with a "single lamp single wavelength" design tended to have a statistically significantly smaller average error than sensors with other designs except for "single lamp dual wavelength" sensors, which did not have a statistically significantly lower accuracy. Sensor age was not consistently a statistically significant predictor of error.
Errors based on the CO2 concentrations displayed by building energy management systems were generally very close to the errors determined from sensor displays (when available). The average of the absolute value of the difference between 113 paired estimates of error was 25 ppm; however, excluding data from two sensors located within the same building, the average difference was 10 ppm. These findings indicate that the substantial measurement errors found in this study are sensor errors, not errors in translating the sensor output signals to the energy management systems.
Laboratory-based evaluations of nine sensors with large measurement errors did not identify definite causes of sensor failures. The study did determine that four of the nine sensors had an output signal that was essentially invariable with CO2 concentration; i.e., the sensors were nonfunctional yet still deployed. The evaluations did identify slight soiling or corrosion of optical cells and, in two sensors, holes in the fabrics through which CO2 diffuses into optical cells that may possibly have contributed to performance degradations. In one of two cases when the manufacturer's calibration protocol could be implemented, sensor accuracy was clearly improved after the protocol was implemented.
The Iowa Energy Center recently released the results from a laboratory-based study of the accuracy of 15 models of new single-location CO2 sensors. Although their report does not provide summary statistics, their findings are broadly consistent with the findings of the field studies of CO2 sensor accuracy described in this report. Many of the new CO2 sensors had errors greater than 75 ppm and errors greater than 200 ppm were not unusual.
In 13 buildings, the facility manager provided data on the CO2 set point concentration above which the demand controlled ventilation system increased the rate of ventilation. The reported set point concentrations ranged from 500 ppm (one instance) to 1100 ppm. The buildingweighted- average set point concentration was 860 ppm. When asked, no facility manager indicated that they had calibrated sensors since sensor installation.
In a pilot study of the accuracy of multi-location CO2 measurement systems, data were collected from systems installed in two buildings. The same manufacturer provided the multi-location measurement systems used in both buildings. In the first building, for the range of CO2 concentrations of key interest, the average and standard deviation in error in the indoor minus outdoor CO2 concentration difference were 14 ppm and 39 ppm, respectively, and in 16 of 18 cases the error was 36 ppm or smaller. In the second building, the measured CO2 concentrations were consistently approximately 110 ppm greater than the CO2 concentration measured with the reference CO2 instrument. Outdoor CO2 concentrations measured by the building's measurement system averaged approximately 510 ppm which is approximately 110 ppm larger than the typical outdoor air CO2 concentration. In both of these buildings, the error in the difference between indoor and outdoor CO2 concentration, which is the appropriate control input for demand controlled ventilation, was small except at a couple measurement locations.
The purpose of the multi-point measurements of CO2 concentrations in occupied meeting rooms was to provide information for selecting sensor installation locations. Data were analyzed for 30 to 90 minute periods of meeting room occupancy. The Title 24 standard requires that CO2 be measured between 0.9 and 1.8 m (3 and 6 ft) above the floor. The results of the multi-point measurements varied among the meeting rooms. In some instances, concentrations at different wall-mounted sample points varied by more than 200 ppm and concentrations at these locations sometimes fluctuated rapidly. These concentration differences may be a consequence, in part, of the high concentrations of CO2 (e.g., 50,000 ppm) in the exhaled breath of nearby occupants. In four of seven data sets, the period-average CO2 concentration at return grilles were within 5% of the period-average of all CO2 concentration measurements made at locations on walls; for the other three data sets the deviations were 7, 11, and 16%. Return-air CO2 concentrations were not consistently higher or lower than the average concentration at locations on walls. In four data sets, the period-average return-air CO2 concentration was between the lowest and highest periodaverage concentration measured at wall locations, while in the other three data sets the period average concentrations were lowest at the return grilles. There was no consistent increase or decrease in CO2 concentrations with height.
Together, the findings from the laboratory studies of the Iowa Energy Center and the current field studies described in this report indicate that many CO2 based demand controlled ventilation systems will, because of poor sensor accuracy, fail to meet the design goals of saving energy while assuring that ventilation rates meet code requirements. Given this situation, one must question whether the current prescriptions for demand controlled ventilation in the Title 24 standard are adequate. However, given the importance of ventilation and the energy savings potential of demand controlled ventilation, technology improvement activities by industry as well as further research are warranted. Some possible technical options for improving the performance of demand controlled ventilation are listed below:
- Manufacturers of single-location CO2 sensors for demand controlled ventilation applications change technologies to improve CO2 sensor accuracy. Sensor costs are likely to increase.
- Users of CO2 sensors for demand controlled ventilation applications perform sensor calibrations immediately after initial sensor installation and periodically thereafter. Research is needed to determine if such a protocol would lead to acceptable accuracy and whether costs are acceptable.
- Demand controlled ventilation systems employ existing CO2 sensors that are more accurate, stable, and expensive than the sensors traditionally used for demand controlled ventilation. To spread the cost of these sensors, multi-location sampling systems may be necessary. The pilot scale evaluations of this option included in this project are too limited for conclusions but suggest that these systems may be more accurate. System costs will need to be reduced.
- Demand controlled ventilation systems utilize sensors that count occupants, as opposed to sensors that measure CO2 concentrations.
With respect to selecting locations for CO2 sensors in meeting rooms, this research does not result in definitive guidance; however, the results suggest that measurements at return-air grilles may be preferred to measurements at wall-mounted locations.