ASHRAE Interpretation Gives Green Light to CO2 Control
By Mike Schell, Engelhard Corporation (reprinted with permission)
The role of Carbon Dioxide (CO2) demand controlled ventilation (DCV) in indoor air quality and ventilation control has been clarified in a recent interpretation to ANSI/ASHRAE 62-1989 "Ventilation For Acceptable Air Quality". While the proposed and massive revision to the standard, often referred to as 62-R, is dead as a doornail, incremental changes and modifications to the standard have always been allowed through specific requests for clarification or "interpretation" from the ASHRAE 62-89 Committee.
One such interpretation on the use of CO2, (IC 62-1989-27) was accepted this past January and published in the April issue of the ASHRAE Journal. This little known interpretation, which is now provided as an addendum to the standard, describes the proper procedure on how to apply CO2 control to modulate ventilation based on actual occupancy.
Over the past few years there has been a lot of confusion surrounding the use of CO2. In ANSI/ASHRAE Standard 62-89, there is reference to a guideline level of 1,000 parts per million (ppm). Appendix D of the Standard also provides some background on how CO2 levels can be related to occupancy and the cfm-per-person ventilation rate of a space. Previous interpretations regarding CO2 have clarified that CO2 should not be looked at as a stand-alone contaminant and that a level of 1,000 ppm does not necessarily constitute good air quality. Adding to the confusion is the fact OSHA has set a limit of 5000 ppm as a maximum 8-hour exposure for industrial settings.
Much of the confusion surrounding CO2 DCV is related to the fact that the Standard addresses two distinct approaches for providing ventilation for acceptable air quality. The first and most often used approach is the "Ventilation Rate Procedure" that provides recommended cfm-per-person ventilation rates intended to ensure acceptable air quality. The second, more esoteric approach called the "IAQ Procedure" allows innovative designers to design systems that specifically control only pollutants that may be of concern. Those systems designed to the Air Quality procedure often utilize a variety of air quality related sensors to activate or verify that acceptable air quality is maintained. Given CO2 is a sensor based approach, many assume that it is best applied under the IAQ procedure, when in fact it is actually best applied using the Ventilation Rate procedure.
Ventilation Control Based On Actual Occupancy
A designer typically using the ventilation approach, would determine the maximum occupancy of the space and multiply that times the recommended cfm-per-person outside air ventilation rate for that particular application. The resulting "Design Ventilation Rate" would be provided continuously to the space during occupied hours. One subsection of the "Ventilation Rate Procedure" allows the designer to modulate ventilation rates below the calculated "Design Ventilation Rate" rates if occupancy is variable or intermittent. An important corollary to this section is that the ASHRAE recommended cfm-per-person rate must be maintained for each occupant of the space. One example of an occupancy based ventilation approach used for years is in movie theaters, is where the ventilation rate is adjusted based on the number of tickets sold for any given show.
Occupancy based ventilation can ensure that recommended cfm-per-person ventilation rates are maintained yet energy savings can be derived from reducing over ventilation resulting from continuous ventilation at some arbitrary ventilation rate based on assumptions of maximum occupancy.
Enter CO2 based Demand Controlled Ventilation. The production of CO2 by people involved in the same level of metabolic activity (e.g. office work) is very similar and typically within about 15% for adults aged 16 to 65. (Children will have a different production rate of CO2 based on their age.) Simply put, doubling the number of people in a space, doubles the amount of CO2 production. As each person enters or exits a room the CO2 production rate will change in a known and predictable increment. This may be combined with the fact that outside levels of CO2 in most urban environments remain at a fairly stable 350 to 450 range. As a result, an indoor CO2 concentration can provide an indication of how much outside air (at a fixed and low CO2 concentration) is being used to dilute the production of CO2 by people. Demand Controlled Ventilation employs a control strategy that can take minute-by-minute CO2 levels and modulate outside air ventilation to ensure that a targeted cfm-per-person ventilation rate is maintained at all times.
Making CO2 Control Work
The ASHRAE interpretation for CO2 control establishes criteria on how to properly apply a demand controlled ventilation strategy and conform to the variable and intermittent provisions of the standard. The criteria may be summarized as follows:
o Sensor location and control strategy should be selected to maintain the ASHRAE recommended cfm-per-person ventilation rates for the application as provided in Table 2 of the standard.
o It is not enough just to control and react to CO2 levels. A control strategy must be used that ensures the delivery of ventilation is responsive to changes in occupancy within the lead and lag times established by the standard. Just opening and closing a damper at 1,000 ppm will not satisfy this requirement for most applications.
o If necessary, there should be some strategy to control and reduce any appreciable buildup of contaminants that might occur during unoccupied hours. This is because CO2 is related to people and may not be indicative of other pollutants that may build up when the system is turned off for the night.
o During occupied hours, it is recommended that some sort of base ventilation rate occur to ensure that general non-occupant related contaminants are controlled. Practice in applying DCV control systems suggests that a good rule of thumb is to provide a based rate 15% to 30% of the calculated "Design Ventilation Rate". While not specifically addressed in the interpretation, experience in CO2 control also dictates that designers of new buildings may want to provide an initially higher base ventilation rate during the initial 6 months of occupancy to accommodate the out-gassing of new furnishings.
o If CO2 DCV is being used, the practice of lowering ventilation based on diversity calculations (a black art in my opinion) cannot also be applied.
o Dilution via natural or mechanical ventilation is the only acceptable approach to be used with CO2 DCV. This means that the standard will not allow special systems that just filter out CO2 . (CO2 is not being used as a indicator of air quality but as an indicator of occupancy and the per-person ventilation rate of the space).
It is important to note that this interpretation applied specifically to CO2 control. Mixed gas or so-called "VOC sensors" can not be applied using this interpretation because they are non-specific and react to very general changes of gases in the space. They are not specific to occupancy and are best utilized to identify incidence of non-predictable contaminant releases in the space. Because mixed gas sensors are impossible to calibrate and are subject to continuous and unpredictable drift, their best application is for detecting short-term, large-magnitude changes of general gas levels in air. They should never be used to proportionally control ventilation as a CO2 sensor might be used for occupancy.
Advantages Of CO2 DCV
CO2 demand controlled ventilation optimizes and resolves the traditional conflict between reducing ventilation to save energy while maintaining adequate ventilation for air quality. Some of the benefits are outlined below:
o Energy can be saved by eliminating over-ventilation as a result of design or when spaces are partially or intermittently occupied. As one northwestern utility has found, energy paybacks can equal or exceed the paybacks offered by many lighting retrofits. In another example, $30,000 in CO2 controls installed in a 25 story Minneapolis building have yielded over $160,000 in savings over the first 8 months of this year.
o A carefully designed CO2 control strategy can maintain ASHRAE established, cfm-per-person ventilation rates at all times. Even if a space is not intermittently occupied, CO2 control can ensure that a space is not over or under-ventilated for current occupancy levels. It is the only simple method of ensuring that fresh air is actually distributed to spaces in proportion to their occupancy.
o CO2 control does not make a distinction between fresh air resulting from infiltration or mechanical ventilation. If an open window or leaky construction is providing plenty of fresh air, CO2 control will reduce mechanical ventilation proportionally, again providing opportunity for energy savings.
o A CO2 control system operated in conjunction with a building energy management system can record and store CO2 concentrations over time. This can provide the owner with a documented record of building ventilation that can be used to troubleshoot or verify optimum building ventilation levels.
o While a CO2 system can be used to maintain ventilation rates recommended by the ANSI/ASHRAE Standard 62-1989, a control strategy can be selected to allow ventilation rates ranging from 5 cfm-per-person and up.
Demand Controlled Ventilation using CO2 is not an IAQ cure-all. However, when applied as part of a conventional building control system, it can save energy and maintain ASHRAE recommended ventilation rates at all times. All types of spaces are good candidates for CO2 DCV including school classrooms (often under-ventilated) and office spaces (often over designed and over ventilated).
Mike Schell is Director of Marketing and Business Development for Engelhard Sensor Technologies, based in Santa Barbara, California.