Though not as straightforward as it once was, CO2-based DCV remains a practical way to reduce costs
Editor's note: Following is an update of the November 2005 (Volume 34, No. 5) edition of Engineers Newsletter, "CO2-Based Demand-
Controlled Ventilation With ASHRAE Standard 62.1-2004." Published periodically by Trane's Applications Engineering group, Engineers Newsletter is intended to aid engineering professionals in the design and application of HVAC systems by providing "reliable, objective, and technologically current information in a non-commercial format." Engineers News-letter is archived at www.trane.com/engineersnewsletter.
Demand-controlled ventilation (DCV) can reduce the cost of operating an HVAC system. But implementing DCV based on indoor levels of carbon dioxide (CO2) is not as straightforward under the 2007 version of ANSI (American National Standards Institute)/ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard 62.1, Ventilation for Acceptable Indoor Air Quality, as it was under previous versions. The good news is that DCV remains do-able and practical, especially for spaces such as gymnasiums and meeting rooms, where people and their activities are the main sources of contaminants. This article reviews Standard 62.1's requirements for dynamic reset and outlines several methods of implementing DCV using CO2 sensors.
Dynamic Reset
In Section 6.2.7, "Dynamic Reset," Standard 62.1 permits an HVAC system to "reset the design outdoor-air intake flow (Vot) and/or space or zone airflow as operating conditions change." Although the standard does not provide details for implementation, any system-control approach that responds to varying conditions must be capable of providing at least the required minimum breathing-zone outdoor airflow whenever the zones served by a system are occupied. The standard lists three types of dynamic reset:
Reset based on occupancy. Standard 62.1 allows the resetting of intake airflow in response to changes in zone population. This control strategy--DCV--responds to the actual need, or "demand," for ventilation by regulating the rate at which an HVAC system brings outdoor air into a building. Ventilation demand can be assessed one of three ways:
Reset based on ventilation efficiency. Standard 62.1 also allows intake airflow to be reset in response to changes in ventilation efficiency. In a multiple-zone variable-air-volume (VAV) system, ventilation efficiency depends on zone- and system-level primary airflows and is higher at part load than it is at design (worst-case) conditions. This control strategy is known as ventilation reset.
Reset based on economizer operation. Lastly, Standard 62.1 allows the resetting of minimum primary airflow at VAV boxes in response to changes in intake airflow. For example, when a system is in economizer (free-cooling) mode, the amount of outdoor air in the primary air is greater than is necessary to meet minimum ventilation requirements, so the minimum primary-airflow settings on VAV boxes can be reduced. If a zone requires reheat during economizer operation, this strategy can reduce both fan and reheat energy.
Let's take a closer look at what may be the most common application of dynamic ventilation reset--DCV based on CO2 readings--to understand how it works and how Standard 62.1 affects its implementation.
Applying CO2-based DCV
In CO2-based DCV, CO2 is monitored as a byproduct of respiration, rather than as an indoor contaminant. The rate at which individuals produce CO2 varies with their diet and health, as well as the duration and intensity of their physical activity. The more exertion an activity entails, the more CO2 that is produced.
Appendix C of Standard 62.1 provides the following mass-balance equation to predict the difference between indoor and outdoor concentrations of CO2 at steady-state conditions, given a constant per-person ventilation rate and a constant CO2-generation rate:
Vo = N ÷ (Cs – Co)
where:
Vo = outdoor-airflow rate, cubic feet per minute (cfm) per person
N = CO2-generation rate, cfm per person
Cs = indoor CO2 concentration, parts per million (ppm)
Co = outdoor CO2 concentration, ppm
Implementing CO2-based DCV, then, is a matter of estimating the CO2-generation rate of occupants (N), measuring the difference between indoor and outdoor CO2 concentration (Cs – Co), and using that to determine the rate at which ventilation air (Vo) is delivered to a space on a per-person basis.
In most locations, the outdoor concentration of CO2 seldom varies from the nominal value by more than 100 ppm.1 Because of this and in lieu of installing an outdoor CO2 sensor, most designers use either a one-time reading of outdoor CO2 concentration or a conservative value from historical readings. This simplifies control, lowers installed cost, and usually increases accuracy by avoiding the potential inaccuracy of an outdoor sensor.
Impact of Standard
1989 until 2004. The 1989 through 2001 versions of what then was known as Standard 62 based required ventilation rates on either the number of occupants in a zone (cfm per person) or the floor area of a zone (cfm per square foot).
For example, consider a lecture classroom with a design population of 65. To comply with the standard's requirement of 15 cfm of outdoor air per person, the classroom would need to receive 975 cfm of outdoor air (15 cfm per person × 65 people). If the population dropped to 20, the required amount of outdoor air would drop as well, to 300 cfm (15 cfm per person × 20 people).
Assuming a constant CO2-generation rate of 0.0105 cfm per person, a 700-ppm difference between indoor and outdoor CO2 concentrations would correspond to 15 cfm of outdoor air per person delivered under steady-state conditions (Figure 1).
 |
| FIGURE 1. CO2-based DCV under the 1989 through 2001 versions of ASHRAE Standard 62. |
Standard 62 required that a breathing zone receive a constant rate of outdoor airflow per person--15 cfm in our classroom example--regardless of the number of people actually in the space. Therefore, the desired differential between indoor and outdoor CO2 concentrations would remain constant, regardless of the number of people actually in the space, as well (Figure 1). By controlling to that constant differential, CO2-based DCV maintained a constant per-person ventilation rate to a space during periods of reduced occupancy (Figure 2).
 |
| FIGURE 2. CO2-based DCV under the 1989 through 2001 versions of ASHRAE Standard 62. |
(Note: Assumptions simplify DCV, but also introduce inaccuracy. Remember that CO2-generation rate varies with occupant activity level, diet, and health; required ventilation rate varies by space type under the standard; and outdoor CO2 concentration can vary by location.2)
2004 to present. In 2004, the method of determining breathing-zone ventilation rate (Vbz) was changed. Now, required rates are based on the number of occupants in a zone (cfm per person) and a zone's floor area (cfm per square foot). Therefore, two ventilation rates are prescribed: one for people-related sources (Rp) and one for building-related sources (Ra).
Vbz = (Rp × Pz) + (Ra × Az)
where:
Rp = required outdoor-airflow rate, cfm per person
Pz = zone population, number of people
Ra = required outdoor-airflow rate per unit area, cfm per square foot
Az = zone floor area, square feet
For our example lecture classroom, Standard 62.1 requires 7.5 cfm of
outdoor air per person plus 0.06 cfm of outdoor air per square foot of floor area. With a design population of 65 and a floor area of 1,000 sq ft, the delivery of 550 cfm of outdoor air [(7.5 cfm per person × 65 people) + (0.06 cfm per square foot × 1,000 sq ft)] is required. With 20 people in the classroom, the delivery of 210 cfm of outdoor air [(7.5 cfm per person × 20 people) + (0.06 cfm per square foot × 1,000 sq ft)] is required.
Figure 3 reveals two important effects of changes implemented with the 2004 version of the standard:
 |
| FIGURE 3. Potential DCV savings, Standard 62.1 vs. Standard 62. |
• By accounting for people- and building-related sources separately, the standard now results in lower breathing-zone ventilation rates for most occupancy categories (550 cfm vs. 975 cfm for our example lecture classroom). For densely occupied spaces--those that historically benefited most from CO2-based DCV, such as auditoriums, gymnasiums, conference rooms, lecture classrooms, and cafeterias--the rates are dramatically lower (Table 1).
 |
| TABLE 1. Minimum ventilation rates, Standard 62 vs. Standard 62.1. |
• As zone population decreases, the required breathing-zone ventilation rate drops less rapidly. In our example lecture classroom, it drops by 7.5 cfm for every person who leaves the zone under Standard 62.1, as opposed to 15 cfm per person under Standard 62.
Those two effects point to less potential for energy savings for most space types with CO2-based DCV under Standard 62.1.
Assuming a CO2-generation rate of 0.0105 cfm per person, the difference between indoor and outdoor CO2 concentrations for our example lecture classroom is 1,250 ppm at design occupancy. As the number of people in the space decreases, the desired difference between indoor and outdoor CO2 concentrations changes because the effective outdoor-airflow rate--on a cfm-per-person basis--no longer is constant. With 20 occupants, Standard 62.1 requires 210 cfm of outdoor air, which equates to 10.5 cfm per person, compared with 8.5 cfm per person at design occupancy. At 10.5 cfm per person, the desired difference between indoor and outdoor CO2 concentrations drops to 1,000 ppm with 20 occupants (Figure 4).
 |
| FIGURE 4. CO2-based DCV under Standard 62.1. |
Under Standard 62.1, effective cfm-per-person ventilation rate varies with population. Therefore, the desired difference between indoor and outdoor CO2 concentrations also varies. Controlling to a constant differential based on design occupancy will underventilate a zone at partial occupancy.
The bottom line is that CO2-based DCV is more difficult to implement under Standard 62.1. More difficult, but not impossible. CO2-based DCV in a single-zone system. In a single-zone HVAC system utilizing CO2-based DCV, the CO2 sensor typically is installed on a wall in the breathing zone (Figure 5). For expedience, the outdoor CO2 concentration usually is assumed to be constant, which allows the indoor concentration, rather than the difference between the indoor and outdoor concentrations, to be measured and used to modulate the position of the outdoor-air (OA) damper and provide the space with the proper amount of ventilation air on a per-person basis.
 |
| FIGURE 5. CO2-based DCV in a single-zone HVAC system. |
Standard 62.1's control strategy for CO2-based DCV is more complex than Standard 62's. Following is an explanation of two possible approaches: proportional control and single-set-point control.
Proportional control. Appendix A of "Standard 62.1-2007 User's Manual"3 discusses a method of implementing CO2-based DCV in a single-zone system. Essentially:
1) Find the required intake flow of outdoor air for the design zone population.
Vot-design = Voz = [(Rp × Pz) + (Ra × Az)] ÷ Ez = [(7.5 × 65) + (0.06 × 1,000)] ÷ 1.0 = 550 cfm
2) Find the required intake flow of outdoor air when the zone is unoccupied.
Vot-min = [(7.5 × 0) + (0.06 × 1,000)] ÷ 1.0 = 60 cfm
3) Find the target indoor CO2 concentration at design outdoor-air intake flow.
Cs-design = Co + [N ÷ (Vot-design ÷ Pz-design)] = 350 ppm + [0.0105 ÷ (550 cfm ÷ 65 people)] = 1,600 ppm
4) Set the target indoor CO2 concentration at minimum outdoor-air intake flow equal to the outdoor CO2 concentration.
Cs-min = 350 ppm
When actual indoor CO2 concentration equals design indoor CO2 concentration (1,600 ppm for our example lecture classroom), actual outdoor-air intake flow should equal design outdoor-air intake flow (550 cfm). When actual indoor CO2 concentration equals minimum indoor CO2 concentration (350 ppm), actual outdoor-air intake flow should equal minimum outdoor-air intake flow (60 cfm). When actual indoor CO2 concentration is between its minimum and design values, a controller should adjust outdoor-air intake flow proportionally between its minimum and design values:
Vot = [(Cs-actual – Cs-min) ÷ (Cs-design – Cs-min)] × (Vot-design – Vot-min) + Vot-min
As Figure 6 shows, the proportional-control approach yields an outdoor-air intake flow that equals or exceeds the requirement of Standard 62.1. This strategy is easy to implement, but overventilates zones at partial occupancy. A modulating OA damper, as well as a controller with two CO2 limits (Cs-design and Cs-min) and two OA-damper limits corresponding to intake airflows (Vot-design and Vot-min), are required.
 |
| FIGURE 6. Proportional-control strategy for CO2-based DCV per Standard 62.1. |
(Note: A simple improvement to this approach is to use a value other than zero for minimum population. In most cases, this will result in actual intake values closer to the minimum values required by the standard [less overventilation] than the values achieved with the approach described in "Standard 62.1-2007 User's Manual."3)
Single-set-point control. Following is an alternative control strategy that may result in less overventilation for some occupancy categories:
1) Pick a reasonable value (other than zero) to represent minimum occupancy (Pz-min), and find the required intake flow of outdoor air for that population.
Pz-min = 25 people
Vot-min = [(7.5 × 25) + (0.06 × 1,000)] ÷ 1.0 = 250 cfm
2) Find the target indoor CO2 concentration at minimum outdoor-air intake flow.
Cs-min = Co + [N ÷ (Vot-min ÷ Pz-min)] = 350 ppm + [0.0105 ÷ (250 cfm ÷ 25 people)] = 1,400 ppm
Intake flow is adjusted to maintain indoor CO2 concentration at the minimum value (1,400 ppm). If the OA damper reaches minimum outdoor-air intake flow (250 cfm), and zone population drops, the OA damper will maintain minimum outdoor-air intake flow. This will overventilate the zone, causing indoor CO2 concentration to drift downward. Conversely, as current population nears design, the zone will be overventilated.
As Figure 7 shows, the single-set-point approach results in an outdoor-air intake flow that equals or exceeds the ventilation rate required by Standard 62.1. The approach is simple to implement and, depending on the characteristics of the zone, may result in less overventilation at partial occupancy than the proportional-control method. Like the proportional-control method, it requires a modulating OA damper; however, the controller needs only one OA-damper set point (Vot-min) and one CO2 set point (Cs-min).
 |
| FIGURE 7. Single-set-point-control strategy for CO2-based DCV per Standard 62.1. |
CO2-Based DCV in a Multiple-Zone VAV System
CO2-based DCV alone. One approach to implementing CO2-based DCV in a multiple-zone VAV system is to install a CO2 sensor in every zone. A BAS monitors all of the sensors, determines how much outdoor air must be brought in at the air handler to satisfy the critical zone (and, thus, overventilate all other zones), and repositions the OA damper accordingly.
Installing a CO2 sensor in every zone is costly, especially considering that most of the zones always will be overventilated, regardless of operating conditions. Installing a sensor in "noncritical" zones offers no added value.
Alternatively, some designers install a single CO2 sensor in the return-air duct of a multiple-zone system and use that sensor to vary the amount of outdoor air brought in at the air handler. This sensor measures average CO2 concentration, so some spaces may be underventilated while others are overventilated. Whether this approach provides adequate ventilation is a subject of debate among designers.
Ventilation reset alone. Another control strategy for multiple-zone VAV systems--ventilation reset--resets intake airflow in response to changes in system ventilation efficiency.
Each VAV-box controller senses primary airflow and calculates its outdoor-air fraction. The BAS totals primary airflows and required outdoor airflows from all boxes and determines the highest outdoor-air fraction reported. Then, it solves the equations in Appendix A of Standard 62.1, calculating system ventilation efficiency and the required system-level intake flow of outdoor air. The new intake-flow set point is communicated to the air-handler controller, which adjusts the OA damper to bring in the required amount of outdoor air (Figure 8).
 |
| FIGURE 8. Control points for ventilation reset in a multiple-zone VAV system. |
In direct-digital-control VAV systems, this strategy is fairly easy to implement because all of the necessary real-time information already is available digitally (so no new sensors are required). All of the equations are defined in Appendix A of the standard and can be solved dynamically to find the required outdoor-air intake flow.
CO2-based DCV combined with ventilation reset.4 For most multiple-zone VAV systems, the best approach often is CO2-based DCV combined with ventilation reset. Using this strategy, CO2 sensors are installed only in zones that are densely populated with widely varying patterns of occupancy (e.g., conference rooms).
The sensors are used to reset the ventilation requirements for their respective zones. The other zones--which are not densely populated and/or do not
experience significant variations in occupancy--are assumed to require their design ventilation rates whenever they are occupied. The BAS uses ventilation-reset equations to determine how much outdoor air must be brought in at the air handler to satisfy all of the zones served.
In the VAV system in Table 2, Zone 1 is a conference room that is densely populated, with widely varying patterns of occupancy, while zones 2 and 3 are general office spaces that are more sparsely and more consistently occupied. The data in the top section of the table concern the use of ventilation reset only. Each zone is assumed to require its design outdoor airflow, regardless of actual population.
 |
| TABLE 2. Effect of ventilation-control strategies in a single-duct VAV system at part load. |
The data in the lower section of Table 2 concern the use of a CO2 sensor in Zone 1 for the reduction of required outdoor airflow from the design value of 500 cfm to 200 cfm when the actual population is less than design. Zones 2 and 3 still require their design outdoor airflows. While sensing CO2 and finding the current value of Zone 1 outdoor airflow lowers the average outdoor-air fraction (Xs), it increases system ventilation efficiency (Ev) and lowers the required intake airflow from 2,370 cfm to 1,530 cfm.
Combining CO2-based DCV with ventilation reset:
• Can ensure that each zone receives the proper amount of ventilation without a CO2 sensor being installed in every zone. CO2 sensors are used only where they are most beneficial. When other zones are unoccupied, time-of-day schedules or occupancy sensors are used to reduce ventilation.
• Enables documentation of actual ventilation-system performance by communicating ventilation airflows for every zone to the BAS.
Summary
DCV can reduce the cost of operating an HVAC system--especially where contaminants result primarily from people (or their activities) and occupancy varies greatly. Although it explicitly allows DCV based on CO2, Standard 62.1 diminishes the value of CO2-based DCV's implementation for most space types by reducing required design ventilation rates. Standard 62.1 complicates implementation of CO2-based DCV because effective cfm per person and, therefore, desired indoor/outdoor CO2-concentration differential vary as zone population changes. CO2-based DCV most commonly is used in single-zone systems serving densely occupied spaces with varying populations. In multiple-zone VAV systems, combining CO2-based DCV with ventilation reset--using CO2 sensors only in densely occupied zones with widely varying populations--provides a cost-effective, reliable, and energy-efficient system.
References
1) Schell, M.B., Turner, S.C., & Shim, R.O. (1998). Application of CO2-based demand-controlled ventilation using ASHRAE Standard 62: Optimizing energy use and ventilation. ASHRAE Transactions, 104, 1213-1225.
2) Murphy, J., & Bradley, B. (2002). Using CO2 for demand-controlled ventilation. Engineers Newsletter, 31(2). Available at http://www.trane.com/commercial/library/vol31_3/adm_apn004_en.pdf
3) ASHRAE. (2007). Standard 62.1-2007 user's manual. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
4) Murphy, J., & Harshaw, J. (2007). Rooftop VAV systems. La Crosse, WI: Trane.
John Murphy, LEED AP, is an applications engineer for Trane whose areas of expertise include energy efficiency, dehumidification, air-to-air energy recovery, psychrometry, and ventilation. He is chair of American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Technical Committee (TC) 8.10, Mechanical Dehumidification Equipment and Heat Pipes, and a member of ASHRAE TC 1.12, Moisture Management in Buildings. The late Brenda Bradley was an information designer for Trane.