CO2-Based Demand-Controlled Ventilation and Its Implications for Interior Design

CO2-Based Demand-Controlled Ventilationand Its Implications for Interior Design

Seunghae Lee, Ph.D., Oregon State University

ABSTRACT

CO2-based demand-controlled ventilation (DCV) is a ventilation method that resetsoutdoor air supply rates using CO2 as an operating parameter (ASHRAE, 2007a). Eventhough CO2 itself is not harmful, high levels of CO2 concentration in indoor environmentsdisplace oxygen in the air and can cause a deficiency of oxygen for breathing. Themost important aspect of CO2 in DCV is that it is a good indicator of occupancy inindoor spaces. Therefore, CO2 is an effective parameter for controlling ventilation basedon occupancy level. CO2-based DCV has been increasingly used in recent years tooptimize the energy consumption in ventilation systems as technological advances makeits implementation more feasible and interest in environmentally responsible buildingdesign grows. This study aims to provide information about CO2-based DCV andsuggest implications for interior design practitioners and educators. To achieve thispurpose, this study reviewed previous studies and examined the current practices inCO2-based DCV in order to suggest guidelines for CO2-based DCV implementation forinterior designers. The guidelines focused on proper CO2 sensor location design, takinginto consideration the activities and other equipment in the room. In addition, a studyof CO2-based DCV in the lecture halls of a university campus was conducted. Physicalmeasurements were done in the lecture halls, and trends data were collected from theuniversity’s facility management department to compare the measurements. A perceivedindoor environmental quality survey was also done to explore the occupants’ responsesabout CO2-based DCV.

This study aims to provide information on CO2-baseddemand-controlled ventilation (DCV), an emergingtechnology for environmentally responsible interiordesign, in order to help interior designers understandthe technology and its impact on building users.As interest in environmentally responsible buildingdesigns increases, various building-related technolo-gies have been developed and increasingly used inrecent years to improve energy efficiency and reduceenergy use. When a building’s energy performance ismore effective, less greenhouse gases generated fromenergy production are emitted (United States GreenBuilding Council [USGBC], 2009). On the otherhand, it is also important to maintain the indoor envi-ronmental quality (IEQ) to a desired level for occu-pants while keeping the energy performance effective,and there is a very close relationship between the IEQand the building’s mechanical systems. The Leader-ship in Energy and Environmental Design (LEED)Reference Guidelines by the USGBC (2009) suggeststhat: (1) the building’s mechanical system may impact

tenants’ controllability in adjusting occupants’ IEQ,(2) a high IEQ may increase occupants’ productivity,and (3) when the IEQ is improved, building ownersmay have less liability, and building occupants’ healthand well-being may be improved.

Therefore, it is critical for interior design practitionersand educators to understand the emerging technolo-gies for environmentally responsible interior designand explore their implications. Although interiordesigners will not need to deal with technical issuesintensively over the course of design projects, they stillhave to have a general knowledge of various technicalfields, such as mechanical, electrical, and plumbing,because they will need to communicate with otherprofessionals and understand how to handle theimpact of technical matters on the interiors (Pile,2007). Interior designers understand user needs, activ-ities, and equipment location requirements; therefore,the early involvement of interior designers in projectprogramming and project phases is important for

© Copyright 2012, Interior Design Educators Council,Journal of Interior Design 19 Journal of Interior Design 37(2), 19–33

CO2-BASED DEMAND-CONTROLLED VENTILATION

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The early involvement of interior designers in project programming and project phases isimportant for successful and effective CO2-based DCV implementation.

successful and effective CO2-based DCV implemen-tation. To achieve this, interior designers should beaware of this innovative ventilation system whenworking with other professionals in building designand construction. Interior designers who have a basicunderstanding and knowledge of engineering disci-plines can be more dynamic and valuable membersof a collaborative design team (Stieg, 2006).

For interior designers who represent clients and areresponsible for the health, well-being, and safetyof end users, in particular, it is critical to haveknowledge about the impact of technical systemson occupants. In this effort to build environmentallyresponsible interior design, it is also important todevelop strategies for effective and efficient energy usewhile at the same time keeping the IEQ to a desirablelevel. As Americans spend an average of 90% of theirtime indoors, the IEQ may impact occupants’ healthand well-being significantly (USGBC, 2009). The IEQincludes several elements, such as air quality, thermalconditions, acoustics, and lighting. The protectionof indoor air from contaminants is essential for thehealth of building occupants. The building ventila-tion system is the critical component in maintainingthe indoor air quality (IAQ). DCV is a ventilationmethod that controls the intake of the outdoor air(OA) supply based on occupancy indicators, such astime schedule, the actual number of occupants, orthe number of occupants counted using occupancysensors (Mui & Chan, 2006). CO2-based DCV isa recent technological development in DCV thatpermits the resetting of the OA intake flow dependingon the indoor CO2 concentration level, controllingventilation based on actual demand and thus savingenergy (Murphy & Bradley, 2008). Even though CO2itself is not harmful, high levels of CO2 concentrationin indoor environments displace oxygen in the airand can cause oxygen deficiency, thus impeding theability to breath. The most important aspect of CO2in DCV is that it is a good indicator of occupancy inindoor spaces (American Society of Heating, Refrig-erating, and Air-Conditioning Engineers [ASHRAE],2007b). Therefore, this method is most effective inenvironments such as lecture halls, ballrooms, con-ference rooms, auditoriums, and theaters where the

occupancy is intermittent and often well below themaximum design occupancy. In contrast, most heat-ing, ventilation and air-conditioning (HVAC) designsfor OA delivery are based on the peak occupancy.

ASHRAE Standard 62.1-2007 specifies minimumventilation rates and other measures to provide IAQthat minimizes adverse health effects for humanoccupants (ASHRAE, 2007a). ASHRAE also providesa users’ manual that describes the CO2-based DCVimplementation methods in detail as the technologyis increasingly used in practice for energy saving. TheLEED certification system includes IEQ as one of itscriteria and suggests that the CO2-based DCV can beused for OA delivery monitoring, which is one of thecredits in IEQ (USGBC, 2009). While CO2-basedDCV is an effective practice that saves energyfor environmentally responsible building design andmaintenance, it should be carefully implemented tomaintain the IAQ. The recently updated Council forInterior Design Accreditation (CIDA) ProfessionalStandards 2011 includes Standard 12: EnvironmentSystems and Controls, which demonstrates howimportant it is for interior designers to understand thetechnical aspects of interior design practices. One ofthe standard’s learning expectations is for students tounderstand the principles of IAQ. CIDA suggests CO2monitoring as one way to understand IAQ principles.Therefore, it is important for interior design researchto address this issue and provide the information forinterior design practitioners and educators about thegeneral practice of the technology and its impact onthe IAQ and occupants. This study is relevant to thecurrent issue of energy use and environmental control,which is central to the discussion of sustainabilityand environmental impact. The literature onenvironmentally responsible interior design presentsCO2-based DCV as a ventilation control system thatnot only controls IAQ but also reduces energy use(Bonda & Sosnowchik, 2007; Kibert, 2008; Winchip,2007). In order to examine the current practicesand issues of CO2-based DCV and its impact onbuilding occupants, a study of CO2-based DCVwas conducted. On the basis of the results of thestudy, implications for interior design practitionersand educators are discussed.

Journal of Interior Design 20 Volume 37 Number 2 2012


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The implementation of CO2-based DCV involves two major technologies: CO2 sensorsand an air handling system.

CO2-Based DCV: Practices,Standards, and ResearchThe examination of CO2-based DCV in this studyincludes the types of CO2 sensors, sensor locationlayout design, interfaces with the building system, andrequirements based on industry standards includingASHRAE 62.1-2007 and LEED environmentallyresponsible design standards. The review of previousstudies focuses on issues such as IAQ, energy savings,and the technical aspects of CO2-based DCV.

To comply with building codes that regulate a min-imum amount of fresh air from outside in order tomaintain adequate air quality for building occupants,the ventilation system is designed to operate on thebasis of the maximum number of occupants expectedin a space (Schell, 1998). For example, when a venti-lation system for a theater is designed, it assumes thenumber of occupants when the theater is fully occu-pied and the seats are all filled, such as during a perfor-mance. However, many spaces in the building are notoccupied by its maximum assumed number of people,and, therefore, a ventilation that runs at its maximumcapacity wastes energy (U.S. Department of Energy,2004). As a result, expensive and unnecessary over-ventilation occurs. For the effective use of energy,engineers employ various ventilation strategies thatcan control the ventilation variably to keep the airquality at a desirable level and not operate the systemmore than necessary. CO2-based DCV is one of thosemethods that help save energy by controlling the ven-tilation more effectively (U.S. Department of Energy,2004). Although this ventilation method is fairly newas it is less than two decades old, it has been widelyembraced by the design community given the currentinterest in environmentally responsible designs.

The implementation of CO2-based DCV involvestwo major technologies: CO2 sensors and an airhandling system. For the successful implementationof CO2-based DCV, it is critical to gather informa-tion about these technologies and study them beforeimplementation. Fortunately, CO2-based DCV canbe implemented in a building retrofit as well as in newbuilding construction. CO2-based DCV is not too

complicated to implement and retrofitting an existingsystem for CO2-based DCV is possible. However,it is challenging to update the older HVAC with apneumatic control system to CO2-based DCV as it ismore costly and complicated to update a pneumaticcontrol system into the new system (U.S. Departmentof Energy, 2004). In some applications of CO2-basedDCV, the lighting occupancy sensor can be connectedwith CO2 sensors so that the minimum airflow iskept at zero when the area is not occupied.

It is particularly important for interior designers tounderstand CO2 sensors since they are part of adesign team that determines the types and locationsof the sensors. This decision requires an understand-ing of the activities in the room and the locationsof other equipment. Approximately 60,000 sensorsare sold per year in the United States, and the marketis growing (U.S. Department of Energy, 2004).There are mainly two types of CO2 sensors availableon the market in terms of installation methods: awall-mounted sensor and a duct-mounted sensor.Wall-mounted sensors are more practical and advan-tageous, since the duct-mounted system averages theCO2 level rather than representing individual spaces.For a duct-mounted CO2 sensor system, a singleCO2 sensor is often installed in the return air duct.Therefore, the sensor monitors the CO2 level from theair that has been mixed and delivered from all areasin the space; it does not represent underventilated oroverventilated areas but only represents their average.Thus, although there is an area in the interior whereoccupants are concentrated, the CO2 sensor cannotdetect if there is a vacant area in the space to averageout the CO2 level to a lower number than that in thearea of concentration. It is less costly and easier toimplement the duct-mounted sensor type, but it maynot represent underventilated areas (USGBC, 2009).

The criteria for CO2 sensor location selection forwall-mounted sensors are similar to the criteria forthermometer location selection. The major point isto select a location from which the CO2 level can bemeasured accurately and which is representative ofthe area or zone that is served by the air handlingunit (AHU). Thus, CO2 sensors should not be



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Much research has been conducted to study CO2-based DCV over the last 20 years.

located where people may breathe directly into thesensor, such as near doorways, operable windows,air vents, coffee service areas, or water coolers(Carrier, 2001; Persily, Braun, Emmerich, Mercer, &Lawrence, 2003). The guidelines from Carrier (2001)also suggest that wall-mounted CO2 sensors be placedin a reasonably central location within a zone. TheLEED guidelines suggest installing CO2 sensors invertical breathing zones, which are 3 to 6 ft abovethe floor. However, CO2 sensors usually cannot belocated in the breathing zone because the sensors mayinterfere with the occupants’ activities in the roomand the measurements may not be accurate when thesensors are too close to the occupants (Chao & Hu,2004). It is suggested that multiple CO2 sensors beinstalled to monitor the CO2 level more accurately ina space as the CO2 level can vary depending on theconcentration of people (USGBC, 2009). The industryguidelines from Carrier also suggest that a single CO2sensor may cover up to 5000 sq ft. When an areaexceeds 5000 sq ft, multiple CO2 sensors should beinstalled to monitor the CO2 level in the space.

Much research has been conducted to study thesubject of CO2-based DCV over the last 20 years.Such studies focused on topics such as the technicalimplementation and technologies of CO2-based DCVand the impact of CO2-based DCV on IAQ andenergy savings. For example, some studies focused onthe ventilation control strategy for CO2-based DCV(Mankibi, 2009; Wang & Xu, 2004) to find a moreeffective approach.

Wang and Xu (2004) suggested that the IAQ canbe maintained at the same level or even improvedwith CO2-based DCV compared to the traditionalventilation system as DCV can introduce morefresh air. Mankibi (2009) supported this resultand demonstrated that CO2-based DCV can offerbetter results in terms of the IAQ and heat energyconsumption.

Previous studies have suggested energy savings withDCV (Chao & Hu, 2004; Schell, 1998; Wang &Xu, 2004). For example, Chao and Hu (2004)reported a possible 8.3% to 28.3% increase in

daily electrical energy savings with CO2-based DCVcompared to the traditional fixed-rate ventilationcontrol strategy. Wachenfeldt, Mysen, and Schild(2007) showed similar results. In their study onCO2-based DCV in schools, they suggested that thetechnology reduced the total heating energy demandby 21%. Another study on CO2-based DCV inschools showed that this ventilation method reducedthe energy usage by 38% in the average classroom(Mysen, Berntsen, Nafstad, & Schild, 2005). Studieshave evaluated the potential of CO2-based DCVfor energy savings in buildings that have not beenconsidered as suitable and profitable for CO2-basedDCV previously. These include a study on CO2-basedDCV in a multifamily building using the softwaresimulation method (Pavlovas, 2004) and on CO2-based DCV in office buildings (Mysen, Rydock, &Tjelflaat, 2003). Hendricks, Drayer, and Lee (2006)also experimented with CO2-based DCV in offices tocompare the energy usage to that of the traditionalventilation method; they found that CO2-based DCVmade possible an increase of about 34% in annualelectricity energy savings when it was used along withtime-controlled ventilation in the office module.

Studies about the responses of occupants in buildingswith DCV are limited. In one such study (Chao &Hu, 2004), occupants were asked about the IAQ, and90% of those in a building with CO2-based DCVreported that it was acceptable. The IAQ is a criticalelement of IEQ as studies show that in classroomenvironments, increased ventilation can result inhigher thermal comfort and air quality (Norback& Nordstrom, 2008). A study on the associationsbetween CO2 levels and student attendances showedthat a 1000 ppm difference in the CO2 level wasassociated with a .5% to .9% decrease in annualaverage daily attendance (Shendell et al., 2004). A100 ppm increase in the CO2 level was shown to beassociated with combined mucous membrane, dryeyes, sore throat, sinus congestion, sneezing, andwheezing symptoms as a high CO2 level is assumedto indicate low ventilation (Erdmann & Apte,2004). Rudnick and Milton (2003) suggested thatcontinuous measurements of CO2 levels can be usedto estimate the likelihood of airborne transmission of



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A review of literature shows that CO2-based DCV has been increasingly implementedmainly due to energy saving purposes. However, there is limited evidence in occupant

responses to IAQ.

infection indoors. There is reported evidence that theclassroom ventilation, represented by the CO2 level,has a significant association with test results in math(Shaughnessy, Haverinen-Shaughnessy, Nevalainen,& Moschandreas, 2006). Evidence from previousstudies about CO2 as an indicator of the ventilationrate further supports the ventilation control strategythat uses CO2 levels. In addition to CO2, the volatileorganic compound (VOC) levels showed increases inthe report on perceived poor IAQ (Claeson, Nordid,& Sunesson, 2009).

A review of literature shows that CO2-based DCV hasbeen increasingly implemented mainly due to energysaving purposes. However, there is limited evidence inoccupant responses to IAQ. Thus, it is timely and crit-ical to examine occupants’ responses to IAQ in class-rooms that control ventilations based on CO2 level.

This study will explore CO2-based DCV implementedin a large lecture hall by examining occupants’responses with regard to IAQ, temperature, andhumidity to better understand how people feel aboutand perceive the IEQ in spaces that have CO2-based DCV. Although physical measurements areimportant as objective tools with which to examinethe IEQ, building occupants are a critical source ofinformation about IEQ and its impact on comfortand productivity (Zagreus, Huizenga, Arens, &Lehrer, 2004). The results from this investigationwill provide information to interior designers whoare concerned with occupants’ health and well-beingin environmentally responsible interior spaces.

Research MethodsTo understand the impact of CO2-based DCV onbuilding occupants, the researchers examined build-ing occupants’ perceptions of indoor environmentalaspects in spaces with a conventional ventilationmethod and in spaces with CO2-based DCV. Thestudy was conducted in a large public university inthe Midwest. Over the past 10 years, the school’sfacilities management department has implementedCO2-based DCV in classrooms and auditoriums to

provide the desired air quality and save energy. Thenumber of buildings that implemented DCV hadgrown to 43 locations by 2009. The school’s facili-ties management department implements CO2-basedDCV and uses the criteria listed below.

1. Ideally, the AHU and CO2 sensor serve only onespace. If there are additional zones, individualCO2 sensors are required for each space.

2. The occupancy of the space varies with time asthe system saves energy by lowering air supplywhen the occupancy is low.

3. The OA duct and damper must be large enough toprovide the desired ventilation for full occupancyto supply enough air for the peak occupancy.

4. A modulating mixed damper control of outdoor,return, and relief air is required where applicable.The dampers must be in control from the baseminimum to the maximum desired flow. TheCO2-DCV’s new base ventilation OA rate shouldbe equal to 25% of the full ventilation capacity(25% × 15 cfm/person × full occupancy) tosupply minimum air for the base ventilation.

5. There should always be return or relief airflow atthe CO2 sensor to sense CO2 constantly.

6. The CO2 sensor should require 24 VAC power.7. The existing dampers and actuators should be

in good condition and have low leakage as thesensor may not detect the CO2 level accuratelywhen there is leakage.

8. The CO2 sensor should be located in the returnduct of the AHU or in the room. For an accuratereading, an accessible location as near as possibleto the room should be chosen, with wiring(4–20 mA and 24 V power) connected to thesensor. Duct sensor locations with significant airleakage between the room and the sensor shouldbe avoided, or the CO2 sensor should be mountedin the room.

The perceived IEQ survey was conducted in twolecture halls, one with traditional ventilation and theother with CO2-based DCV. The two classroomsin this study were similar in size and design. Bothhad a tiered theater-style floor for student seating.A total of 305 responses were used for the dataanalysis, including 101 responses from the occupants



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To understand the impact of CO2-based DCV on building occupants, the researchersexamined building occupants’ perceptions of indoor environmental aspects in spaces with

a conventional ventilation method and in spaces with CO2-based DCV.

of the building with CO2-based DCV. The surveyexamined satisfaction with temperature, humidity,and air quality (i.e., stuffy, stale, clean, or odorousair). The responses on satisfaction were measuredusing a 5-point Likert scale (1 = very dissatisfiedto 5 = very satisfied). The perceived interferenceof uncomfortable temperature, humidity, and airquality to learning was determined using a 5-pointLikert scale (1 = strongly disagree to 5 = stronglyagree). In addition, the instrument also includedquestions on demographics of the participants. Also,items determined whether or not the participants hadany respiratory disease or smoked. The survey wasconducted immediately after class. The indoor envi-ronmental conditions, such as temperature, humidity,CO2 level, and VOC levels, were measured at the timeof the data collection using a PP Monitor Stand AloneSystem (SAS) manufactured by PPM Technology. Forthe lecture hall with CO2-based DCV, data on theventilation system, such as temperature, humidity,and CO2 level, were collected from the facilitiesmanagement department for comparison with thedata measured in the classroom. Research questionsinclude (1) the IAQ measured in a lecture hall withtraditional ventilation method is different from theIAQ measured in a lecture hall with CO2-based ven-tilation, (2) the temperature, CO2 level, and humiditylevels measured by the CO2 sensor in the ventilationcontrol system are different from those collected bythe measurement device measured in the lecture hall,(3) occupants’ responses to IEQ in the lecture hallwith traditional ventilation method are different fromthose with CO2-based ventilation, and (4) there aregender differences in occupants’ responses to IEQ.

The following statistical analyses analyzed the data,using SPSS 18 for Windows. First, the descriptivestatistics summarized the data. Next, zero-order cor-relation analyses were employed to determine thebivariate relationships between the independent vari-ables and the dependent variables. The independentvariables include ventilation method, age, and gen-der. The dependent variables include satisfaction withtemperature, satisfaction with humidity, perceivedinterference of temperature with learning, perceivedinterference of humidity with learning, satisfaction

with air quality, and perceived interference of airquality with learning. After the correlations wereexplored, a one-way within-subject analysis of vari-ance (ANOVA) was conducted to examine the venti-lation control method effects on the perceived IEQ.

ResultsThis study reports the results from the indoorenvironmental measurements that were taken at thetime of the perceived IEQ survey in both lecturehalls, the trends data from the facilities managementdepartment for the lecture room that uses CO2-basedDCV, and the perceived IEQ survey.

Physical MeasurementsThe results that were collected from measurementsof IEQ, such as CO2, CO, formaldehyde, humidity,temperature, and total VOC levels, showed that thedata from the lecture hall with CO2-based DCVwere not very different from the data from thelecture hall with the traditional ventilation method.Measurements from the lecture hall with traditionalventilation were collected twice at different classtimes. None of the contaminant chemicals, suchas CO2, CO, formaldehyde, and total VOCs, wereshown to be at hazardous levels. The CO2 levelswere under 1000 ppm in all measurements, whichis ideal for interior spaces (Schell, 1998). The levelof formaldehyde ranged from .012 to .018, whichis considered neither dangerous nor ideal. Daisey,Angell, and Apte (2003) suggested that studies haveshown an acute formaldehyde toxicity level rangingbetween .055 and .08 ppm, and the most strictformaldehyde reference exposure level is .002 ppm.The temperature in both lecture halls was around 69◦

Fahrenheit, while the relative humidity was lower inthe lecture hall with CO2-based DCV than in thelecture hall with traditional ventilation (Table 1).

The temperature, CO2 level, and humidity measure-ments from the trends data provided by the facilitiesmanagement department showed similar results.The trends data include the temperature, CO2, and



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Occupants in the lecture hall with CO2-based DCV reported more satisfaction withtemperature compared to occupants in the hall with a traditional ventilation system.

Table 1. Physical measurements in the lecture halls.

CO2 (ppm) CO (ppm) Formaldehyde (ppm) Humidity (% RH) Temperature (F)Total volatile organic

compound (ppm)

Traditional 1 747.00 .74 .01 40.80 68.90 .25Traditional 2 974.00 .75 .02 59.30 70.70 .19CO2-based 784.00 .55 .02 25.10 69.62 .24

Figure 1. Trends data: CO2.

humidity levels from various sensors in the lecturehall and return air ducts. The ventilation systemuses these data to control the ventilation, and thecontrol is maintained automatically. Trends data forthe lecture hall with the traditional ventilation systemwere not available, as the lecture hall does not usesuch data to control the ventilation and so does nothave air monitors. These trends data, including thetemperature, humidity, and CO2 levels, were veryclose to the measurements taken in the lecture hallat the time of the perceived indoor environmentalsurvey. Figure 1 shows the CO2 levels in the lecturehall with CO2-based DCV on the day the perceivedindoor environmental survey was taken. The CO2level increased during class, reflecting the occupancy

in the room. The level was under 1200 ppm, whichshows that the ventilation is controlled properly.Figure 2 shows the temperature and relative humiditylevels from two sensors in the lecture hall and twosensors in the return air duct. The measurements fromthe sensors in the lecture hall were similar to thosetaken by the investigator at the time of the perceivedIEQ survey (4:30 pm, December 9, 2009). Figure 3shows the damper position to allow the air in poundsper square inches. The damper opening is controlledby the temperature and CO2 level in this lecture hall.Figure 3 shows that the damper was open when thelecture hall was occupied, as the damper positionis controlled by the CO2 level, the time, and thetemperature.



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Gender differences appeared in regard to satisfaction with the indoor temperature. Morefemale occupants reported dissatisfaction with temperature and more perceived

interference of temperature with learning in both lecture halls.

Figure 2. Trends data: temperature and humidity.

Perceived Indoor Environmental Quality Survey

Correlational analyses showed relationships betweenvarious factors. They revealed significant and positiverelationships between ventilation type (traditionalventilation = 1, CO2-based DCV = 2) and satisfac-tion with temperature. This means that occupants inthe lecture hall with CO2-based DCV reported moresatisfaction with temperature compared to occupantsin the hall with a traditional ventilation system.Occupants in the lecture hall with CO2-based DCValso reported less perceived interferences of tem-perature with their activities than occupants in thelecture hall with traditional ventilation did. Similarrelationships existed for the responses on humidity.Significant correlations existed between satisfactionwith humidity and ventilation methods (Table 2). Inaddition, there were significant relationships betweenperceived interference of humidity and ventilation

methods. No significant relationship was foundbetween ventilation type and satisfaction with airquality or perceived interference of air quality withactivities in the room.

Gender differences appeared in regard to satisfactionwith the indoor temperature. More female occupantsreported dissatisfaction with temperature and moreperceived interference of temperature with learning inboth lecture halls. Occupants who smoked or had arespiratory disease did not report any significantlydifferent level of satisfaction with temperature,humidity, and air quality, or interferences of thosefactors with learning in both lecture halls.

It was interesting to note that occupants who weremore satisfied with temperature also reported greatersatisfaction with humidity or air quality, and lessinterference of temperature with learning. Similarly,



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Figure 3. Trends data: damper opening.

occupants who reported more satisfaction withhumidity reported more satisfaction with air qualityand less interference of air quality with activities.These results support the findings from a previousstudy that showed a significant influence of tempera-ture and humidity on the perceived air quality (Fang,Clausen, & Fanger, 1998).

On the basis of the results of the correlationalanalyses, ANOVA tests compared the responses ofoccupants in the lecture hall with CO2-based DCVwith those occupying in the lecture hall, which incor-porated a traditional ventilation method. The firstANOVA examined occupant satisfaction with tem-perature, humidity, and air quality, and perceivedinterference of temperature, humidity, and air qual-ity with learning in the room. The results showeddifferences in the responses in terms of satisfactionwith temperature and humidity, and perceived inter-ference of temperature and humidity with learning

depending on ventilation method (Table 3). Thenext ANOVA examined differences in the responsesbetween males and females. The results showed a sig-nificant gender difference in the satisfaction with tem-perature and perceived interference of temperaturewith learning (Table 4). The third ANOVA exploredoccupants by age. The results showed differencesin the responses on satisfaction with temperature,humidity, and air quality, and perceived interfer-ences of temperature, humidity, and air quality withactivities in the room, depending on age (Table 5).

The results from the statistical analyses showedthat occupants in the lecture hall with CO2-basedDCV did not report air quality differently comparedto occupants in the lecture hall with traditionalventilation. However, positive relationships existbetween CO2-based DCV and satisfaction withtemperature and humidity. For interior designers,these results suggest that CO2-based DCV not



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Table 3. ANOVA: ventilation methods.

Sum of squares df Mean square F Significance

Temperature (satisfaction) Between groups 11.42 1 11.42 14.60 .00Within groups 251.16 321 .78 — —

Total 262.58 322 — — —Temperature (interference) Between groups 10.99 1 10.99 12.15 .00

Within groups 287.50 318 .90 — —Total 298.49 319 — — —

Humidity (satisfaction) Between groups 3.26 1 3.26 5.69 .02Within groups 182.98 319 .57 — —

Total 186.24 320 — — —Humidity (interference) Between groups 5.25 1 5.25 6.66 .01


Air quality (satisfaction) Between groups .06 1 .06 .07 .79Within groups 262.65 320 .82 — —

Total 262.71 321 — — —Air quality (interference) Between groups 1.76 1 1.76 1.96 .16


∗∗Correlation is significant at the .01 level (two-tailed).∗Correlation is significant at the .05 level (two-tailed).

Table 4. ANOVA: gender.






Total 185.99 319 — — —Humidity (interference) Between groups .28 1 .28 .35 .55


Air quality (satisfaction) Between groups .21 1 .21 .25 .62Within groups 262.32 319 .82 — —






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For interior designers, these results suggest that CO2-based DCV not only results ineffective energy use but also helps occupants feel satisfied with the indoor environment.

Table 5. ANOVA: age.






Total 186.24 320 — — —Humidity (interference) Between groups 9.05 3 3.02 3.86 .010


Air quality (satisfaction) Between groups 6.66 3 2.22 2.76 .04Within groups 256.05 318 .81 — —




only results in effective energy use but also helpsoccupants feel satisfied with the indoor environment.

Conclusion and ImplicationsThe purpose of this study was to examine CO2-based DCV by employing a methodology to studythe perceptions of building users. Among the manyissues with the CO2-based DCV technology, theappropriate location and number of CO2 sensorsis considered to be the most critical in relation to theinterior designers’ responsibilities in design projects.There are several possible locations for CO2 sensors,such as on walls or in return ducts. Currently, theASHRAE recommends placing CO2 sensors in thebreathing zone adjacent to the thermostat, ‘‘regionwithin an occupied space between three planes: 3and 72 in. above the floor and more than 2 ftfrom the walls or fixed air-conditioning equipment(ASHRAE, 2007a).’’ However, in practice, sensorsare usually located on the wall or in return ducts.To lower the implementation and maintenance cost,

many applications place the sensor in the return ductinstead of on the wall of the space (USGBC, 2009).When the sensor is placed in the return duct, the airin the room flows into the duct and gets mixed. In thiscase, the accuracy of the method is debatable becauseleakages of air in the return air duct can cause thesensor to reflect a lower CO2 level than the actualCO2 level in the room. When deciding where to placeCO2 sensors, it is important for interior designers toconsider the activities in the room and the locationof other equipment. Another consideration in CO2

sensor placement for CO2-based DCV is the numberof CO2 sensors in the room. As CO2-based DCVis implemented in relatively larger spaces, one CO2

sensor may not be enough to measure the overall CO2

level in a room. It is suggested that a single sensorcan cover up to 5000 sq ft. ASHRAE standards andLEED guidelines suggest installing multiple sensorsfor more accurate detection of CO2 levels. Currently,wireless sensor systems are available on the market.These are used to feed CO2 levels from multiplelocations in a room in order to control the ventilation



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The findings indicate that occupants are more sensitive to temperature than to humidity orair quality differences in occupant responses.

more effectively. When CO2 sensors are installed onthe wall, they should be reasonably centered in theroom (Carrier, 2001) and not located where peoplemay breathe directly into the sensor, such as neardoorways, operable windows, air vents, coffee serviceareas, or water coolers (Carrier, 2001; Persily et al.,2003). CO2 sensors can be installed and operated inconjunction with other sensors, such as occupancysensors for lighting (USGBC, 2009).

Studies on CO2-based DCV have focused on thetechnical implementation (Mui & Chan, 2006;Schell, 1998) and energy savings (Chao & Hu,2004; Hendricks, Drayer, & Lee, 2006; Mysen et al.,2005; Pavlovas, 2004; Wachenfeldt et al., 2007;Wang & Xu, 2004). There is very scant evidencefrom empirical studies about how CO2-based DCVaffects the perceptions and satisfaction of buildingusers. However, it is important to understand theimpact of this technology on building users whosehealth and well-being should not be compromised toachieve energy efficiency (USGBC, 2009), especiallyconsidering that the energy efficiency–IAQ dilemmais a well-known problem in energy-efficient buildingdesign (Becker, Goldberger, & Paciuk, 2007). Onthe other hand, studies on DCV and its impact havedemonstrated that the DCV strategy can improve theIAQ (Chao & Hu, 2004; Wang & Xu, 2004).

Results from this study showed that there wasno serious contaminant in both lecture halls, andthe temperature and humidity levels were withinthe ideal range for indoor activities. The findingsfrom the perceived indoor environmental surveysuggest that the CO2-based DCV did not impactoccupants’ satisfaction with air quality negatively asthere were no significant differences in satisfactionwith air quality between occupants exposed to CO2-based DCV and those exposed to a traditionalventilation system. Rather, occupants in the lecturehall with CO2-based DCV were more satisfied withtemperature compared to those in the lecture hall withthe traditional ventilation method. This implies thatCO2-based DCV controls the airflow more variablyand based on demand compared to other ventilation

methods; therefore, it provides better control of airin response to changes.

The findings indicate that occupants are more sensi-tive to temperature than to humidity or air quality.It is important to note age and gender differencesin occupant responses. Women felt less satisfactionwith temperature than men did, and older occupantsreported more interference of temperature with learn-ing. When interior designers work with mechanicalengineers to consider this CO2-based DCV strategyfor sustainable design purposes, it is important toconsider occupants’ satisfaction with and perceptionsof indoor environments, and that CO2-based DCVis beneficial not only in energy saving but also inoffering a comfortable environment for occupants.

This study demonstrates that CO2-based DCV canbe an effective ventilation strategy in classroomenvironments both for efficient energy use and foroccupants’ comfort and well-being. Ventilation mayhave negative effects on IAQ and climate when itis not properly designed, installed, and maintained(Seppanen & Fisk, 2004). A previous study suggestedproviding more ventilation than is required bystandards to improve perceptions of IAQ andreduce sick building syndrome symptoms (Wargocki,Wyon, Sundell, Clausen, & Fanger, 2000). However,the results from this study suggest that when theventilation is optimized and controlled on the basis ofdemand, occupants do not feel less satisfied but rathermore satisfied with the temperature and humidity.The associations of satisfaction with temperature andhumidity to air quality suggest that it is importantthat occupants feel satisfied with the temperature andhumidity because this may eventually lead to satisfac-tion with air quality. The observed higher satisfactionwith temperature and humidity supports the resultsfrom a previous study, which reported that about50% of occupants in a lecture hall with CO2-basedDCV felt that the air was fair and 90% of occupantsfelt that the air was acceptable (Chao & Hu, 2004).

Thus, this study recommends CO2-based DCV forsimilar university settings. The results from this studyprovide significant information as it compared the



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Women felt less satisfaction with temperature than men did, and older occupants reportedmore interference of temperature with learning.

responses of occupants exposed to CO2-based DCVwith those exposed to the traditional ventilationmethod.

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Received February 15, 2011; revised July 1, 2011; accepted October 17, 2011





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CO2-Based Demand-Controlled Ventilation and Its Implications for Interior Design