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Demand-Controlled VentilationLeading by example,aving energy andaxpayer dollars Using CO2 Sensorsn federal facilities

Preventing energy losses from over-ventilation while maintaining indoor air quality

Executive Summary

Demand-controlled ventilation (DCV) using carbon dioxide (CO2) sensing is a combination of twotechnologies: CO2 sensors that monitor CO2 levels in the air inside a building, and an air-handlingsystem that uses data from the sensors to regulate the amount of ventilation air admitted. CO2 sensors continually monitor the air in a conditioned space. Given a predictable activity level, such asmight occur in an office, people will exhale CO2 at a predictable level. Thus CO2 production in the

space will very closely track occupancy. Outside CO2 levels are typically at low concentrations ofaround 400 to 450 ppm. Given these two characteristics of CO2, an indoor CO2 measurement can beused to measure and control the amount of outside air at a low CO2 concentration that is being introduced to dilute the CO2 generated by building occupants. The result is that ventilation rates can bemeasured and controlled to a specific cfm/person based on actual occupancy. This is in contrast to thetraditional method of ventilating at a fixed rate regardless of occupancy.

Building codes require that a minimum amount of fresh air be provided to ensure adequateair quality. To comply, ventilation systems often operate at a fixed rate based on an assumedoccupancy (e.g., 15 cfm per person multiplied by the maximum design occupancy). The resultis there often is much more fresh air coming into buildings than is necessary. That air must beconditioned, resulting in higher energy consumption and costs than is necessary with appropriate ventilation. In humid climates, excess ventilation also can result in uncomfortable humid

ity and mold and mildew growth, making the indoor air quality (IAQ) worse rather than better.A lack of adequate fresh air, on the other hand, can make building occupants drowsy anduncomfortable. To avoid the problems of too much or too little fresh air, the heating, ventilation, and air-conditioning (HVAC) system can use DCV to tailor the amount of ventilationair to the occupancy level. CO2 sensors have emerged as the primary technology for monitoring occupancy and implementing DCV. Energy savings come from controlling ventilationbased on actual occupancy versus whatever the original design assumed.

Handheld CO sensor with2

ata logging. Application Domain

CO2 sensors have been available for about 12 years. An estimated 60,000 CO2 sensors are sold annually for ventilation control in buildings, and the market is growing. There is a potential for millionsof sensors to be used, since any building that has fresh air ventilation requirements might potentially

benefit from the technology.CO2-based DCV has the most energy savings potential in buildings where occupancy fluctuates duringa 24-hour period, is unpredictable, and peaks at a high levelfor example, office buildings, government facilities, retail stores and shopping malls, movie theaters, auditoriums, schools, entertainmentclubs and nightclubs.

CO2 sensors are considered a mature technology and are offered by all major HVAC equipment and controls companies. The technology is recognized in ASHRAE Standard 62, the International Mechanical

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Code (which establishes minimum regulations for mechanical systems), andsome state and local building codes.Although the first CO2 sensors sold wereexpensive, unreliable, and difficult tokeep calibrated, manufacturers say they

have largely resolved those problems.The unit cost of sensors has droppedfrom $400 to $500 a few years ago to$200 to $250 (not including installation). As market penetration increases,prices are expected to fall further.

Several manufacturers produce CO2 sensors for DCV. Most manufacturers ofthermostats and economizers are integrating CO2 sensors into their products,and major manufacturers of packagedrooftop HVAC systems offer factory-installed CO

2

sensors as an option.

Benefits

DCV saves energy by avoiding the heating, cooling, and dehumidification ofmore ventilation air than is needed. CO2sensors are the most widely acceptedtechnology currently available for implementing DCV. Additional benefits ofCO2-based DCV include

Improved IAQBy increasing ventilation if CO2 levels rise to an un

acceptable level, Improved humidity controlIn

humid climates, DCV can preventunnecessary influxes of humidoutdoor air that makes occupantsuncomfortable and encouragesmold and mildew growth.

Estimated Savings

The potential of CO2-based DCVfor operational energy savings has

been estimated in the literature atfrom $0.05 to more than $1 per squarefoot annually. The highest paybackcan be expected in high-density spacesin which occupancy is variable andunpredictable (e.g., auditoriums, some

school buildings, meeting areas, andretail establishments), in locations withhigh heating and/or cooling demand,and in areas with high utility rates.

Design Considerations

CO2 sensing is a fairly simple technology, and installation of the sensors themselves is not complicated. IncludingCO2-based DCV in a new HVACinstallation should not add significantlyto the difficulty of commissioning thesystem. However, retrofitting an existingsystem for DCV may be more problematic, particularly for an older system withpneumatic controls. Applying a CO2based DCV strategy using ASHRAE 62is more complicated than simply installing CO2 sensors and using them to control dampers. In variable-air-volumesystems, particularly, fairly complexcalculations and control algorithmsmay be necessary to program the control system properly for DCV. The useof a more complex control algorithm

often provides increased savings andimproved IAQ; while it increases thelevel of commissioning, the results outweigh the extra initial time and expense.

Maintenance Impact

Maintenance of the sensors themselvesis not generally reported to be a problem. Manufacturers offer sensors thatrecalibrate themselves automaticallyand that are guaranteed not to need

Disclaimer

calibration for up to 5 years. However,it is recommended that calibrationbe checked periodically by comparingsensor readings during a several-hourperiod when the building is unoccupied with readings from the out

door air. Many sensor models areable to sense calibration problemsand alert maintenance personnel ifthey are malfunctioning.

Costs

Costs for sensors have dropped byabout 50% over the last several years.Sensors typically cost about $250 to$260 each, uninstalled. For a newsystem, the installed cost will generally be about $600 to $700 per zone.For a retrofit system, the cost willdepend on what type of control system the building has. A controls contractor estimates installed costs forretrofit applications at from $700 to$900 per zone for systems with anexisting DDC programmable controller and from $900 to $1200 per zonefor systems with pneumatic, electronic, or application-specific DDCs.Installation costs for wireless systemsare minimal beyond the cost of theactual sensor and gateway that can

serve multiple sensor units.In addition to the installation of thesensors, other components such asvariable frequency drives and controlinput and output hardware often areneeded to control the whole building,incrementally increasing the overallinstalled project cost beyond just thesensor installation cost.

This report was sponsored by the United States Department of Energy, Energy Efficiency and Renewable Energy,

Federal Energy Management Program. Neither the United States Government nor any agency or contractor thereof,

nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for

the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents

that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or

service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorse

ment, recommendation, or favoring by the United States Government or any agency or contractor thereof. The views

and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or

any agency or contractor thereof.

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Contents

Abstract ............................................................................................................................................. 3

About the Technology ....................................................................................................................... 3

Application Domain

Energy-Saving Mechanism

Other Benefits

Variations

Installation

Federal-Sector Potential ..................................................................................................................... 7

Estimated Savingsand Market Potential

Laboratory Perspective

Application ....................................................................................................................................... 8

Application Screening

Where to ApplyWhat to Avoid


Maintenance Impact

Equipment Warranties

Codes and Standards

Costs

Utility Incentives and Support

Technology Performance ................................................................................................................. 12

Field Experience

Energy SavingsMaintenance Impact

Case Study ...................................................................................................................................... 13

Facility Description

Existing Technology Description

New Technology Equipment Selection

Building Upgrade Assessment

Life-Cycle Cost

Post-Implementation Experience

The Technology in Perspective ........................................................................................................ 19

The Technologys Development

Technology Outlook

Manufacturers ................................................................................................................................. 20

Who is Using the Technology.......................................................................................................... 20

For Further Information .................................................................................................................. 21

Appendix A Federal Life-Cycle Costing Proceduresand the BLCC Software.............................................................................................. 23

FEDERAL ENERGY MANAGEMENT PROGRAM

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2 FEDERAL ENERGY MANAGEMENT PROGRAM

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Abstract

Demand-controlled ventilation (DCV)using carbon dioxide (CO2) sensorscombines two technologies: advancedgas sensing and an air-handling systemthat uses data from the sensors to regulate ventilation. CO2 sensors continually monitor the air in a conditionedspace. Since people exhale CO2, thedifference between the indoor CO2 concentration and the level outside thebuilding indicates the occupancy and/or activity level in a space and thus itsventilation requirements. The sensorssend CO2 readings to the ventilationcontrols, which automatically increaseventilation when CO2 concentrationsin a zone rise above a specified level.

Either too little or too much fresh airin a building can be a problem. Overventilation results in higher energyusage and costs than are necessary withappropriate ventilation while potentiallyincreasing IAQ problems in warm,humid climates. Inadequate ventilation leads to poor air quality that cancause occupant discomfort and healthproblems. To ensure adequate air quality in buildings, the American Societyof Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)

recommended a ventilation rate of1520 cfm per person in ASHRAEStandard 62-1999. To meet the standard, many ventilation systems aredesigned to admit air at the maximumlevel whenever a building is occupied,as if every area were always at full occupancy. The result, in many cases, hasbeen buildings that are highly over-ventilated. The development of CO2based DVC was driven in part bythe need to satisfy ASHRAE 62 with

out overventilating.Non-dispersive infrared CO2 sensors arethe type most widely used. All majormakers of heating, ventilation, and air-conditioning (HVAC) equipment andHVAC controls offer the sensors, eitheras separate units or as a part of packagedHVAC systems. Although earlier sensorswere plagued by reliability and calibration problems, those issues seem to havebeen largely resolved in newer models.

Typically, a CO2 sensor is installed onthe wall like a thermostat. Newer products entering the market combine thermostats and CO2 and relative humiditysensors, the three primary indicatorsof human comfort, in one unit. Sensors

may be installed inside ductwork ratherthan wall-mounted, but in-duct installation is not recommended for all applications. In a conventional wired CO2sensor system, wires are run from thesensors to the HVAC controls or tothe damper actuators. With wirelessCO2 sensors, the data are transmittedto the building automation system viaa wireless gateway for use in the control algorithm. Properly functioningmodulating dampers are necessary.Pneumatic controls may need to be

replaced with electronic or direct digital controls. In a retrofit installation,dampers may need to be repaired orupgraded to work with the sensors.

In all applications of CO2-based DCV,a minimum base ventilation rate mustbe provided at all times the building isoccupied. A higher rate may be neededfor buildings in which building materials, contents, or processes releasechemicals into the air. DCV should beused only in areas where human activ

ity is the main reason for ventilating thespace. Industrial or laboratory spacesare unsuitable for CO2-based ventilation control.

The potential of CO2-based DCV forenergy savings is estimated at from$0.05 to more than $1 per square footannually. The highest payback can beexpected in high-density spaces in whichoccupancy is variable and unpredictable(e.g., auditoriums, some school buildings, meeting areas, and retail establishments), in locations with highheating and/or cooling demand, andin areas with high utility rates. Casestudies show DCV offers greater savingsfor heating than for cooling. In areaswhere peak power demand and peakprices are an issue, DCV can be usedto control loads in response to real-time prices. In those locations, DCVmay enable significant cost savingseven with little or no energy savings.

CO2-based DCV does not interfere witheconomizers or other systems that introduce outdoor air into a building forcooling. Economizer operation overrides DCV when conditions warranteconomizer use. Buildings that use

evaporative cooling may not benefitfrom DCV during the cooling season.

Costs for sensors have dropped by about50% over the last several years as thetechnology has matured and becomemore widely used. Sensors typically costabout $250 to $260 each, uninstalled.For a new system, the installed costwill generally be about $600 to $700per zone. For a retrofit system, the costwill depend on what type of controlsystem the building has and the degreeof difficulty of installing signal andpower wiring for a wired system. Acomplete wireless sensor system canbe deployed quickly and without additional cost as such systems are batterypowered. Given the advances in battery technology and microprocessor-controlled power management, sensorscan be expected to operate for 23 yearsbefore they require a battery change.

About the Technology

Demand-controlled ventilation (DCV)using carbon dioxide (CO2) sensing isa combination of two technologies:CO2 sensors that monitor the levels ofCO2 in the air inside a building, andan air-handling system that uses datafrom the sensors to regulate the amountof outside air admitted for ventilation.DCV operates on the premise thatbasing the amount of ventilation airon the fluctuating needs of buildingoccupants, rather than on a pre-set,fixed formula, will save energy and at

the same time help maintain indoor airquality (IAQ) at healthy levels.

CO2 sensors continually monitor theair in a conditioned space. Becausepeople constantly exhale CO2, the difference between the indoor CO2 concentration and the leve l ou tside thebuilding indicates the occupancy and/or activity level in a space and thus itsventilation requirements. (An indoor/


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Figure 1. The relationship between CO2 and ventilationrates, assuming office-type activity.

consumption and costs thanwould be necessary withappropriate ventilation. Inaddition, in humid climates,an overload of fresh air canresult in uncomfortable

humidity and mold andmildew growth, makingthe indoor air qualityworse rather than better.

A lack of adequate freshair, on the other hand, canmake building occupantsdrowsy and uncomfortableas occupancy-related contaminants accumulate.A CO2 level of around1100 ppm (a differentialof 700 ppm, assuming the

outdoor CO2 differential of 700 ppmis usually assumed to indicate a ventilation rate of 15 cfm/person; a differential of 500 ppm, a 20 cfm/personventilation rate, etc.) The sensors sendCO2 readings to the air handling system, which automatically increasesventilation when CO2 concentrationsin a zone rise above a specified level.

Building codes specify that a minimumamount of fresh air be brought into a

building to provide for adequate airquality. The American Society of Heating, Refrigerating and Air-ConditioningEngineers (ASHRAE) recommends aventilation rate of 15-20 cfm per personin ASHRAE Standard 62.1 To complywith codes, building ventilation systemsoften operate at constant or predetermined rates regardless of the occupancylevel of the building. Many systems ventilate buildings at the maximum levelall the time, as if every area were alwaysfully occupied. Others are programmedto accommodate expected occupancy,varying the ventilation rate by time ofday but without regard to the actualoccupancy level. The result often ismore ventilation air coming into buildings than is necessary. That air mustbe heated, cooled, or dehumidified/humidified, resulting in higher energy

outdoor air CO2 level isaround 400 ppm) indicates that theventilation rate has dropped belowacceptable levels and that contaminants in the air are increasing.

To avoid the problems of too much ortoo little fresh air, a heating, ventilation, and air-conditioning (HVAC)system can employ DCV to adjust theamount of ventilation air supplied toan indoor space according to the occupancy level. CO2 sensors have emerged

as the primary technology for monitoring occupancy and implementing DCV.

Application Domain

CO2 sensors have been available forabout 12 years. An estimated 60,000CO2 sensors are sold annually for ventilation control in buildings, and a manufacturer estimates the market is growingat about 40 to 60% per year. There isa potential for millions of sensors to beused, since any building that has freshair ventilation requirements couldpotentially benefit from this refinedcontrol technology.

DCV using CO2 sensors has the mostenergy-saving potential in buildingswhere occupancy fluctuates during a24-hour period, is unpredictable, andpeaks at a high level, for example, office

buildings, government facilities, retailstores and shopping malls, movie theatres, auditoriums, schools, entertainment clubs and nightclubs. In buildingswith more stable occupancy levels, DCVcan ensure that the target ventilation

rate per person is being provided at alltimes. DCV is more likely to reduceenergy costs in areas with high utilityrates or climate extremes.

CO2-based DCV can operate in conjunction with economizers or othersystems that introduce outdoor airinto a building for heating or cooling.However, energy savings may be lesswhere economizers are in use, depending on climate, occupancy schedule,and building type.

Buildings that use evaporative coolingmay not benefit from DCV during thecooling season, and those that use heatexchangers to transfer heat betweenincoming and outgoing air may notrealize significant energy savings.

In the next revision of its building code,California will begin requiring CO2based DCV in all buildings housing25 people or more per 1000 ft2. A proposed change in the Oregon buildingcode would require DCV for HVACsystems with ventilation air requirements of at least 1500 cfm, servingareas with an occupant factor of 20 orless. ASHRAE 90 requires CO2 sensorsfor DCV in high-density applications.The U.S. Green Building Code givespoints in its Leadership in Energy andEnvironmental Design (LEED) ratingsystem for use of CO2-based ventilationcontrol in buildings.

CO2 sensors are considered a maturetechnology and are offered by all majorHVAC equipment and controls compa

nies. The technology is recognized inASHRAE Standard 62, the InternationalMechanical Code (which establishesminimum regulations for mechanicalsystems), and some state and localbuilding codes. The first CO2 sensorssold were expensive, unreliable, and

1An ASHRAE standard is usually designated by a standard number and the year in which it was last revised. Standard 62 is under continuous maintenance and is constantly being revised.


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difficult to keep calibrated accurately.However, manufacturers say they havelargely resolved those problems-theunits currently available generally areself-calibrating and similar to thermostats in price and reliability. A few years

ago, sensors cost around $400 to $500each; the cost now is usually about$200 to $250 per sensor (not includinginstallation). As market penetrationincreases, sensor prices are expectedto fall further.

Several manufacturers produce CO2 sensors for use in DCV. Most manufacturersof thermostats and economizers are integrating CO2 sensors into their products,and major manufacturers of packagedrooftop HVAC systems offer factory-installed CO2 sensors as an option.

CO2-based DCV is suitable only whenthere is a means of automatically adjusting the ventilation air supply (e.g.,variable-speed fans or some variabledamper arrangement). If this control isnot presently available, the savings fromDCV may justify the modifications toaccommodate this degree of control.

CO2 sensors designed for DCV aresuitable only for controlling occupant-related ventilation. DCV does noteliminate the need for a base rate ofventilation to prevent degradation ofair quality from contaminant sourcesunrelated to building occupancy, suchas emissions from building materials.Thus CO2-based DCV may not beappropriateor may require higher target ventilation settings-in new buildingsor others where there are contaminantsnot related to human occupancy, as itmay not provide sufficient fresh air todilute those contaminants. The CO2sensors used for DCV are not appro

priate to monitor CO2 for medical orindustrial purposes that demand precise air quality control.

DCV should be used only in areas wherehuman activity is the main reason forventilating the space. Industrial orlaboratory spaces that are subject toindoor air quality (IAQ) degradationfrom a wide variety of sources are unsuitable for CO2-based ventilation control.

Energy-Saving Mechanism

The energy savings from CO2 sensorsfor DCV result from the avoidance ofheating, cooling, and dehumidifyingfresh air in excess of what is neededto provide recommended ventilation

rates. Many HVAC systems ventilateat a constant fixed level, usually thelevel prescribed for full occupancy, andthus provide more fresh air per occupant than the designed ventilation ratemuch of the time. Moreover, systemsusing fixed ventilation rates cannotaccurately account for unanticipatedair infiltration into a building (e.g., fromleakage or opened windows) and adjustthe fresh air intake accordingly.

DCV based on CO2 sensing allows real-

time control of the ventilation levelsaccording to building occupancy. If abuilding is only 50% full, then only50% of the design-rate ventilation air,not 100%, is pulled in. CO2 sensors arethe most widely accepted technologycurrently available for implementingDCV. They do this by increasing theventilation rate whenever the CO2 levelin a space reaches a predetermined levelthat represents a differential betweenthe indoor and outdoor CO2 levels. Theoutdoor CO2 level is slightly dependent

on local conditions and elevation, butcan generally be assumed to be around400 ppm. An indoor level of 1100 ppmof CO2 thus represents a 700-ppm differential and indicates a ventilation rateof 15 cfm per person in the occupiedspace. A differential of 500 ppm in thesame space would indicate a ventilationrate of 20 cfm per person.

The technology most often used in CO2sensors is non-dispersive infrared spectroscopy. It is based on the principle

that every gas absorbs light at specificwavelengths. Carbon dioxide sensorscalculate CO2 concentrations bymeasuring the absorption of infraredlight (at a wavelength of 4.26 microns)by CO2 molecules. The sensor apparatus incorporates a source of infraredradiation, a detector, and electronics todetect the absorption. Air from the areabeing monitored diffuses into a chamber

Figure 2. Typical non-dispersive infraredspectroscopic CO2 sensor.

that has a light source at one end and alight detector at the other. Selectiveoptical fibers mounted over the lightdetector permit only light at the 4.26micron wavelength absorbed by CO2to pass through to the detector. AsCO2 levels rise, more infrared light isabsorbed and less light is detectable.

Photo-acoustic CO2 sensors also are

available. In these sensors, also, airdiffuses into a chamber in the sensorsand is exposed to light at the wavelength absorbed by CO2. As the CO2molecules absorb light energy, theyheat the air chamber and causes pressure pulses. A piezo-resistor senses thepulses and transmits data to a processor that calculates the CO2 level.

Electrochemical sensors measure thecurrent transmitted across a gap filledwith an electrochemical solution. CO2decreases the pH of the solution, free

ing conductive metal ions. A weakelectrical current can then flow acrossthe gap; the current signals an increasein the CO2 level.

Mixed-gas sensors also can detect CO2along with other gases in the air, butthey have not proved to be effectivefor DCV because they do not measureCO2 specifically and cannot be tied to


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ventilation rates as explicitly as a CO2sensor can.

Sensors generally are either wall-mounted in the space to be monitoredor mounted inside the duct system.(Wall-mounted sensors usually are rec

ommended because duct-mounted unitsprovide data on the average CO2 concentration in multiple spaces rather thanon the CO2 levels in individual areas.)

The sensors monitor CO2 concentrations continually and send data to thesystem for controlling the ventilationequipment. Various types of control systems can be used to incorporate CO2sensing: simple setpoint control thatactivates a fan or damper when CO2levels exceed a setpoint; proportional

control, in which sensor data adjustventilation air volume through a rangeof levels; proportional-integral control,in which fresh air intake is controllednot only by the CO2 level but also bythe rate at which the level is changing;and two-stage controls for zone-basedsystems in which both temperature sensors and CO2 sensors control ventilation.

Other Benefits

Potential secondary benefits of CO2based DCV include:

Improved IAQ: By increasing thesupply of fresh air to the buildingif CO2 levels rise to an unacceptablelevel, the technology could preventunder-ventilation that results inpoor air quality and stuffy rooms.

Improved humidity control: In humidclimates, DCV can prevent unnecessary influxes of humid outdoor airthat causes occupants to be uncomfortable and encourages the growthof mold and mildew.

Records of air quality data: Sensorreadings can be logged to provide areliable record of proper ventilationin a building. Such records can beuseful in protecting building ownersagainst ventilation-related illness ordamage claims.

Reduced operational running timesfor the major HVAC equipment:Improving the ability to conditionthe building could delay start-timesof the HVAC equipment during

morning pre-conditioning periodsby as much a s several hours on aMonday morning in humid climates,resulting in incremental energy andcost savings.

Variations

DCV can be implemented using methods other than CO2 sensing to indicateoccupancy levels or air quality conditions. For example, humidity sensors,motion detectors, particle counters,volatile organic contaminant sensors,

and mixed-gas sensors can be used toregulate DCV. Time-controlled ventilation (e.g., using programmed timeclocks) is also an option for buildingsthat are occupied only during certaintimes, for example, office buildings

and schools. Based on a review of theliterature, it appears that CO2 sensorsare becoming the industry standard fortypical DCV applications.

CO2 sensors are of three main types:infrared, electrochemical, and photo-

acoustic. Based on the literature, infrared sensors appear to be the type mostcommonly used for DCV applications.

Combination sensing units are available that package wall-mounted CO2sensors with thermostats or withhumidity sensors.

There are numerous variations in thetypes of HVAC systems and controlsystems with which CO2-based DCVis implemented. CO2-based DCVcapability can be added to an existing

system by installing sensors and connecting them with the air handlingsystems. Increasingly, new HVAC systems are being factory-equipped withinput and controls strategies to acceptCO2-based DCV.

Installation

Typically, a CO2 sensor is installedon the wall like a thermostat. Newerproducts entering the market combine thermostats and CO2 and relative

humidity sensors, the three primaryindicators of human comfort, in oneunit. Sensors may be installed insideductwork instead of on a wall, butin-duct installation is not recommendedfor applications where the sensor would

All of these types of systems can be modified to accomplish a DCV strategy

Control Type Minimum Modification Recommended Modification

All Apply ASHRAE Standard 62-1999 with CO2sensors provided in zones with high critical

outside air (OA) calculations

Provide CO2 sensors in all zones, using thehighest zone to increase the fresh air provided

by the associated air-handling unit (AHU)Pneumatic Provide electronic-to-pneumatic transducer

to limit the OA damper positionUpgrade the AHU to DDCprogrammable control

Electronic Provide electronic device to limit the OAdamper position

Upgrade the AHU to DDCprogrammable control

DDCapplication specific Provide electronic device to limit the OAdamper position

Upgrade the AHU to DDCprogrammable control

DDC-programmable Modify the control program to providenew DCV strategy

Same as minimum


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Figure 3. CO2 equilibrium levels at various ventilation rates.

average readings from several differentareas. With hardwired sensors (thoserequiring both power and signal wiring),wires are run from the sensors to thebuildings HVAC control system or tothe actuator that controls the fresh airsupply. Wireless sensors require neitherpower nor signal wiring, and the sensordata are fed into the HVAC controlsvia a gateway using one of several com

munication protocols.In a building retrofit, dampers may needto be repaired or upgraded to work withthe sensors. Properly functioning modulating dampers are necessary. Pneumaticcontrols will need to be replaced withelectronic or direct digital controls(DDCs); it may also be worthwhileto replace existing electronic controlswith DDCs. Actuator modules thatdo not have input points for the sensors will need to be upgraded.

Part of the preparation for implementing DCV is to monitor the air in theoutdoor areas surrounding the building for at least a week to establish theCO2 concentration in the ventilationair. The differential between the CO2levels of the outdoor and indoor airwill determine the amount of freshair needed to meet ventilation requirements. CO2 concentrations in all zonesinside the conditioned space also should

be monitored for several days to indicate existing ventilation levels in thezones. This monitoring may identifyproblems with air-handling systemsthat need to be corrected before DCVis implemented.

To ensure that the sensors are correctlycalibrated and working properly, ahand-held monitor should be used tocheck CO2 concentrations inside the

buildings regularly for a few days afterthe system begins operating. The readings from the monitor should be compared with the readings the sensors aresending to the ventilation system controls. Sensors that prove to be improperly calibrated should be recalibrated.

In all applications of CO2-based DCV,a minimum ventilation rate should beprovided at all times the building isoccupied. A rate of 2030% of the original design ventilation rate for the space

at maximum occupancy is often recommended as a new baseline ventilationrate. A higher rate may be needed forbuildings in which building materials,contents, or processes release chemicalsinto the air.

Federal-Sector Potential

Federal Technology Alerts target technologies that appear to have significant

untapped federal-sector potential andfor which some installation experienceexists. CO2 sensors are recognized ashaving potential to reduce energyconsumption and costs resulting fromover-ventilation of buildings while

guarding against under-ventilation.Estimated Savingsand Market Potential

CO2-sensors for demand-controlledventilation are recognized as havingpotential to reduce energy consumption, costs, and emissions.

Demand-controlled ventilation has notbeen assessed by the New TechnologyDemonstration activities. There are noknown estimates of the savings poten

tial of CO2-based DCV for the federalbuilding sector. Therefore, this reportcannot adequately quantify the energy-savings potential of the application ofthis technology for the federal sector.

The literature about DCV includesnumerous estimates and models of thesavings potential of DCV, as well asaccounts of monitored savings in specific applications. The predicted andactual savings vary widely dependingon climate, type of HVAC system withwhich DCV is implemented, occupancy

patterns in the space in which is it implemented, and other operating conditions.The capability of the building staff tokeep equipment adequately maintainedand operating properly also may affectsavings significantly.

The potential of CO2-based DCV foroperational energy savings has beenestimated in some of the literature atfrom $0.05 to more than $1 per squarefoot annually. The highest payback canbe expected in high-density spaces in

which occupancy is variable and unpredictable (e.g., auditoriums, some schoolbuildings, meeting areas, and retailestablishments), in locations with highheating and/or cooling demand, and inareas with high utility rates.

A report by Lawrence Berkeley NationalLaboratory cited five case studies in largeoffice buildings with CO2-based DCV,all of which reported energy savings


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that resulted in payback times of from0.4 to 2.2 years. Two of the studies werecomputer simulations. One of those,conducted in 1994, simulated a 10-flooroffice building located in Miami,Atlanta, Washington, D.C., New York,

and Chicago. The simulation predictedlarge gas savings for heating and smallerelectricity savings, resulting in predictedpayback times for the different locations of from 1.4 to 2.2 years.

A 1999 study modeled the impactof DCV and economizer operationon energy use in four building types(office, retail, restaurant, school) inthree locations representing differentclimates: Atlanta; Madison, Wisconsin;and Albuquerque. For cooling, predictedsavings attributed to DCV dependedgreatly on location-savings were largerin Atlanta and Madison because humidity made economizer operation lessbeneficial. In low-humidity Albuquerque, economizer operation was muchmore significant than DCV in reducing cooling energy demand. In all threelocations, DCV resulted in large savingsin heating energy27%, 38%, and42% for the office building in Madison,Albuquerque, and Atlanta, respectively;from 70% in Madison to over 80% in

Atlanta and Albuquerque for the school;and over 90% in all three locations forthe retail and restaurant spaces. Similarresults were obtained for 17 other U.S.locations modeled. In all locations, theoffice building showed the most modest savings.

A recent proposal for a change to theOregon building code to require DCVin some applications cites a DOE2analysis of implementing a DCV strategy in a middle school gymnasium.It predicts energy cost savings of$3700 per year from DCV and asimple payback of about 6 months.

No estimate of market potential forCO2-based DCV in the federal sectorwas available.

Laboratory Perspective

Research on CO2-based DCV at thenational laboratories has been limitedso far. The technology is generally

regarded as a valid operational strategythat offers potential for energy andcost savings and protection of IAQ inmany facilities. A study conducted byLawrence Berkeley National Laboratoryconcludes that sensor-regulated DCV

is generally cost-effective in buildingsin which the measured parameter (e.g.,CO2) is the dominant emission, theoccupancy schedule and levels are varied and unpredictable, and the heating/cooling requirements are large. Monitoring of CO2-based DCV systems isunder way at some national laboratoriesto quantify savings and analyze effectson IAQ.

Some caveats have emerged from laboratory experience. Calibration drift hasbeen observed to be a problem in somesensors. The management of energymanagement systems so that theyadmit ventilation air at an appropriate CO2 level is another. A researcherat Oak Ridge National Laboratorywho works with building projects atfederal facilities noted that a skilled,well-trained building maintenancestaff is essential to proper functioningof the sensors and associated controlsystems. If other HVAC control systems in a building frequently function

improperly, there probably will beproblems with CO2 sensors and DCVcontrols, also. He advises that a testinstallation be tried first to ensure thatthe building staff understand how thedevices work, that the devices functionproperly, and that the staff can handlea larger DCV implementation.

Application

This section addresses technical aspectsof applying the technology. The range

of applications and climates in whichthe technology can be best applied areaddressed. The advantages, limitations,and benefits in each application areenumerated. Design and integrationconcerns for the technology are discussed, including equipment andinstallation costs, installation details,maintenance impacts, and relevantcodes and standards. Utility incentivesand support are also discussed.

Application Screening

DCV based on CO2 sensing offers themost potential energy savings in buildings where occupancy fluctuates during a 24-hour period, is somewhatunpredictable, and peaks at a high

level. It is also more likely to reduceenergy costs in locales that requireheating and cooling for most of theyear and where utility rates are high.

Savings opportunities are the greatest inbuildings with low average occupancylevels compared with the design occupancy levels. Larger savings are likelyin buildings that supply 100% outsideair to conditioned spaces than in buildings that supply a mixture of outsideand recirculated air.

Types of buildings in which CO2-basedDCV is likely to be cost-effective includelarge office buildings; assembly rooms,auditoriums, and lecture halls; largeretail buildings and shopping malls;movie theaters; restaurants, bars andnightclubs; banks; outpatient areas inhospital; and hotel atriums or lobbies.

In primary and secondary schools, whereoccupancy is variable but predictable,time-controlled DCV may be more

Figure 4. Percent of total ventilation capacity percent building occupancy.


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cost-effective. College classroom buildings and large lecture rooms with variable occupancy through the day and theweek are more appropriate candidates.

CO2-based DCV does not interfere witheconomizers or other systems that intro

duce outdoor air into a building forcooling. Economizer operation overrides DCV when conditions warranteconomizer use. The energy and costsavings for such arrangements dependon climate, occupancy schedule, andbuilding type. Warm, humid climates(e.g., the Southeast) offer the mostpotential to save cooling energy withDCV because high humidity reducesopportunities for economizer cooling.In dry climates such as the desert Southwest, economizer operation may savemore cooling energy than DCV. Additional cooling savings from DCV maybe insignificant in such climates, andDCV without economizer coolingmay even result in an energy penalty.

Buildings that use evaporative coolingmay not benefit from DCV during thecooling season.

Case studies generally show greatersavings for heating than for coolingin all climates. Heating savings aregreatest where heating the ventilation air accounts for a large portionof the energy demand. A buildingwith exchangers to transfer heatbetween incoming and exhaust air hasless potential for savings from DCVbecause the heat exchangers reducethe energy penalty of ventilation air.

In areas where peak power demand andpeak prices are an issue, DCV can beused to control loads in response to real-time prices. In those locations, DCVmay enable significant cost savings

even with little or no energy savings.DCV can be implemented only in buildings in which the outside air supply canbe automatically adjusted. If the air-handling system lacks that capability,it must be upgraded before CO2-basedDCV can be implemented. It is recommended that pneumatic controls bereplaced with electronic or digital controls to implement CO2-based DCV.

The cost of installing CO2 sensors forDCV depends on how easily the existing control system can incorporatethem. If the existing system has digitalcontrols and available input points onthe control modules for wired sensors,

costs will be lower and implementation easier. Wireless sensors may bemore easily integrated into an existingcontrol system, as no new input control modules are needed.

Modeling is recommended, if feasible,to estimate the energy savings andcost-effectiveness of DCV in a specific situation.

CO2 from human respiration must bethe dominant pollutant in a space forCO2-based DCV to be appropriate.

Otherwise, it could lead to insufficientventilation. Occupancy-based DCV isinappropriate for spaces with high levels of contaminants unrelated to humanoccupancy, including industrial orlaboratory spaces. If CO2-based DCVis used in a new building or other spacecontaining materials that release irritating emissions (e.g., a clothing orcarpet store), the target ventilationrate must be high enough to ensurethat emissions are adequately diluted.

An HVAC system with CO2

-basedDCV needs to be capable of a dailypre-occupancy (e.g., early morning)purge of inside air to avoid exposingoccupants to emissions (e.g., frombuilding materials) that accumulatewhile ventilation is at a low level.

Although DCV was developed mainlyas an energy-efficiency technology, italso is useful to ensure acceptable IAQ.In a properly operating system, CO2based DCV ensures that the targetventilation rate per person is being

provided at all times. Thus it may beappropriate for spaces where there areIAQ concerns. It may also help controlthe growth of mold and mildew byreducing unnecessarily large influxesof humid outdoor air.

Where to Apply

Buildings where occupancy fluctuates during a 24-hour period, is

somewhat unpredictable, and peaksat a high level.

Locales that require heating andcooling for most of the year.

Areas where utility rates are high.

Areas where peak power demandand prices are high.

Buildings with low average occupancy compared with the designoccupancy.

Large office buildings; assemblyrooms, auditoriums, and lecturehalls; large retail buildings and shopping malls; movie theaters; restaurants, bars and nightclubs; banks;outpatient areas in hospital; andhotel atriums or lobbies.

Warm, humid climates.

Buildings in which heating/cooling/dehumidifying the ventilation airaccounts for a large portion of theenergy demand.

Buildings in which the outside airsupply can be automatically adjusted

Buildings with HVAC systems thathave, or can be upgraded to, electronic or digital controls.

Spaces in which CO2 from humanrespiration is the only or dominantpollutant.

Spaces that do not have high levelsof contaminants unrelated to humanoccupancy (e.g., industrial or laboratory spaces).

Buildings in which poor IAQ resultingfrom under-ventilation or excessivehumidity resulting from under- orover-ventilation is a concern.

Precautions

The following precautions should bekept in mind in considering the useof CO2-based DCV.

Sensors should be placed wherethey will provide readings that areclose to actual conditions in thespaced to be controlled.

Wall-mounted sensors are generallypreferable to duct-mounted sensors.


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Sensors should not be mountedin locations where people willregularly breathe directly on them(e.g., at standing level near the coffee machine).

Dont buy carelesslychoose sensors

with a reputation for performing wellin terms of self-calibration, drift,accuracy, and reliability.

A competent, well-trained maintenance staff is important to a successful implementation.

In a retrofit, it is important to makea thorough study of the HVAC system and troubleshoot for existingventilation problems before installing a DCV system.

A new base ventilation rate should beprovided at any time the building isoccupied. An often-cited guideline is20 to 30% of the original design rate.

Applications in moderate climates,especially where economizer operationcontributes substantially to cooling,may show little energy/cost savings.

Spaces where there are high levels ofcontaminants not related to occupancy may demand a higher ventilation rate than would be provided

by DCV based solely on CO2 levels.

During the first year or so in a newbuilding, it is advisable to maintaina higher ventilation rate than CO2sensing would indicate to ensureproper dilution of emissions fromnew materials.


CO2 sensing is a fairly simple technology, and installation of the sensorsthemselves is not complicated. Sensorvoltage, power, and control outputrequirements are similar to those usedcommonly by thermostats. For wiredsensor installations, the type of wire usedfor the signal wiring is often critical;some controls manufacturers specifythe wire gauge, type, and shielding/grounding requirements to prevent

signal irregularities and errors. Withwireless sensors, these considerationsare not a concern, as data are transmitted over a Federal CommunicationsCommission-approved frequency. Thewireless sensors are self-powered anduse on-board power management toalert the building operator when thebattery needs to be changed.

Most HVAC equipment suppliers arenow offering systems designed to accommodate DCV and accept readings from

CO2 sensors, so including CO2-based

DCV in a new HVAC installationshould not add significantly to the difficulty of getting the system into operation. However, retrofitting an existingsystem to work with DCV may be moreproblematic, particularly for an older

system with pneumatic controls.The sensors typically are mounted onwalls like thermostats. Some manufacturers offer thermostat/sensor combinations. Units that monitor temperature,CO2, and humidity are also available;they are useful with systems that includedesiccant dehumidification to controlthe humidity load in ventilation air.

Data from the sensors are fed to thebuildings HVAC control system or toan actuator that controls the amount of

ventilation air that is admitted. In aretrofit installation, it may be necessaryto repair or upgrade dampers so theywill work in a more dynamic, modulating fashion in response to the sensors.Properly functioning dampers that canbe automatical ly controlled are essential. Pneumatic controls will need tobe replaced with electronic controls orDDCs; it may also be worthwhile toreplace existing electronic controls withDDCs. Actuator modules that do nothave input points for the sensors will

need to be upgraded to add input points.

Air handler unit and variable-air-volume controls are used for communication between the sensors and the air-handling system.

Control Type Control Medium Sequence

Pneumatic Air pressure from 3 to 15 psi Hard-piped: changes require additionalhardware and physical modifications tothe control air tubing

Electronic Electronic signals from 2 to 10 V DC, 0 to10 V DC, 4 to 20 mA are the most common.Other signals are 3 to 9 V DC, 6 to 9 V DC,0 to 20 V phase cut

Hard-wired: changes require additionalhardware and physical modifications tothe control wiring

Direct digital control (DDC)application-specific Electronic signals from 2 to 10 V DC, 0 to10 V DC, 4 to 20 mA are the most common.Other signals are 3 to 9 V DC, 6 to 9 V DC,0 to 20 V phase cut, and via wireless transmission to multi-protocol enabled gateways

Programmed into an electronic controllerthat can be configured: some systems mayneed additional hardware to accomplishdesired sequences

DDCprogrammable Electronic signals from 2 to 10 V DC, 0 to10 V DC, 4 to 20 mA are the most common.Other signals are 3 to 9 V DC, 6 to 9 V DC,0 to 20 V phase cut and via wireless transmission to multi-protocol enabled gateways

Programmed into an electronic controllerthat can be reprogrammed to accomplishdesired sequences


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The application of a CO2-based DCVstrategy using ASHRAE 62 is morecomplicated than simply installing CO2sensors and using them to controldampers. In variable-air-volume systems, particularly, fairly complex cal

culations and control algorithms maybe necessary to program the controlsystem properly for DCV.

DCV is compatible with economizersor other systems that can bring outdoor air into a building for cooling.Economizer operation overrides DCVwhen conditions favor economizeruse. DCV used in conjunction withan enthalpy economizer (one regulated by relative humidity) probablywill save more energy than DCV witha temperature-regulated economizer.

Heat exchangers that transfer heatbetween supply air and exhaust airreduce the energy demand caused bylarge inflows of ventilation air; therefore, DCV may not reduce energyuse in buildings with heat exchangers.Evaporative cooling systems may notbenefit from DCV during the cooling season.

Maintenance Impact

Maintenance of the sensors them

selves is not reported to be a problem.Although earlier sensor models hadreliability problems, the literaturegenerally reports that newer modelstypically are reliable and accurate.Manufacturers offer sensors thatrecalibrate themselves automaticallyand that are guaranteed not to needcalibration for up to 5 years. However,it is recommended that calibration bechecked periodically by comparing sensor readings during a several-hour periodwhen the building is unoccupied withreadings from the outdoor air. Manysensor models are able to sense calibration problems and alert maintenancepersonnel if they are malfunctioning.

Some users report problems in getting the sensors calibrated initially. Ahand-held monitor should be used toensure that the installed sensors aremeasuring CO2 concentrations in theirzones accurately.

CO2-based DCV is a more sophisticatedtechnology than many maintenance personnel are accustomed to, a buildingresearcher noted. A facility with a well-trained staff who know how to maintainand troubleshoot controls is the best

candidate for a CO2-based DCV system.Equipment Warranties

The prospective user should ask potential suppliers, contractors, and installersabout warranties for specific equipmentmodels. Warranties for CO2 sensorsvary widely among different manufacturers. Those reviewed offer warrantyperiods for parts and labor rangingfrom 90 days from the date of shipmentto 5 years from the date of purchase.A warranty period of 12 to 18 months

for parts and labor appears to be fairlycommon. Some sensors are guaranteedto remain calibrated for at least 5 years,and at least one manufacturer offers alifetime calibration guarantee.

Codes and Standards

The use of CO2-based DCV is acceptedin both ASHRAE 62 and the International Mechanical Code (IMC). TheIMC is referenced by the BuildingOfficials and Code Administrators

International, the Southern BuildingCode Congress International, and theInternational Code Conference ofBuilding Officials, which togetherestablish the model code languageused in local and state building codesthrough the United States. The commentary on IMC Section 403.1 states,The intent of this section is to allowthe rate of ventilation to modulate inproportion to the number of occupants.CO2 detectors can be used to sense thelevel of CO2 concentrations which are

indicative of the number of occupants...and this knowledge can be used to estimate the occupant load in a space.

ASHRAE 62 recommends DCV for allventilation systems with design outsideair capacities of greater than 3000 ft3

per inch serving areas with an averagedesign occupancy density of more than100 people per 1000 ft2.

The California Building Standards Codewas amended in June 2001 to requireCO2-based DCV in some high-densityapplications during periods of partialoccupancy. A proposed change to theOregon Building Code (NR-HVAC-7)

would require HVAC systems to includeprovision for DCV during periods whenspaces are only partially occupied.

Costs

Costs for sensors have dropped by about50% over the last several years as thetechnology has matured and becomemore widely used. Sensors typically costabout $250 to $260 each, uninstalled.For a new system, the installed cost willgenerally be about $600 to $700 perzone. For a retrofit system, the cost

will depend on what type of controlsystem the building has. A controlscontractor estimates installed costs forretrofit applications at from $700 to$900 per zone for systems with anexisting DDC programmable controller and from $900 to $1200 per zonefor systems with pneumatic, electronicor application-specific DDCs. Installation costs for wireless systems are minimal beyond the cost of the actual sensorand gateway that can serve multiplesensor units.

In addition to the installation of thesensors, other components such asvariable frequency drives, controlinput and output hardware are oftenneeded to control the whole building,incrementally increasing the overallinstalled project cost beyond just thesensor cost.

Utility Incentives and Support

No specific information was foundregarding utility incentives that are in

place for CO2-based DCV. Some states(e.g., California) are considering suchan incentive program. Most electricand natural gas utility companies haverebate programs to promote energy-efficiency technologies that resultin overall improvement in buildingperformance, and DCV systems wouldquality for incentives under some ofthose programs.

FEDERAL ENERGY MANAGEMENT PROGRAM 1

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DCV is one of the technologies thatutilities might consider for financialincentives in working with federal customers through a utility energy servicecontract on energy-efficiency projectsat federal sites.

Technology Performance

Field Experience

Three building operators whose facilities have installed CO2 sensors forDCV and a controls engineer whosecompany oversees HVAC installationswere contacted for this report regarding their experience with the use ofCO2-based DCV. All were generallypleased with the performance theyhave observed, although one reportedproblems with getting the sensors properly calibrated and wired.

Purdue University has installed CO2sensors in 12 large auditoriums andlecture halls (100 to 500 seats) to addressboth air quality issues and energy costs.The sensors were added to existing airhandling systems, and modulating outside damper actuators were added wherethey were not already in place. The existing air-handling units were 15 to 50 yearsold. Some of the rooms had pneumatic

dampers with electronic controls; thosecontrols were not replaced unless theywere in bad condition. The large auditoriums had DDC systems. All theexisting control modules had enoughavailable inputs to add the sensors.

Purdues controls systems engineerreported that there were minor problems with retrofitting the older air-handling systems, but they were thesame kinds of problems that wouldhave surfaced with a conventional

HVAC retrofit-a lack of original orupdated building plans and plans thatdid not match the existing systems.We had to do some legwork to verifythe existing configurations. It wouldbe easier with new systems, she said.

Purdue has found the equipmentinstalled to be reliable, and it has performed as expected. There has been

a negligible impact on the maintenance staff because the sensors installedrecalibrate themselves automatically.Purdue had previously installed sensorsfrom another manufacturer that provedunsatisfactory because they could not

be calibrated properly or could notmaintain their calibration, the controlsengineer said. The calibration issueis very important in sensor selection,she said. The maintenance for theother equipment installed has beencomparable to maintenance on thesystems replaced.

Once the sensors began operating,they revealed that some of the lecturerooms had been underventilated bythe old systems. The sensors workedwell in resolving IAQ issues. Purduehas not monitored energy use sincethe sensors and new dampers wereinstalled, but trended data have shownthat the ventilation dampers modulateto lower or minimal positions when thelecture rooms are unoccupied or partlyoccupied and on weekends. Before thesensors were installed, the dampers wereopen to ventilate for full occupancy during the occupied cycle time (6 A.M.to 10 P.M. weekdays and 8 A.M. to4 P.M. Saturdays).

Purdue continues to install CO2-basedDCV in new applications in large roomswith variable occupancy. It will be addedin a renovation of the air handling system in a 900-person-capacity ballroomof the Purdue Memorial Union and isbeing reviewed for application in dining halls on campus.

A CO2 sensor was installed in an officebuilding on the Beaufort Marine CorpsAir Station in South Carolina to regulate the makeup air system. It is the

first of several DCV retrofits plannedon the base as part of an energy servicesperformance contract and a MarineCorps-funded controls system upgradefor the station. The air station is locatedin the low country of South Carolina,a particularly humid climate with aheavy cooling load.

The office building, a one-story41,354 ft2 facility with no windows,contains offices and a flight simulator.It was designed for 500 people, andthe ventilation system supplied enoughair to meet ASHRAE standards for 500,

but the occupancy is rarely over 100,said Neil Tisdale, utilities director forthe base. Thats a lot of air to be conditioning for no reason. The sensorwas added as part of a makeup-airsystem retrofit. The system alreadyhad modulating dampers and DDCs.Adding CO2-based DCV was fairlysimpleinstalling a sensor and modifying the control program to regulatethe damper with input from the sensor. The CO2 sensor was placed in thereturn air path.

The system has been operating for ayear, Tisdale said, and he is not awareof any maintenance problems or malfunctions. He ventured a rough estimate that the DCV system reduces thecooling load for the building by 20 tons,saving roughly of 12 megawatt hoursof power annually. The load for thebuildings chiller, sized to provide coolair for 500 people, was reduced somuch that part of the chilled waterwas diverted and used to cool an adja

cent 11,000 ft2

building.A large hospital in Houston, Texas, hasinstalled CO2 sensors in its auditoriumsto ensure good IAQ and control energycosts, according to the hospitals manager of energy services. The sensorswere added to existing systems, all ofwhich had digital controls; integratingthe sensors was not difficult, he said.The sensor input is not yet being usedto control the dampers automatically;building staff are monitoring the sensors to see whether they will control airchanges properly before switching toautomatic control. Were going to takeit a step at a time and expand gradually, the energy services manager said.The switch to direct control of dampers by the sensors will take minutesto implement and will probably occurduring the coming year, he said.


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When the hospital begin installing thesensors, it was discovered that about halfof them were not wired properly andneeded to be rewired, he said. Manyof the units were calibrated incorrectlyand had to be recalibrated using a hand

held meter. Some of them were showing 4000 to 5000 ppm of CO2. Youcant just set them up and assume theyllwork properly, he cautioned.

However, he expects the sensors andDCV to perform well now that theinitial problems have been addressed.They may be added in other areas suchas busy foyers and other gathering areasonce the conference room installationis in full operation.

The owner of a digital controls company

who oversees HVAC installations sayshis companys clients who have installedCO2-based DCV systems have beengenerally pleased with their performanceand found DCV to be a valuable addition. A new HVAC installation thatincorporates CO2 sensors and DCV is ofminimal cost and difficulty and requiresonly a few additional calculations to setup the air handling program properly,he said. He cautions that sensors shouldbe installed to cover every CO2 zone inan area because high occupancy, and a

corresponding high CO2 level, in onezone may not be reflected in the sensorreadings from the adjacent zone.

Although his firm has not done formalstudies of savings from DCV, it has seena reduction in outside air requirementsin all DCV installations because noneof the facilities are at the design occupancy all the time. The largest energysavings are likely in buildings designedto meet ASHRAE 62 requirements,he said, because they are likely to be

admitting more unneeded outside air.He notes that i f a building has previously been chronically under-ventilated,installing DCV might increase ratherthan decrease energy usage because itwould bring in more outside air. However, DCV will correct IAQ problemsand reduce liability for IAQ-relatedillnesses in such situations and may correct problems with mold growth.

The maintenance impact of installingCO2 sensors is usually minimal if thesensors are self-calibrating, he said.Sensor calibration can be checked periodically by comparing indoor sensorreadings with metered outside air read

ings after a building has been unoccupied for about 5 hours.

Energy Savings

None of the building operators interviewed had measured energy consumption before and after the installation ofCO2 sensors. However, both Purdue staffand the controls company owner haveobserved lower outside air requirementsin facilities using DCV, which generallyresults in reduced energy demand.

Maintenance ImpactThe maintenance impact of installing CO2 sensors has been minimalat Purdue. The sensors used are self-calibrating and have performed reliably.The hospital in Texas reported problemswith getting several of the sensors calibrated initially and had to use hand-heldmonitors to recalibrate them. In addition, some of the sensors were wiredimproperly and had to be corrected.

Case StudyThis case study describes the methodology used to assess the cost and energysavings implications of retrofitting abuilding for CO2-based ventilation control. This methodology can be appliedto any building that has a track recordof energy usage. In this case, the preliminary assessment of the buildingshowed significant energy savings wereavailable. The performance of the building after 6 months of operating with

CO2 control is also presented and showsthat the predicted performance wasslightly conservative compared withthe actual savings realized.

Facility Description

This case study addresses a privatelyowned (non-government) 30-storyClass A office building in Birminghamthat was retrofitted with a CO2-based

ventilation control system in 2001. Ithad an existing state-of-the-art digitalbuilding control system installed severalyears earlier that was fully functional.The building was also upgraded toqualify for the EnergyStar label awarded

by EPA to buildings having met qualifying energy efficiency standards.

Utility costs for the building were verylow, $0.48/kWh during the base yearof 1999; overall energy costs for the baseyear were $1.61 ft2/year, representing117,992 Btu/ft2/year. This energy performance was compared with that ofother similar buildings in that regionusing the DOE Energy InformationAdministration (EIA) energy intensityindices, which provide a general guideto average energy usage for differenttypes of buildings in a region. The casestudy building was found to consumeabout 30% more than those found inthe EIA indices. The fact that the energycost was higher than average providedsome preliminary indication that despitethe low utility costs and EnergyStarrating, there were some opportunitiesfor further energy conservation.

Of the total energy consumed by thebuilding in the year 2000, 84.55%was spent on electricity for a total of

13,966,500 kWh or $670,742 annually;the maximum peak load was 3,318 kW.Of the remaining total energy consumedby the building, 13.81% was spent onsteam for a total of 7,810,000 lb at acost of $122,581 annually.

Existing TechnologyDescription

The building HVAC system wasdesigned with two air-handling units(AHUs) per floor, located on oppositesides of the building. Variable air volume (VAV) boxes served interior zones,and linear diffusers were located onperimeter zones. The linear diffusershave re-heat capability; the interior VAVboxes do not. Each AHU fan motorwas controlled by a variable-frequencydrive (VFD) to supply VAV boxes onthe floor. Each AHU was on a time-ofday schedule provided by the automation system, wherein the units were


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scheduled on and off to correspondwith the staff occupancy for that particular floor on a Monday throughFriday basis, with limited Saturdayhours of operation. Sunday was mostoften scheduled off all day for most

floors. Because of the nature of the client operations, a few floors operated24 hours per day, 7 days per week.

Outside air is drawn into each AHU ateach floor through a ducted grille. Thecombined general building and bathroom exhaust are provided by twin fans(of differing sizes) exhausted at the rooflevel by connecting all floors throughtwin vertical shafts. Both exhaustfans were of a constant-speed design,exhausting the full design load airregardless of the intake of outside airinto the building. Although a limitednumber of floors operated 24 hoursper day, both exhaust fans ran constantly at the full design load. Thisfeature probably also contributed tothe higher than normal operating costfor the building.

The building had a DDC building airsystem that was deemed capable ofexecuting a DCV strategy with someadditional programming, and had sufficient input/output capacity to add

the required components (e.g., CO2sensors, peripheral I/O modules).

New Technology EquipmentSelection

The building automation system (BAS),an existing Siemens System 600, hadbeen installed several years previously.It was deemed capable of accepting thedata from the new CO2 sensors, able toexecute the new control strategy, andto have sufficient spare point capacityto provide the required input/output(I/O) from/to the new devices. Fournew Telaire series 8002, dual-beam CO2sensors were installed per floor, excepton the second floor, which had a number of enclosed meeting/conferencerooms. One CO2 sensor was installedin each of these rooms. One outsideCO2 sensor was installed to provideaccurate indoor/outdoor CO2 differential readings.To facilitate the additional

I/O point requirements, new moduleswere installed within each S600 cabinet.For the CO2 sensors, analog input modules accepting a 4-20 mA signal wereneeded, for the new fully proportionalelectric actuators, replacing the old relay

(open/closed) actuators required analog modules driving a 0-10 V signal.

The existing building exhaust fans wereretrofitted with Magnatek VFDs to balance the aggregate total of outside-airintake. The Magnatek VFDs were specified with factory-installed Siemens S600FLN (field level network) cards to facilitate the connection to the S600 BASwithout having to hardwire each I/Opoint into the S600 control cabinet; thisarrangement saved labor and the expenseof individual I/O modules while simultaneously providing much more motorperformance data to the building owner.

Completing the BAS componentchanges were the addition of oneSetra building static pressure sensorthat was essential to ensure a positivepressure in the whole building relativeto the outside.

No other major control componentswere required for the case study building; however, the application of a newand fairly complex control strategy wasessential to execute the DCV application. The DCV control algorithm isthe most critical aspect of the project,as failure to implement an effectivecontrol sequence will yield lower energysavings performance.

Building Upgrade Assessment

Building owners must be able to evaluate the energy-saving potential andcost of specific initiatives to weighthe value of various building upgrade

options. The methodology used toassess the potential of CO2-based ventilation control involved five basic steps.

1. Spot measurement of CO2 levels

2. Trend logging of CO2 and otherenvironmental factors

3. Estimation of the savings potential

4. Implementation assessment

5. Payback analysis

1. Spot measurement of CO2

As indicated previously, CO2 concentrations can be correlated to cfm/personventilation rates inside a space. For a preliminary assessment of ventilation ratesfor this building, a number of spot mea

surements of CO2 concentrations weremade on each floor. A hand-held CO2monitor was used that is capable of calculating the cfm/person ventilation ratebased on inside/outside differential CO2concentrations. The monitor assumesoutside concentrations are 400 ppm,but it will also allow the baseline concentration to be set based on an actualoutside measurement.

Spot measurements for CO2 were madein the mid-morning to late morning and

late afternoon hours on weekdays, afteroccupancy had stabilized in the building. They were taken while the buildingwas not operating in economizer mode.

The results of the spot measurementsshowed that most areas of the buildingunder study had CO2 concentrationsbelow 700-800 ppm, correspondingto ventilation rates in the range of28 to 35 cfm/person. These rates werewell over the original design target of20 cfm/person. The chart shows thecorrelation between peak CO

2

levels

Figure 5. Handheld CO2 sensor with cfm/person calculation and data logging.


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and cfm/person ventilation rates, assuming office-type activity and an outsidelevel of 400 ppm. Based on the spotmeasurements, it appeared that thisbuilding could be a good candidate forenergy savings through better control

of ventilation. These results warrantedfurther investigation.

2. Trend logging of environmentalfactors

Trend logging was conducted over7 typical days in the building to ensurethat representative conditions werebeing measured. Ideally, measurementsshould be made in the major occupancyzones on each floor. If many locationsare involved and monitoring devices arelimited, multiple measurement sessions

may be necessary. Devices used formeasurement should measure CO2 andshould be able to log concentrations to a

CO2 & Ventilationcfm/person CO2

UnderVentilated

Ideal

OverVentilated

Outside air

5

10

15

20

25

30

35

40

50

100

25001500120011001000900850800750700650600500400

Figure 6. CO2 to ventilation rate conversion,assuming 400 ppm outside and office-typeactivity (1.2 MET).

Nov 2000

database every 15 minutes for a periodof a week (at least 672 data points perparameter measured). Typically, data-logging sensors come with software toadjust, download and graph the results.

Several locations in the building weremonitored over 7 days. Results of trendlogging verified that the building was

over-ventilated compared with theoriginal design. The ventilation ratecan be determined by looking at portions of the graph that show extendedperiods of operation where the CO2levels have stabilized; these indicatethat the amount of CO2 produced bypeople has reached equilibrium withthe ventilation rate of the space. Theseperiods are represented by the flat areasat the peak of each of the daily trendlogs. In almost all locations, CO2 levelsand calculated ventilation rates weresimilar to those recorded during spotmeasurements. In general, peak levelsaround 930 ppm indicate a ventilationrate of 20 cfm/person; peak levels near1100 ppm would indicate ventilationrates of 15 cfm/person. At the peakvalues recorded in the chart (674 ppmon average), the effective delivered

amount of fresh air per person was morethan 34 cfm/person, or 170% of thedesign need of 20 cfm/person. Duringperiods of lower occupancy throughoutthe day and during Saturday morningoperation, the effective delivered cfm/person considerably exceeded design needsand resulted in the use of excess energy

to condition the surplus outside air.Over-ventilation in the space may bedue to either space densities being beloworiginal design conditions or the upwardadjustment of outside air delivery to thebuilding over design conditions. Thebuilding operator indicated that duringsummer months, occupants in thebuilding often complained of uncomfortable humidity levels, indicating thatperhaps the existing system was introducing more outside air than the system was originally designed for. Thisoften happens when building operatorstweak the building control system orair intakes to respond to complaints orto better tune the feel of the building.Based on hundreds of measurementsin buildings throughout the country,over-ventilation appears to be a common problem in office buildings.

Figure 7. CO2 trend-logged results from 7 days of monitoring of one location in the building.


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In summary, the work required toupgrade the building included

Installing a minimum of four CO2sensors and two VFD drives oneach of the air intakes for each floorand interfacing these devices to the

building control system. Programming the building control

system to take the CO2 transmittersignal and regulate air delivery on eachfloor based on in-space CO2 levels.

Moving and replacing the buildingstatic pressure sensor.

Installing variable-speed drives onthe building exhaust fans.

It is important to note that for CO2control to work in a building, all other

HVAC-related systems must be in goodoperating order. CO2-related investigations may identify problems not previously recognized and add cost to anupgrade project. In this building, theassessment identified problems withthe location of the building pressurization sensor, which had to be relocatedand replaced. Logging of CO2 concentrations in the spaces also indicatedthat the time-of-day operating scheduleson some floors did not match actualoccupancy patterns, and a change wasrecommended. In some cases, a regularcheck of logged CO2 data from permanently installed sensors can help keeptime-of-day schedules relevant to currentoccupancy patterns. In some cases, CO2data have been used to detect end-ofday occupancy and initiate setbackoperation when inside levels approachoutside concentrations.

5. Payback analysis

The total cost of the building upgrade,

including equipment and labor, wasestimated at $178,800, which alsoincluded $15,000 for the for the pre-project trend analysis. Based on theprojected cost savings of $81,293 theCO2 upgrade project was projected toyield a 2.2-year payback. Based on this

Summary of BLCC 5.1-03 life-cycle cost analysis

Study Period 15 yearsDiscount Rate3%

Initial Investment: CashRequirements

Future CostAnnual & non-annual

recurring costEnergy related costTotal

Base: CurrentOperational/No

CO2 Control

$

$

$9,414,476$9,414,476

Retrofit of SelfCalibrating Saving from

CO2 Control CO2

$178,800.00 $ (178,800.00)

$ $

$8,460,875 $953,602$8,460,875 $953,602

$953,602$178,800

5.3315.66%3 years3 years

Net SavingsPV of non-investment savingsIncreased total investment

Savings-To-Investment Ratio

Adjusted Internal Rate of Return

Simple Payback

Discounted Payback

methodology and analysis, the projectwas initiated by the building owner inthe late spring of 2001 and completedin July of that year.

Life-Cycle Cost Analysis

A life-cycle cost analysis of this projectusing BLCC 5.1-03 was performedusing energy and cost data for the building collected for the year 2000 but basedon an April 2003 start date, as requiredby the BLCC program. This procedureprobably results in an analysis based onslightly lower costs in the analysis thanprevail currently. The study period of15 years was selected because this is thetypical life of most electronic controldevices. If a longer period is desired, theuser should account for replacementof the sensors at the end of year 15(approximate cost is $250 to $350 perreplacement sensor, including labor).In performing the CO2 portion of thestudy, no annual or periodic costs were

assumed. In this case, we assumed useof self-calibrating sensor that requireno maintenance or calibration over theiroperating life. Some sensors do requireperiodic calibration at 3 to 5 years, andusers are urged to consider this fact inthe selection and cost analysis of their

particular installation. The cost ofcalibration will vary depending on themanufacturer and procedures requiredAbout a third of the sensors sold todayhave a self-calibrating feature that eliminates maintenance requirements.

The total cost of the building upgrade,including equipment and labor, wasestimated at $178,800 (including$15,000 for pre-project trend analysis). The projected annual cost savings

was $81,293. The upgrade project wasprojected to yield a 2.2-year payback.

Post-Implementation Experience

After 6 months of operation, energydata from the building were collectedto determine how the CO2 retrofit andother improvements to the buildingwere performing. Unfortunately, because of changes resulting from a company reorganization, a full 12 monthsof performance data was not availablefor analysis. Figure 6 provides a sum

mary of the energy usage for each half-year period during 2000 and 2001.The CO2 ventilation control systemwas operated from July to Decemberof 2001. As can be seen from the data,the reductions in energy consumption


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were in excess of the savings predicted.The difference is probably due to anumber of factors, including these:

Replacement and relocation ofthe building pressurization sensorprobably impacted energy savings,

but savings were not predicted forthis improvement.

Time-of-day schedules were reprogrammed based on actual occupancy,and building exhaust fans wereadjusted to minimum levels duringunoccupied hours. The change thatprobably contributed additionalenergy savings to savings predicted.

The year 2001 was milder than2000 and had 20% fewer coolingdegree days and 4% fewer heating

degree days.

According to the facility manager, thetenants were satisfied with the comfortlevels in the space following the retrofitand no longer complained of highhumidity levels in the building duringsummer months. Because the loggedCO2 data showed the space to be significantly over-ventilated, reducingventilation with CO2 control reducedthe amount of humid outside air drawninto the building and allowed the cool

ing system to maintain better controlof humidity.

The following charts show energy performance, electricity costs, and steamcosts before and after the CO2-basedDCV retrofit.

Before-and-after energy performance

*CO2 Control in Operation

Energy Cost 2000JanJune JulyDec

2001JanJune JulyDec*

Electricity kwhElectricity ($)

6,357,000$305,136

7,609,500$365,256

6,606,000$317,088

6,219,000$298,512

Steam (therm)Steam ($)

71,918$64,007

75,838$67,496

84,523$75,225

22,960$20,434

Total ($)Annual ($)

$369,143 $432,752$801,895

$392,313 $318,946$711,260

6 month savings comparing JulyDec 2000 vs 2001Electric ($)Steam ($)Total ($)

$66,744$47,061

$113,805

Figure 8. Shown are the monthly costs (right scale) for the baseline months 2000(light line) and for the months of 2001 (dark line) calculated using 2000 rates. The variance (left scale) is shown by the columns. A major vector change occurred beginningin July (as the first phases of DCV were coming on-line) that is known not to be weatherrelated (the minor variance in Feb/Mar is known to reflect a shift in the weather load).


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Figure 9. Shown are the monthly costs (right scale) for the baseline months in 2000(lighter line) and the cost for the months of 2001 (darker line) calculated using 2000rates. The variance (left scale) is shown by the columns. Since July, savings haveoccurred each month as a direct result of the CO2 retrofit project, whereas the variancein March 2001 is known to be due to a shift in weather load.

Before and After Steam Usage for Case Study Building

The Technologyin Perspective

The Technologys Development

CO2-based DCV is an emerging technology with currently limited butincreasing market penetration. Theliterature on CO2-based DCV consistently predicts that the technology willbecome a common ventilation strategy.

CO2 sensors for DCV regulation area relatively mature product, havingbeen on the market since about 1990.The first CO2 sensors were expensive,unreliable, and difficult to keep calibrated. However, manufacturers claimto have resolved those problemsunitscurrently available generally are self-calibrating and similar to thermostats

in price and reliability. Although manufacturers say most sensors are designedto operate for 5 years without maintenance, regular inspection to verifycalibration and proper operation isstill recommended.

Interest in DCV was spurred byASHRAE 62, which increased therequirement for ventilation air inbuildings to safeguard IAQ. Building

managers became concerned that theincreased ventilation level was drivingup energy costs by introducing unnecessarily large amounts of fresh air thathad to be cooled, heated, dehumidified.CO2-controlled DCV offers a methodof regulating ventilation levels so as tosatisfy the requirements of ASHRAE62 without over-ventilating.

DCV based on CO2 sensing is recognized as a viable strategy by the HVACand building industries:

CO2 sensors are sold by all majorHVAC equipment and controlscompanies.

More than a dozen manufacturersproduce CO2 sensors for DCV.

Most manufacturers of thermostatsand economizers now integrate COsensors into their products, and majormanufacturers of packaged rooftopHVAC systems offer factory-installedCO2 sensors as an option.

ASHRAE 90 requires CO2 sensorsfor DCV in high-density applications.

In the next revision of its building

code, California will begin requiringCO2-based DCV in all buildingshousing 25 people or more per1000 ft2.

A proposed change in the Oregonbuilding code would require DCVfor HVAC systems with ventilationair requirements of at least 1500 cfm,serving areas with an occupant factor of 20 or less.

ASHRAE Standard 62, the IMC(which establishes minimum regula

tions for mechanical systems), andsome state and local building codesrecognize CO2-based DCV.

The U.S. Green Building Code givespoints in its LEED rating system foruse of CO2-based DCV.

Technology Outlook

CO2-based DCV appears likely tobecome a commonplace ventilationstrategy for large commercial and institutional buildings as building operators

seek ways to balance building codeventilation requirements and IAQ concerns with the need to control energydemand and operating costs. Equipment development is encouraging theadoption of the technology. Prices ofCO2 sensors for DCV prices havedropped by about 50% since 1990,


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and as market penetration increases,are expected to fall further. Most HVACequipment makers are including factoryinstallation of CO2 sensors for DCVas an option in their large packaged systems. At least one major manufacturer

has adapted its equipment line toaccommodate CO2-based DCV inevery piece of ventilation equipmentit makes. This implicit endorsement ofthe technology by equipment makersmakes its adoption simpler and lessexpensive, at least in new installations,and will almost certainly contributeto developing a broader market forCO2-based DCV.

The emergence of wireless, self-poweredmulti-parameter (CO2/temperature/humidity) sensors designed for permanent installation is expected to furtherincrease the speed of adoption of DCV.They have the potential to lower thetotal cost of deployment, makinginstallation simpler and faster andimproving paybacks.

As installers, HVAC engineers, andmaintenance personnel accumulatemore hands-on experience with thecontrols and operational issues CO2based DCV presents, and gain moreexpertise in troubleshooting and resolv

ing problems, the comfort level withthe technology should increase. Theincreased familiarity is likely to increaseits acceptance.

Manu

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