26763263 demand controlled ventilation

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Demand Controlled VentilationSystem DesignDemand Controlled VentilationSystem DesignPROVIDING THE RIGHT AMOUNT OF AIR, IN THE RIGHT PLACE, AT THE RIGHT TIMEPROVIDING THE RIGHT AMOUNT OF AIR, IN THE RIGHT PLACE, AT THE RIGHT TIME

SAVING ENERGY COSTS WHILE OPTIMIZING INDOOR AIR QUALITYSAVING ENERGY COSTS WHILE OPTIMIZING INDOOR AIR QUALITY


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2.1 The CO2 DCV Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.2 Atmospheric CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.3 Indoor CO2 Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52.4 CO2 Differential and Ventilation Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62.5 CO2 Control Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.6 CO2 as a Contaminant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72.7 DCV Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

1.1 Why a Handbook on Ventilation Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.2 CO2 as an Important Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.3 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

T A B L E O F C O N T E N T S

1. IN TR O D U C T I O N

2 . C O 2 B A S I C S

3.1 Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93.2 The Evolution of Mechanical Ventilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93.3 CO2 Control ... A New Idea? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93.4 Ventilation Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93.5 CO2 and ASHRAE Standard 62 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103.6 DCV and Building Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113.7 The Maturation of a Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

3 . V E N T I LATION , BUI LDI NG CO DES AN D CO 2

4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154.2 CO2 Control and Standard 62 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154.3 DCV Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154.4 Design Steps for DCV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154.5 Step 1: Is the Space Appropriate for DCV? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .164.6 Step 2: Determining Outdoor Air Ventilation Requirements . . . . . . . . . . . . . . . . . . . .17

4.6.1 DCV or Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .174.6.2 Constant Volume Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .184.6.3 Multiple Zone VAV Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

4.7 Step 3: Calculating Base Ventilation Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . .184.8 Step 4: Select DCV Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .204.9 Step 5: Locating CO

2Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

4.9.1 In-Space or Duct Mounted Sensors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .204.10 Sensor Location - Constant Volume Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

4.10.1 Sensor Selection - VAV Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

4 . D C V A P P L I C A T I O N G U I D E L I N E S

S E C T I O N I

B a c k g r o u n d o n C O 2 a n d V e n t i l a t i o n C o n t r o l

S E C T I O N I I

D C V A p p l i c a t i o n F u n d a m e n t a l s

C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N

Use of the information contained in this Manual is voluntary, and reliance on it should only be undertaken after independent review of itsaccuracy, completeness, and timeliness. Carrier, including its employees and agents, assumes no responsibility for consequences resultingfrom the use of the information herein, or in any respect for the content of such information, including but not limited to errors or omissions.Carrier is not responsible for, and expressly disclaims liability for, damages of any kind arising out of use, reference to, or reliance on suchinformation. No guarantees or warranties, including, but not limited to, any express or implied warranties of merchantabilities or fitness fora particular use or purpose, are made by Carrier with respect to such information.


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5. DC V CO NT ROL ST RAT E G I E S

E X A M P L E 1 : S I N G L E Z O N E R E TA I L S PA C E

S E C T I O N I I I

D e s i g n E x a m p l e s

5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .235.2 Step 1: Consideration of Outdoor Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

5.2.1 Direct Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .235.2.2 Measurement or Assumption of Outside CO2 Concentrations . . . . . . . . . . . . .23

5.3 Step 2: Establishing the CO2 Equilibrium Anchor Point . . . . . . . . . . . . . . . . . . . . . . .235.4 Step 3: Control Strategy Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

5.4.1 Three Control Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .245.4.2 Consideration of Control Response Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

5.5 Set Point Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255.6 Proportional Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255.7 Proportional-Integral Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255.8 Two Stage Control for Zone Based VAV Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

5.8.1 Establishing Zone and System Set Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . .265.8.2 Sizing of Zone Heating Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N

6.1 Application: Retail Clothing Store . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .296.2 Step 1: Is the Space Appropriate? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

6.3 Step 2: Determine Ventilation Requirements for the Space . . . . . . . . . . . . . . . . . . . . . .296.4 Step 3: Determine Base Ventilation Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .296.5 Step 4: Determine Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

6.5.1 Considering Outside Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .306.5.2 Determine CO2 Control Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.6 Step 5: Locate Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .306.7 Installation Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

E X A M P L E 2 : S I N G L E A I R H A N D L E R S E R V IN G M U LT I P L E Z O N E S S C H O O L

7.1 Application: Four Classrooms Served By One Air Handler . . . . . . . . . . . . . . . . . . . . . .317.2 Step 1: Is the Space Appropriate? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .317.3 Step 2: Determine Ventilation Requirements for the Space . . . . . . . . . . . . . . . . . . . . . .317.4 Step 3: Determine Base Ventilation Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

7.5 Step 4: Determine Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .317.5.1 Considering Outside Concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .327.5.2 Determine CO2 Control Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327.5.3 Selecting the Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

7.6 Step 5: Locate Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .327.7 Installation Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

E X A M P L E 3 : M U L T I P L E Z O N E O F F I C E W I T H V A V

8.1 Application: 14 Zone Office with VAV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .338.2 Step1: Is the Space Appropriate for DCV? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .338.3 Step 2: Determine Ventilation Requirements for the Space . . . . . . . . . . . . . . . . . . . . . .33

8.3.1 Maximum Airflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .348.3.2 Minimum Airflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

8.4 Step 3: Determine Base Minimum Ventilation Rate for DCV . . . . . . . . . . . . . . . . . . .358.5 Step 4: Determine CO2 Control Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .358.6 Step 5: Locating CO2 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .368.7 Installation Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

A P PEN DIX A : Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37A P PEN DIX B: ASHRAE 62 Interpretations IC 62-1999-33 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39A P PEN DIX C: CO2 Equilibrium Anchor Points for Alternative Activity Levels . . . . . . . . . . . . . . . . . .41A P PEN DI X D: Sequences of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43A P PEN DIX E : Guide Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45A P PEN DI X F : Background on CO2 Sensor Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47A P PEN DI X G: DCV Compatible Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53


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1 . 1 WH Y A H A N D B O O K O N V E N T I L AT I O N C O N T RO L

This handbook is designed to give the user astrong understanding of a simple but powerful methodof active ventilation control with carbon dioxide (CO2)that can ensure good air quality to code requirements,save energy and enhance occupant comfort.

Active, zone level ventilation control is both anew and an old concept. Windows were the first effec-tive method of ventilation control. Every building built

before the 1920s incorporated operable windows,which provided both light and a method of ventilationcontrol that allowed variable control of fresh air on aroom-by-room basis. As central mechanical systemsfor heating and cooling became more common, activezone/room based control of ventilation was lost andreplaced by a centrally delivered, fixed ventilation rateintended for the entire building. The result was bettertemperature control but a loss in flexibility in ventila-tion control. This passive, fixed ventilation approachwas applied with mechanical systems, because untilrecently there was not an inexpensive method ofmeasuring and controlling ventilation at the zone level.

As described in this handbook, indoor CO2 levels

have been used as an indicator of outside air ventila-tion rates for over 90 years. Carbon dioxide ventilationcontrol or demand controlled ventilation (DCV) allowsfor the measurement and control of outside air ventila-tion levels to a target cfm/person ventilation rate in thespace (i.e., 15 cfm/person) based on the number of

people in the space. It is a direct measure of ventila-tion effectiveness and is a method whereby buildingscan regain active and automatic zone level ventilationcontrol, without having to open windows. Recenttechnical developments in CO2 sensor design andadvancement in equipment control now provide anopportunity for this active ventilation control within acompetitive market environment.

Zone level control of ventilation can also avoidthe cost often related to over ventilating a buildingcontinually to ensure that one critical zone receivesadequate fresh air under all operating conditions. CO2based ventilation control is a dynamic system thatresponds to how the building is used and occupied.It is a real time control approach that offers a vastimprovement over ventilating a building at a fixed ratebased on some pre-construction constant occupancyassumptions. The fixed ventilation approach dependson a set-it-and-forget-it methodology that is completelyunresponsive to changes in the way spaces are utilizedor how equipment is maintained.

This handbook was undertaken because there is no

single source that designers can rely on to understandand design HVAC systems incorporating CO2 ventilationcontrol. This handbook represents the state of the art inthe use of CO2 based ventilation control and has drawnon numerous technical papers, codes and standards aswell as a wealth of field applied experience.

Carrier Corporation believes that ventilation controlwith CO2 (CO2 DCV) is an important building controltechnique that can be applied in to most buildingsand types of building occupancies. Carrier believesso strongly in this approach that now every piece of

Carrier equipment that provides ventilation has aninput and built-in control strategy to utilize CO2 con-trol. It is a proven method to control outdoor air basedon actual occupancy. DCV meets the code, it savesmoney, and it doesnt guess.

Carrier is particularly excited about the highdegree of comfort that can be provided to multi-zonespaces that utilize VAV systems with active ventilationcontrol with CO2. By integrating zone control of both

temperature and ventilation it is possible to measureand control ventilation to ensure that adequatefresh air is actually delivered to all spaces. Previousapproaches could only ensure adequate fresh air atthe air intake but could not quantify if that fresh air was

actually being distributed to the spaces that needed it.Zone ventilation control with DCV removes thetraditional dependence that ventilation has had onspace conditioning load. It is now possible to controlfresh air and space conditioning to a zone independ-ently using the same VAV box. The result is that thedesigner does not have to oversize outside air intakecapacity to handle low load conditions. Significantenergy can be saved over traditional VAV approaches

1 . 2 C O 2 A S A N I M P O R T A N T C O N T R O L S T R A T E G Y

This Handbook is divided into three sections:Section I which includes Chapters 2 and 3, establish-

es the background necessary to understand how CO2DCV works and how it is applied under current codesand standards.

Section II which includes Chapters 4 and 5addresses the application fundamentals necessaryto properly design a HVAC system with DCV.

Section III addresses three different design examples using the guidelines established in section II.

The Appendix to this handbook also provides avaluable point of reference to the reader and providesmore details on DCV applications for special circum-stances, detailed sequence of operation for varioustypes of equipment, and additional details on thesensors and equipment offered by Carrier for DCV.

1 . 3 O V E R V I E W

This handbook is intended to provide a clearengineering rationale for CO2 based DCV that canbe used by the HVAC industry to properly apply thispromising technology. Much has been written on the

topic of DCV as a good idea. This handbook beginswith the conclusion that CO2 DCV is a good idea andthen answers the nuts-and-bolts questions of how toapply it.

1 . 4 S U M M A R Y

C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N 1


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2 C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N


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B AC KG ROU N D O N CO 2 A N D V E N T I L AT I O N C O N T RO L



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I


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2 . 1 T H E C O 2 D C V C O N C E P T

Carbon Dioxide (CO2) based demand controlledventilation (DCV) is an economical means of providingoutdoor air to occupied spaces at the rates requiredby local building codes and ASHRAE Standard 62,Ventilation for Acceptable Indoor Air Quality.Engi-neers and building owners both lament the high costof conditioning outdoor air, and the inexact controlmethodologies that often result in significant overventilation of spaces. CO

2-based DCV offers designers

and building owners an ability to monitor both occu-pancy and ventilation rates in a space to ensure thereis adequate ventilation at all times. Ty p i c a l l y, mostventilation systems are set up and adjusted only whenthey are installed. DCV offers a higher level of controlin that it monitors conditions in the space and constantly

adjusts the system to respond to real time occupancyvariations. The result is that target cfm-per-personrates as established by local codes and standardsare maintained based on actual occupancy. Costlyover ventilation that typically results from a fixedventilation strategy (design occupancy X cfm/person)is avoided and energy usage can be reduced.

Measurement of CO2 concentrations is anaccepted scientific methodology to determine theactual ventilation ratein a building. The use of CO2 tocontrol ventilation rates in buildings is also recognizedas a valid control approach in ASHRAE Standard 62and in model building codes used as reference bymost local code bodies.

2 . 3 I N D O O R C O 2 C O N C E N T R AT I O N S

Indoors in commercial buildings people are theprincipal source of CO2. Plants, due to their low levelof metabolic activity contribute an insignificant amountof CO2 to indoor spaces. Unvented combustionsources can also contribute to indoor CO2 concen-trations but are generally not present in commercialbuildings. In fact highly elevated levels of CO2 (e.g.,3000 to 5000 ppm) can indicate the presence ofpotentially dangerous combustion fumes. CO2 is oneof the most plentiful byproducts of combustion andcan account for 8% to 15% by volume of the contentof a combustion exhaust.

For ventilation control, it is people as a sourceof CO2 that we are interested in. People exhalepredictable quantities of CO2 in proportion to theirdegree of physical activity. This relationship isshown in Figure 2.2 and is taken from Appendix Dof ANSI/ASHRAE Standard 62-1999, VentilationFor Acceptable Indoor Air Quality.2

2 . 2 AT M O S P H E R I C C O2

CO2 is one of the most common compounds inour atmosphere. It is also cited by many as a generalindicator of the buildup of greenhouse gases andglobal warming. Figure 2.1 summarizes data collected

from the Mauna Loa Observatory in Hawaii over thepast 40 years.1 The chart shows the gradual increaseof CO2 concentrations by 1 to 2 ppm per year. Givenits isolated location in the middle of the Pacific Ocean,these concentrations likely represent the lowest con-centrations that will be found worldwide. In urbanareas outdoor CO2 levels typically range from 360up to as high as 450 to 500 ppm due to the presenceof localized sources of CO2 which can include anycombustion device or process. Higher outdoor levelscan also be measured when in close proximity to asource of CO2 such as an idling vehicle or a furnaceor combustion exhaust.

Because of its low molecular weight CO2 willreadily diffuse and equalize within an open space.

F I G U R E 2 . 1C O2 C O N C E N T R ATI ON S ME AS URE D INH AWAI I OV ER 40 Y E A R S

As a result, outside CO2 levels tend to be ubiquitousand fairly constant over large geographic regions.Because of this consistency, it is possible to use CO2as a baseline reference for outside air for the purposeof measuring and controlling ventilation.

F I G U R E 2 . 2C O2 P ROD UC TI ON AN D AC T I V I TY LE V E L

Physical Activity - MET Units

0 1 2 3 4 5

40

30

20

10

1.25

1.00

0.75

0.50

0.25

Activity Level

Very Light ModerateLight


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Because CO2 production is so consistent andpredictable, it can be used as a good indicator ofgeneral occupancy trends. For example, if the numberof people in the space is doubled, the amount of CO2produced will double. If one or two people leave aspace the CO2 production will decrease correspondingly.It is important to note that an indoor CO2 measurementdoes not provide enough information to actually countpeople but it can be used in combination with outside

air concentrations to calculate, measure and controlventilation rates.

An indoor CO2 measurement is a dynamicmeasure of the number of people in a space (exhalingCO2) and the amount of outside air at baseline CO2concentration that is being introduced for dilution viamechanical ventilation and/or infiltration. The result isthat it is possible to determine cfm/person ventilationrates in a space by measuring the CO2 differential.

2 . 4 C O 2 D I F F E R E N T I A L A N D V E N T I L A T I O N R A T E SFigure 2.3 shows the typical pattern of buildup of

CO2 in a space with office type activity (1.2 MET). Thechart assumes a steady-state condition where a con-stant occupancy is present and the ventilation rate isconstant. Once people enter a room, CO2 concentra-tions will begin to increase. These levels will continue toincrease until the amount of CO2 produced by the spaceoccupants and the dilution air delivered to the space arein balance. This is called the equilibrium point.

F I G U R E 2 . 3C O2 E QUI LIB RIU M LEV ELS AND PER PE R S O N

V E N T I L AT IO N R AT E S

As can be seen from Figure 2.3 the equilibriumpoint corresponds to a specific ventilation rate perperson in the space. The equilibrium level for aparticular space can be calculated using a simplemass balance equation found in Appendix D ofANSI/ASHRAE Standard 62-1999.3

Vo= N/(Cs Co)

Where:Vo = outdoor air flow rate per personN = CO2 generation rate per person

Cs = CO2 concentration in the spaceCo = CO2 concentration outside

Note that the activity level for the space is a compo-nent of calculating N. This will be discussed in greaterdetail later in this handbook.

The equation can also be restated so that the equilib-rium level (Ceq) for a particular ventilation rate can becalculated.

Ceq= Cs= Co+ N/Vo

The relationship between indoor/outdoor CO2 differen-tial and ventilation rate is independent of populationdensity. However, population density will affect thetime it takes for CO2 to build up to an equilibrium level.This equation only applies when equilibrium conditionsexist. This is particularly important when trying to inferspace ventilation rates from a spot measurement whennon steady-state conditions exist.4 To make an accu -rate determination of cfm/person rates one should takeCO2 measurements when occupancy has stabilized.Measuring CO2 concentrations that are still in transitionto an equilibrium level can result in over estimation ofthe ventilation rate. Applied properly, spot measure-ments can be extremely useful in helping to qualify ifa space is over or under ventilated.

The ANSI/ASHRASE Standard 62 states that:

Comfort (odor) criteria with respect to humanbioeffluents are likely to be satisfied if the

ventilation results in indoor CO2concentrationsless than 700 ppm above the outdoor air

concentration.5

Appendix D of Standard 62 provides an examplethat shows how this 700 ppm level is derived fromthe 15 cfm per person minimum ventilation rate estab-lished in the standard. The calculation below assumesan activity level of 1.2 MET which would be consideredequivalent to office type activity. Average CO2 p ro-duction at this activity level (as provided in Figure 2.2taken from Appendix D of Standard 62) is 0.30 l/min or0.0106 cfm. Outside CO2 concentrations are assumed

F I G U R E 2 . 4C A R R I E R 7 0 0 1 H A N D - H E L D C O2 M O N I TO R

I

Time

Note:Assumes office type activity level (1.2 MET)

2,500

2,000

1,500

1,000

500

2,120

1,050

700

500

350

0


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2 . 5 C O 2 C O N T R O L C O N S I D E R A T I O N S

When using CO2 to control ventilation rates ina building, the use of the equilibrium level is useddifferently than it would be when using a portablemonitor to determine ventilation rates based on aspot measurement.

While it is possible to provide a CO2 control strat-egy where outside air would only be introduced oncethe equilibrium level is reached, it may not achievethe best results. This is because of the lag time thatmay result between when people enter a space and

when CO2 levels reach the appropriate equilibriumset point (e.g., 700 ppm inside/outside differential).

When applying ventilation control with DCV theequilibrium level actually becomes one component oranchor point in a CO2 control algorithm. The actualchoice of algorithms will be discussed in Chapter 5and is based on providing a control strategy that isresponsive to changes in occupancy so that the targeper person ventilation rate can be provided within areasonable lag time.

to be 400 ppm. If this is the case then the CO2 levelfor a 700 ppm differential would be 1100 ppm.

Ceq= Co+ N/Vo= 0.000400 + (0.0106 / 15)= 0.000400 + 0.000707= 0.001107= 1107 parts per million (ppm)

If the above calculation were performed for20 cfm per person the inside outside differentialwould be approximately 500 ppm or an absolutelevel of 900 ppm (assuming 400 ppm outside).Chapter 5 discusses how this equilibrium levelis used as part of a control strategy.

2 . 6 C O 2 A S A C O N T A M I N A N T

Carbon Dioxide is not considered a health threat-ening contaminant at the levels normally found inbuildings (400 3,000 ppm). In fact, in industrial envi-ronments OSHA has established an 8-hour exposurelevel of 5000 ppm and a 15-minute maximum exposurelimit of 30,000 ppm.6 In commercial buildings CO2 isused as an indicator of the per-person ventilation ratein a space, not as a contaminant.

So why is it that many people have observed thatas CO2 levels rise above 1000 ppm range, increaseddrowsiness, lethargy and discomfort can occur? Thisis because CO2 is an indicator of ventilation. As CO2levels rise above 1100 ppm (a 700 ppm differentialbetween inside and outside) ventilation rates begin to

drop below 15 cfm/person. While CO2 is building upother space and people related contaminants are alsoincreasing. It is these other contaminants that are

creating the physiological effects. The buildup of CO2is an indicator of low ventilation rates that generallywill result in higher levels of all types of contaminantsand a greater level of occupant dissatisfaction.

An excerpt from an interpretation to ASHRAEStandard 62-99 provides a good summary of howCO2 is used in the standard:

700 ppm above outdoors is the steady-statecarbon dioxide concentration differential corre-

sponding to a constant ventilation rate of 15cfm/person of outdoor air in a space occupied by

sedentary adults. Chamber studies have shownthat 15 cfm/person and indoor carbon dioxide

concentrations that are about 700 ppm aboveoutdoors correspond to 80% satisfaction of visitorsto such a space with respect to body odor.7

2 . 7 D C V B E N E F I T S

Compared to a fixed ventilation approach, DCVoffers considerable advantages. Carbon Dioxidebased DCV does not affect the design ventilationcapacity required to serve the space; it just controlsthe operation of the system to be more in tune withhow a building actually operates.

Excessive over-ventilation is avoided while still

maintaining IAQ and providing the required cfm-per-person outside air requirement specified bycodes and standards. Operational energy savingsof $0.05 to $1.00 per square foot annually canresult. This observation has been verified in arecent literature review on CO2 control thatsighted numerous studies where energy savingsfrom DCV control approaches ranged from 5%to 80% versus a fixed ventilation strategy.8

System paybacks can range from a few monthsto two years and are often substantial enough tohelp pay for other system or building upgrades.

The payback from CO2 DCV will be greatest inhigher density spaces that are subject to variableor intermittent occupancy that would have nor-mally used a fixed ventilation strategy (e.g.,theaters, schools, retail establishments, meetingand conference areas).

In spaces with more static occupancies (e.g.,

offices) DCV can provide control and verification thatadequate ventilation is provided to all spaces. Forexample a building operator may arbitrarily and acci-dentally establish a fixed air intake damper positionthat results in over or under ventilation of all or partsof a space. A CO2 control strategy can ensure theposition of the intake air dampers is appropriate forthe ventilation needs and occupancy of the spaceat all times.

In some buildings, infiltration air or open win-dows may be a significant source of outside air.


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A CO2 sensor will consider the contribution ofinfiltration in a space and only require the mech-anical system to make up what is necessary tomeet required ventilation levels. These savingsare in addition to those quoted above.

CO2 monitoring and control is consideredan important part of green building design. Itis one of the criteria that can now be used tomeet the LEED (Leadership In Energy AndEnvironmental Design) criteria for greenbuilding design.9

When integrated with the appropriate buildingcontrol strategy, ventilation can be controlledon a zone-by-zone based on actual occupancy.This allows for the use of transfer air fromunder-occupied zones to be redistributed toareas where more ventilation is required.

A control strategy can be used to maintainany per-person ventilation rate. As a result

this approach is highly adaptable to changingbuilding uses and any changes that may occurin future recommended ventilation rates.

DCV can provide the building owner/managerwith valuable information about occupancytrends and the status of equipment operation.This information can be documented andrecorded by a digital building control system.

CO2

demand control ventilation is a real-time,occupancy based ventilation control approach that canoffer significant energy savings over traditional fixedventilation approaches. Properly applied, it allows forthe maintenance of target per-person ventilation ratesat all times. Even in spaces where occupancy is static,CO2 DCV can be used to ensure that every zone withina space is adequately ventilated for its actual occupancy.Air intake dampers, often subject to maladjustment,or arbitrary adjustments over time can be controlledautomatically, avoiding accidental and costly over orunder ventilation.

1 Atmospheric CO2 concentrations (ppm) derived from in situ air samples collected at Mauna LoaObservatory, Hawaii. Source: C.D. Keeling, T.P. Whorf, Scripps Institution of Oceanography University ofCalifornia, La Jolla, California USA92093-0244.

2 ANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality, ASHRAE, 1999.

3 ANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality, ASHRAE, 1999.

4 Persily, A, Evaluating Building IAQ And Ventilation With Indoor Carbon Dioxide, ASHRAE Transactions, 1997.

5 Section 6.1.3. ANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality, ASHRAE, 1999.

6 OSHA, Chemical Information Manual, OSHA Instruction CPL2-2.43A, July 1, 1991.

7 Interpretation ToANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality,Interpretation No: IC 62-1999-05.

8 Emmerich, S and Persily, A. Literature Review On CO2 Based Demand Controlled Ventilation.

ASHRAE Transactions 1977, American Society Of Heating, Refrigeration and Air Conditioning Engineers.9 US Green Building Council, LEED Green Building Rating System Reference Guide 2.0, June 2001.

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3 . 1 V E N T I L AT I O N

In early part of the last century, in the days beforecentral air-conditioning systems, ventilation was amuch more natural subject than it is today. Not thatventilation has become unnatural, but prior to WorldWar II, central air-conditioning was rare. Buildingswere cooled using natural ventilation and the moderncontaminants that today may lead toSick Building

Syndromewere unknown. In the typical prewar officebuilding, odors, tobacco smoke, and combustionbyproducts from heating appliances were the principalcontaminants of concern. Ventilation was achievednaturally through windows and doors. Infiltration, bydesign, was often the only source of fresh air inwinter months.

3 . 2 T HE EVO LUT I ON OF MEC HAN ICA L V E N T I LAT ION

Willis Carriers research and experimentation inbuilding air conditioning began in 1902 with an idea touse evaporative cooling for humidity control in a print-ing plant. His scientific engineering process helped toestablish further study of the need for devising suit-able equipment for carrying out air conditioningprocesses as well as to identify the need of variousindustries for maintaining atmospheric conditions,independently of external weather variations (Carrier,1936). By 1920 Carriers work had advanced intocomfort cooling applications and created a need todefine thermal comfort and ventilation requirements.10

The important technological advance that mademechanical ventilation possible was the developmentof the electric power industry. But even then, mechan-ical ventilation was slow to catch on. If you look atbuildings constructed in the early part of the 20th cen-tury you will notice that most floor plans are generallyvariations of narrow rectangular areas with the dis-tance from one exterior wall to the opposite exteriorwall not more than about 50 feet. Even the very

largest buildings were arranged with H-shaped,T-shaped, or U-shaped floor plans. The reason wasto keep all the building occupants within reasonabledistance to a window for both ventilation and light.

During World War II, ventilation became animportant issue. Buildings and factories operatingat night during the war had to do so under blackoutconditions. Manufacturing facilities for war productionwere often erected without windows, forcing engi-neers to consider mechanical ventilation as a sourceof fresh air and temperature control. Generally, airwas provided in sufficient volume to keep the average

interior air temperature at about 10F (5.6C) abovethe outdoor air temperature. Even as late as 1957,F.W. Dodge Corporation in their architectural recordbook, Buildings For Industry, discussed ventilationrequirements in national defense terms stating: Ifwindows are provided in the building most of theventilation may be taken care of by them. But in thecase of blackout buildings some mechanical meansmust be provided.11

3 . 3 C O 2 C O N T ROL . . . A NEW ID EA ?

The Mechanical Engineers Handbook(Marks,1916) which featured Willis Carrier as a contributing

editor, was one of the first engineering guides to men-tion CO2 measurement as a reference for ventilationrelative to the number of occupants in a space:

Air is rendered unwholesome by perspiration, by

respiration, excessive heat, humidity, effluvia fromthe human body and other impurities directly or

indirectly imparted by the occupants of a room.The percentage of carbonic acid may be regard-ed as a measure of the vitiation from respirationand from combustion, but not from the heat and

moisture resulting from the same source. Air may

be polluted with dust and other harmful matter ofwhich CO2gives no indication. CO2tests should

be used only for checking the renewal of air andits distribution within the room.The production of

this gas can only be assumed as a basis for cal-culating the air supply where respiration and com-

bustion (gas lights) are the preponderating factorsof vitiation; in such cases the CO2should notexceed 8 or 10 parts in 10,000 [800 to 1,000 ppm].12

The 1929 New York City Building Code echoedMarks Handbook reference to CO2 levels and ventilation

...ventilation consisting of transoms or other similardevices opening into rooms ventilated directly to

the outer air or of other methods capable of main-

taining a carbon dioxide content of the air of notmore than one part in one thousand [1,000 ppm]...13

3 . 4 V E N T I L AT IO N S TA N D A R D S

Recommendations for minimum quantities of out-door air date back to the early 19th century whenThomas Tredgold (1836), an English mining engineer,published an estimate of 4 cfm (1.9 L/sec) per personbased on metabolic needs. In 1895, the A me ric anSociety of Heating and Ventilating Engineers (ASHVE)adopted a minimum recommendation of 30 cfm (14.2

L/sec). Later, in 1914, ASHVE proposed a model coderequiring 30 cfm (14.2 L/sec) per person as the mini-mum. By 1925, 22 states had adopted the requirement

The first ASHRAE Standard 62 appeared in1973 titled Standards for Natural and MechanicalVentilation. The standard provided minimum and recom-mended outdoor airflow rates for the preservation


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1 0 C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N

of the occupants health, safety and well-being in avariety of different spaces. Standard 62-1973 defineda prescriptive approach, meaning that the airflowrates were prescribed (as rules), and became thebasis for most state codes. In 1981 the standard wasupdated and re-titled as ASHRAE Standard 62-1981,Ventilation for Acceptable Indoor Air Quality. The neteffect was a general reduction in outdoor air usage.

In the 1989 update to ASHRAE Standard 62 theminimum acceptable ventilation rate increased from5 cfm (2.4 L/sec) to 15 cfm (7.1 L/sec), which hassince been widely accepted. Evolution of the minimumventilation rate is shown graphically in Figure 3.1.

3 . 5 C O 2 A N D A S H R A E S T A N D A R D 6 2

ANSI/ASHRAE Standard 62-1989 (Standard 62)took the first step in integrating CO2 into modern daystandards by establishing that CO2 concentrationsshould not exceed 1000 parts per million. Appendix Dof the standard was also created as a reference tothe standard to explain the fundamental relationshipbetween CO2 and ventilation as described in Chapter2 of this handbook.

In 1999 Standard 62 was updated to becomeStandard 62-99. In this update the provisions forCO2 were changed slightly. The 1000 ppm level was

modified to a 700 ppm differential. The exact wordingof the standard is as follows:

Comfort (odor) criteria with respect to human bio-effluents are likely to be satisfied if the ventilationresults in indoor CO2concentrations less than

700 ppm above the outdoor air concentration.13

As discussed in Chapter 2, a 700 ppm differentialbetween inside and outside concentrations is consid-ered equivalent to 15 cfm/person when people areinvolved in office-like activity (1.2 MET). So wheredid the original 1000 ppm level come from?

Originally the 1000 ppm guideline, which was

also used in some of the handbooks and standards inthe early 1900s assumed an outside level of around300 ppm (300 + 700 = 1000). With the rise of globalCO2 levels at about 1 to 2 ppm per year the previouslyassumed 300 ppm outside level was no longer correct(See Chapter 2 for more information on rising globalCO2 levels). Currently a value of 400 to 450 is gener-ally used for outside concentrations.

F I G U R E 3 . 2C O2 R E LATED ASH RAE INT ERP RETAT I O N S

F I G U R E 3 . 1

EVO LUTI ON OF MI NIM UM V E N T I LATIO N RAT E S

Source: Janssen 1999

When CO2 was first addressed in Standard 62,some confusion resulted because the Standard wassomewhat ambiguous as to whether CO2 was to beconsidered a contaminant, a surrogate for air quality ora ventilation parameter. To many it was unclear whether

CO2 sensing and control was to be applied under theVentilation Rate Procedure (prescriptive) of the stan-dard or the Air Quality Procedure (performance based).

The confusion led to several requests for inter-pretation to ASHRAE 62 committee. A request for

IC-62-1999-03Ventilation criteria of the Standard is likely to be satisfied ifCO2 levels in EACH space do not exceed 700 PPM aboveoutdoor air C02 levels. The 700 ppm differential noted in thestandard is not a time weighted value or a ceiling value. CO2levels should only be considered during times of occupancy.

IC-62-1999-04The 700 ppm CO2 differential noted in the standard is aguideline based on maintaining adequate ventilation to controlperception of human bioeflluents, not a ceiling value for indoor

air quality.

IC-62-1999-05If filtration means are used under the Air Quality Procedureto remove human bioeffluents and odors the 700 ppmdifferential value for CO2 may not be applicable. It is primarilyintended for use with the Ventilation Rate Procedure.

IC-62-1999-15CO2 control cannot be used to reduce ventilation below Table2 values (CO2 DCV is normally used to maintain table 2 val-ues based on real-time occupancy using the ventilation rateprocedure). CO2 cannot be used as the sole means of claim-ing compliance with the Indoor Air Quality Procedure becausethere are generally other contaminants of concern that mustbe measured and controlled using this procedure. CO2 filtra-tion is not an appropriate way of complying with Standard 62.

IC-62-1999-24

It is not necessary to apply equation 6.1 if the required rates ofacceptable outdoor air in Table 2 are provided to EACH space.

IC-62-1999-32Defines the parameters to apply CO2 based demand con-trolled ventilation. Interpretation 24 above confirms if Table 2rates are supplied to each space then the multiple spaceequation is not needed.

I

Year

4 0

3 5

3 0

25

20

15

1 0

5

0

1825 1850 1875 1900 1925 1950 1975 2000


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C A R R I E R D E M A N D C O N T R O L L E D V E N T I L A T I O N S Y S T E M D E S I G N 1 1

interpretation is a procedure where any individual canask a question to clarify the intent of the Standard.Interpretations are asked as yes or no questions andsubmitted to the ASHRAE committee responsible formaintaining ASHRAE Standard 62.

The use of the CO2 in Standard 62 has beenthe subject of 6 of the 38 interpretations requested ofStandard 62. Figure 3.2 provides a table identifyingand briefly describing the various interpretations thathave addressed CO2 related issues.

All interpretations, including those developedas part of the 1989 standard have now been acceptedas part of the relatively new ASHRAE Standard 62-99.All current interpretations are provided with everycopy of the ANSI/ASHRAE Standard 62 sold byASHRAE. All interpretations are also available onASHRAEs web site (www.ashrae.org).

3 . 6 D C V A N D B U I L D I N G C O D E S

While ASHRAE Standards identify what is goodpractice for HVAC design engineers, local codesultimately dictate how buildings must be designed.In fact, many codes indirectly draw from ASHRAEStandard 62 to establish ventilation requirements inbuildings. ASHRAE Standard 62 is not used directlyin codes because it is not written in the languagenecessary for code enforcement.

The majority of local and state code making bodiesdo not usually have the expertise and resources towrite their building codes from scratch. As a result,

three model codes have been established that developstandardized building code documents that can beadopted in whole or in part by local jurisdictions.

These code bodies are known as BOCA (BuildingO fficials and Code Administrators International),ICBO (International Conference of Building Officials)and SBCCI (Southern Building Code CongressInternational).

Recently these three model code bodies havejointly adopted the International Mechanical Code(IMC) which establishes minimum regulations formechanical systems using prescriptive and perform-ance related provisions. Like the ASHRAE 62 stan-dard, the IMC also provides provisions for modulation

of outside air based on occupancy as long at targetcfm-per person ventilation rates are maintained.This is addressed in section 403.3.1 of the2000International Mechanical Code that states:

The minimum flow rate of outdoor air that the

ventilation system must be capable of supplyingduring its operation shall be permitted to be

based on the rate per person indicated in

Table 403.3 and theactualnumber of occupantspresent. [Emphasis added]15

The IMC has also created a commentary docu-ment to provide clarification to the intent of the code.In reference to section 403.3.1, the commentary usesCO2 control as an example of a ventilation system

that can provide a specific rate per person basedon the actual number of people present. An excerptfrom the commentary is provided below.

The intent of this section is to allow the rate of

ventilation to modulate in proportion to the num-ber of occupants. This can result in significantenergy savings. Current technology can permit

the design of ventilation systems that are capableof detecting the occupant load of the space and

automatically adjusting the ventilation rateaccordingly.

For example, carbon dioxide (CO2) detectors

can be used to sense the level of CO2concentra-

tions, which are indicative of the number of occu-pants. People emit predictable quantities of CO2for any given activity, and this knowledge can beused to estimate the occupant load in a space.16

3 . 7 T H E M A T U R A T I O N O F A T E C H N O L O G Y

Demand controlled ventilation using CO2 hasbeen a well-understood principle for over 100 years.However its application and the sensor technologyprice-performance ratio have only evolved over thepast few years. DCV is now an attractive alternative tothe traditional approach of providing a fixed ventilationrate based on an assumed maximum occupancy.

Ventilation control with DCV is a recognized ven-tilation control approach in ASHRAE Standard 62-99.17

It is a recommended operational approach to controlventilation based on occupancy in the InternationalMechanical Code and as a result is now being updatedinto many local building codes.18 Leading states likeCalifornia have also integrated DCV as part of thestate building code as a method of reducing energyuse yet ensuring indoor air quality.19

Low cost CO2 sensors for DCV first appearedon the market approximately 10 years ago. As anynew technology, these first products encounteredinitial market resistance. In the case of CO2 sensors,price, calibration frequency or their size and appear-ance were issues. There are still products on themarket that have these same problems.

Carrier has discovered that these issues canbe solved with the right technology. Economicalsensors are now available that can self calibrateand offer thermostat-like dependability at thermostat-like prices. The CO2 measurement technology is nolonger a barrier to utilizing this promising ventilationcontrol approach.


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10 Jansen, J. E. 1999. The History of Ventilation and Temperature Control,ASHRAE Journal 41, no. 10: 47-52.

11 Buildings for Industry. 1957. F.W. Dodge Corporation.

12 Marks, Lionel S., Mechanical Engineers Handbook, McGraw-Hill Book Company Inc. 1916.

13 New York City, 1929 NY Building Code.

14 Section 6.1.3. ANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality, ASHRAE, 1999.

15 International Code Council, 2000 International Mechanical Code, 2000.

16 International Code Council, Commentary to International Mechanical Code, 2000.17 ANSI/ASHRAE, Standard 62-1999 Interpretation IC-62-1999-33, American Society Of Heating Refrigeration

And Air Conditioning Engineers.

18 Section 403.3 of the International Mechanical Code and associated Commentary.

19 California Energy Commission, Title 24 Non Residential Building Standards, Section 121, VentilationRequirements, 2001.

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D C V A P P L I C A T I O N F U N D A M E N TA L S



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I


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4 . 1 O V E RV I EW

This chapter provides the framework necessaryfor integrating DCV into a HVAC system designincluding evaluating potential for DCV, sizing systems,establishing a base ventilation rate and locating sensors.

Chapter 5 will provide more details on implementingthe proper control strategy and Chapters 6-8 willprovide examples of DCV designs for three differenttypes of HVAC systems.

4 . 2 C O 2 C O N T R O L A N D S T A N D A R D 6 2

DCV as a ventilation control strategy was clarified

in 1997 in interpretation IC 62-1999-33 (formerly IC62-1989-27). A copy of this interpretation is providedin Appendix B to this handbook. This interpretationidentified the ground rules for using CO2 as a methodof controlling ventilation based on real-time occupancywithin a space.

1 . The use of CO2 is applied using the VentilationRate Procedure of Standard 62, which establishesspecific cfm/person ventilation rates for mostapplications. By definition, ASHRAE Standard 62says that acceptable indoor air quality is achievedby providing ventilation air of the specified qualityand quantity (Table 2 in the Standard) to the

space. The standard states: The Ventilation RateProcedure described in 6.1 is deemed to provideacceptable indoor air quality, ipso facto.

2 . CO2 is applied using the provisions of section6.1.3.4 of the standard that address variable andintermittent occupancy. The CO2 control strategycan be used to modulate ventilation below thedesign ventilation rate while still maintainingTable 2 ventilation rates (e.g., 15 cfm per person).Sensor location and selection of the control

algorithm should be based on achieving the rates

in Table 2. The control strategy should also bedeveloped considering inside/outside CO2 differential

3 . The control strategy must provide adequate lagtime response as required in the Standard.

4 . If CO2 control is used, the design ventilation ratemay not be reduced to consider peak occupanciesof less than 3 hours (often called diversity). Inother words, the variable provision of 6.1.3.4 can-not be applied to lower the estimated maximumoccupancy for the purpose of reducing the designventilation rate while using DCV.

5 . CO2 filtration or bioeffluents removal methodsother than dilution should not be implemented inthe space.

6 . A base ventilation rate should be provided duringoccupied periods to control for non-occupantrelated sources.

7 . Where applicable the multiple spaces provision of6.1 should be applied.

4 . 3 D C V D E S I G N C O N S I D E R A T I O N S

Demand controlled ventilation does not add signif-icant complexity to HVAC design. It is no coincidencethat section 403.3.1 of the International MechanicalCode that is relevant to CO2 control is called SystemOperation. CO2 DCV is primarily an operationalparameter that should not significantly affect thedesign of the system except in the implementationof the control strategies to regulate ventilation levels.

DCV is part of an overall control strategy for abuilding and should be considered complimentary toother building control functions. Such strategies include:

Apre-occupancy purge to clear out contaminantsthat may have built up overnight during systemshut-off.

Economizer operation to take advantage oftimes when outside air can be used for freecooling (will override CO2 control).

High and low temperature limits used to protectequipment from extreme temperatures that maysignificantly exceed design conditions.

4 . 4 D E S I G N S T E P S F O R D C V

DCV is an approach that affects how the systemis operated, not how it is designed. As a result thereare only a few issues to consider when designing aDCV system. There are five simple steps to designingDCV applications:

1 . Verify that the application is appropriate for DCV.

2 . Estimate the building occupancy and calculatethe required outdoor airflow for each space basedon ASHRAE Standard 62 or other appropriate(local) code requirement.

3 . Determine the appropriate base ventilation ratefor non-occupant related sources. This will bethe minimum ventilation rate provided during alloccupied hours.

4 . Determine the appropriate control strategy to usefor the application and equipment used.

5 . Select type of sensor and determine sensor location.


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4 . 5 S T E P 1 : I S T H E S P A C E A P P R O P R I A T E F O R D C V ?

The intent of demand controlled ventilation is notto blindly or irresponsibly reduce outdoor airflow to anoccupied building in the name of energy savings. It isa reasoned, logical application of technology to actu-ally meet the letter of the standard and save energy inthe process. DCV is a reliable method of maintaininga target cfm-per-person fresh air dilution rate basedon actual occupancy. This is in contrast to providing

a fixed dilution rate based on an assumed maximumoccupancy.

DCV has been regularly applied to most buildingswith high occupant densities and frequently variableoccupancy. As CO2 sensing and control devices havebecome more economical, the list of viable applica-tions has grown to include buildings with relativelystatic total occupancies who want to utilize an activeapproach to ensure target ventilation rates are main-tained at all times in all spaces.

It is important that building designers and opera-tors understand the difference between a passive,fixed-ventilation strategy and the active ventilationstrategy of using CO2 to control ventilation on a real

time basis.The former approach requires faith that, assumingthat the building is properly commissioned, airflowswill continue to be delivered at the design ventilationrate for the life of the building. In reality, air intakes

can be adjusted by well meaning building operatorsto tweak the system either for increased comfort orenergy savings. In many cases a building can end upsignificantly over or under ventilated. Occupancy pat-terns and densities may also change over time andrender the originally established fixed rate inappropriate.

Given it is an active control system; DCV auto-matically adjusts ventilation to the appropriate levels

for the space based on actual occupancy. The needfor operator adjustments of the ventilation system isunnecessary. It is for this reason that CO2 DCV isnow being considered in many applications thathave more static occupancies like offices andschool classrooms.

The chart in Figure 4.2 provides recommenda-tions on what type of spaces are most suitable fora DCV control strategy. Most applications indicatedas possible may be suitable applications, but shouldbe evaluated by the HVAC system designer. Separatefactors may govern system selection, such as, manda-tory ventilation requirements other than ASHRAEStandard 62, pressurization between spaces (e.g.,

between kitchens and dining rooms), regular periodicrelease of building-related contaminants that are ahealth hazard to occupants, and extensive require-ments for local exhaust.

F I G U R E 4 . 1D C V D E S I G N S T E P S

1

Determine ifDCV is

Appropriate

2

Determine SpaceVentilation

Requirements

3

Determine BaseVentilation Rate

for

Non-OccupantRelated Sources

5

Select Sensorsand Determine

Sensor Location

4Choose Control

Strategy Set Point Proportional

Proportional-Integral

I


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4 . 6 S T E P 2 : D E T E R M I N I N G O U T D O O R A I R

V E N T I L A T I O N R E Q U I R E M E N T S

4 . 6 . 1 D C V O R D I V E R S I T Y

While CO2 DCV is one method of controlling forintermittent and variable occupancy the ASHRAE stan-dard also provides for another approach for some veryspecific types of uses. It is important to note that thedesigner can apply one or the other methods but notboth. The other method often called diversity allowsfor spaces to be ventilated at their average occupancy(though no less than 50% of maximum occupancy)as long as occupancy duration is 3 hours or less.

It is important to note that while the ASHRAEStandard 62 makes provisions for diversity, the IMC

and most local codes make no provisions for thismethod. Engineers should be certain local codesallow diversity calculations before incorporating it intotheir system design. The primary advantage of inte-grating diversity is that in some limited applicationsdesign ventilation rates and related equipment sizingmay be reduced.

The advantages to using a DCV approach overa diversity assumption include:

Can be applied to most types of spaces regard-less of occupancy characteristics.

Ventilation is controlled based on actual occu-pancy variation. With diversity, ventilationremains fixed for all operating conditions. Ifoccupancy density or patterns change from initialdesign assumptions, DCV will automaticallyadjust the system. If diversity assumptions areused, air quality may be compromised if actualusage is different from design assumptions.

A space CO2 measurement provides a measureof outside air delivery ventilation effectiveness toa particular zone, ensuring the ventilation systemis responsive to occupancy within the space.Assumptions of diversity cannot account for actuaventilation demand or air distribution effectiveness.

The biggest disadvantage of DCV related to adiversity assumption is that use of DCV will only affecthe operation of equipment and can not be used toinfluence equipment sizing.

F I G U R E 4 . 2R E C OM M E N D E D A P P L I C A T I ON S F OR D C V


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4 . 6 . 2 C O N S TANT VO LUM E SY STE MS

With the exception of the use of diversity dis-cussed above, sizing for constant volume systemsis identical to normal practice of multiplying the targetventilation rate for the space times the maximumnumber of occupants in the space. This value becomesthe design ventilation rate and typically is a fixedvolume of air delivered during all occupied periods.When applying DCV, the design ventilation rate becomesthe maximum ventilation position in a modulating

ventilation strategy.

4 . 6 . 3 M U LT IPL E ZONE VAV SYS T EMS

When variable-air-volume (VAV) systems formultiple space occupancies were first conceived, out-door air delivered to each space varied as a functionof the supply airflow. Supply air was varied in propor-tion to the thermostats call for heating or cooling. Inthis case the designer would have to ensure that thesystem, under all operating conditions (e.g., part loadconditions) continued to maintain the proper designventilation rate to each zone. This process is describedin section 6.1.3.1 of ASHRAE Standard 62.

In this type of application, the calculation (e.g.,Equation 6.1 in Standard 62) of required outside aircapacity involves an iterative series of calculations toensure that adequate ventilation air is introduced at thecentral air handler to ensure that even the most criticalspace (least likely to receive adequate ventilation) issatisfied. This often can result in over-ventilation andresulting wasted energy in other non-critical spaces.It can also negate any ventilation reduction that mayhave resulted from the consideration of diversity.

A recent innovation in VAV system design is theability to modulate a zone VAV box to consider bothtemperature control and CO2 for ventilation control.

In this type of system, ventilation is not limited by thespace load requirements but is controlled separatelyusing a CO2 sensor in each major occupied zone. Inthis case the signal from the CO2 sensor modulatesthe local VAV box and the outside air intake to ensureall spaces have adequate ventilation.

A common question that arises is, should adesigner use the multiple spaces sizing approachdescribed in section 6.1.3.1 for ASHRAE Standard 62with a zone level DCV control strategy.

In traditional VAV approaches the designer mustestimate what the critical space will be under variousconditions and then design to worst case circum-stances to ensure the critical space always receivesthe appropriate amount of fresh air. With VAV DCVoutside air delivery is actually measured in everyzone and the VAV box is modulated to control forboth space conditioning load and ventilation.

Since a VAV system with DCV is actually measur-ing ventilation and controlling ventilation rates to thespace independent of temperature, the multiple spacesequation is not necessary. This was clarified in arecent interpretation to the Standard, IC-62-1999-24,which stated that the 6.1.3.1 sizing is not necessary ifthe required ventilation rate is delivered to the space.

In operational situations where a specific spacemay have satisfied cooling needs but still requiresventilation, localized reheat can be applied. The energyimpact of reheat for a zone is usually far less thanover ventilating the entire building to satisfy one zone.

As a result, sizing for a multiple space VAV DCVapplication is similar to current practice for most juris-dictions. The ventilation requirements are calculatedfor the space based on recommended target cfm perperson ventilation rates established by local codes orstandards and multiplied times the maximum intendedoccupancy for the space.

4.7 S T E P 3 : C A L C U L A T I N G B A S E V E N T I L A T I O N R E Q U I R E M E N T SIn the ASHRAE interpretation IC-62-1999-33 that

clarified the use of CO2 DCV with Standard 62, a com-ment was provided that the designer should ensurethat in cases of low occupancy a non-zero base venti-lation rate is provided to handle non-occupant relatedsources whenever the space is occupied.

So, a DCV system must be designed to maintain aminimum ventilation airflow rate to control non-occupantrelated contaminants that may be given off by furnish-ings, equipment or other materials within the space.This should not be confused with the minimum outdoorair rate required by ASHRAE Standard 62 or other

codes. The base ventilation rate is the lowest pointto which CO2 controls may modulate outdoor airflowduring occupied hours.

Minimum ventilation airflow must be adequate toachieve several goals:

Balance supply, exhaust and building pressur-ization requirements

Establish the lowest outdoor air rate permissibleduring times when the building is sparsely occu-pied (i.e., immediately prior to or after businesshours, weekends, and holidays)

The designer should consider the age, condition,and contents of a building when establishing theminimum ventilation airflow rate. A new or remodeledbuilding with newly installed furnishings and finisheswill experience higher concentrations of building-relatedcontaminants than will an older building and may ini-tially require a higher base ventilation rate over thefirst few months of operation.

Retail sales areas, such as furniture and carpetstores, may experience relatively high concentrationsof building-related contaminants regardless of thebuilding age. Experience with DCV systems to date

suggests that the minimum ventilation flow for older(well aged) buildings should not be less than 20 to30% of the design ventilation rate. For new buildingsthis rule of thumb may be higher at about 40 to 50%of the design ventilation rate.

The question of base ventilation rates for sourceswithin a space has been a topic of some debate.However, ASHRAE is about to circulate for commenta series of proposed changes to the ventilation sec-tion of Standard 62 that include establishment of abase ventilation rates for sources within the space,expressed in terms of cfm/ft2. The chart in Figure 4.3combines the draft recommended base ventilation

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F I G U R E 4 . 3BASE V E N T I LATION RATES RECOM MENDED IN A C HANGE TOS TA N D A R D 6 2 C U R R E N T L Y OU T F OR C OM M E N T

rates from the proposed change with current ventila-tion requirements in the Standard 62/IMC and calcu-lates what the base ventilation rate should be as apercent of the OA design capacity.20 Looking at thenumbers, the 20-30% of design appears to be a goodconservative rule of thumb for most applications. It isimportant to note that these levels represent a draft ofa proposed standard and in no way reflects if or when

it will be part of an upcoming standard.Setting the base ventilation rate in a constant vol-

ume or VAV system has caused some confusion onwhether or not to specify a device to monitor airflow inthe outside air duct to ensure that the base ventilationrate is being met during all load conditions. Going tothis level of expense isnt necessary. This setting caneasily be handled during the test-and-balance phaseof the project.

Setting the base ventilation rate in a constant vol-ume system can be done by calculating the base ven-tilation rate as a percentage of the total supply airflowand setting the outside air damper to this percentage.For a more accurate value, the outside air duct can be

traversed for the base ventilation airflow, and then theoutside air damper can be set to match this reading.

The following process will take care of setting thebase ventilation rate for a VAV system:

1 . Set all zone dampers to their design baseventilation rate. (See Chapter 6 on determiningbase ventilation rates for VAV systems.)

2 . Ensure the VFD or inlet guide vanes have

modulated to maintain the design static pressureset point.

3 . Traverse the Outside Air Duct and set the AHUcontrollers Outside Air damper actuator tomaintain the system design base ventilation.This will typically be set as a percentage ofdamper position.

Since the lowest volume of air the VAV systemcan produce at any given time equates to the baseventilation rate, the above setup will ensure that theoutside air damper will always deliver the base venti-lation rate, even at very light loads.


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4 . 8 S T E P 4 : S E L E C T D C V C O N T R O L S T R A T E G Y

The control strategy used to modulate ventilationbased on CO2 levels is the most critical step in the DCVdesign process. The control strategy affects the respon-siveness of the ventilation system to control ventilation

rates based on actual occupancy and will significantlyaffect the possible energy savings achievable over afixed ventilation approach. Chapter 5 provides detailson selecting the most appropriate control strategy.

4 . 9 S T E P 5 : L O C A T I N G C O 2 S E N S O R S

Much of the same logic that goes into selecting

a thermostat location can be applied to selecting anappropriate location for a CO2 sensor. The key is toselect a location where the sensor can accuratelymeasure the CO2 concentration and is representativeof the area or zone served. The exact criteria will varybetween different buildings and system types. In eachcase, the designer must apply good engineering judg-ment to assure that both the sensors and the completeventilation system performs effectively.

In general, a CO2 sensor will be less susceptibleto stratification issues than temperature sensors dueto the tendency of gases to quickly equalize within aspace. A special consideration for CO2 sensor place-ment is to ensure it is not located in an area where

people might be directly breathing on the sensor(e.g., near water cooler/coffee service areas).

4 . 9 . 1 I N - S PACE OR DU CTM O U N T E D S E N S O R S ?

Measurement of CO2 in the space using wall-mounted sensors is preferred for the same reason thattemperature sensors are mounted within the space.

In multiple space applications, duct-mountedsensors may reflect an average of all spaces and willnot provide indication of ventilation requirements inindividual zones. The result is that ventilation to theindividual spaces (i.e., the critical space) may not bemaintained and compliance to ASHRAE 62 require-

ments will be compromised.Space-mounted sensors can also give a good

indication of the ventilation effectiveness in the spaceand will operate the system based on the characteristicsof the space. Duct-mounted sensors cannot indicateventilation effectiveness.

The principal driver for use of duct-mounted sen-sors has been to reduce costs by reducing the numberof sensors required for a job. In the past few years,CO2 sensor pricing has dropped dramatically meaningthat the cost difference between using duct-mountedand multiple space-mounted sensors is a minimalportion of job cost. Some sensors now even combinetemperature and CO2 measurement functionality to

further reduce purchased and installed cost.Some general guidelines for placement of wall-

mounted sensors is provided below.

Select a location that is reasonably centered inthe zone.

When a single sensor serves multiple spaces,the space most sensitive to the ventilation rateshould be selected.

A sensor should be installed in each zone

that is separately controllable (e.g., multi-zonesystems or variable-air-volume systems withmultiple zones).

Avoid locations near doorways, operablewindows or air vents.

F I G U R E 4 . 4C O2 SE NS OR PL AC E M E N T

R E C O M M E N D AT I O N S

Duct-mounted CO2 sensors are best suited tosingle zone systems that run continuously.

Guidelines for installation of duct-mounted sensorsare provided below.

Duct-mounted CO2 sensors should be locatedto serve a single zone, or multiple spaces withina single zone that have similar activity levels.

Locate the sensor as near as possible to thespace being served.

When using duct-mounted sensors for ademand controlled ventilation system, thedesigner must consider ventilation effective-ness in the occupied space (just the sameas is necessary when using the VentilationRate Procedure).

Locate duct-mounted sensors where they areaccessible for inspection and maintenance.

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4 . 1 0 S E N S O R L O C A T I O N - C O N S T A N T V O L U M E S Y S T E M S

A single CO2 sensor is suitable for open areas upto about 5,000 square feet. If a building is designedwith a large open area as a single zone greater than5,000 square feet, multiple CO2 sensors should beused. If a large open area is conditioned with multipleunits (e.g., multiple rooftop units) each separate unitshould be equipped with a CO2 sensor that is locatedcentrally in the area conditioned by that unit.

In systems that have multiple zones, but onlyone location to control the flow of outdoor air (e.g.,constant-volume, single-zone rooftop units), multipleCO2 sensors may be required. This is especially trueif there are different zones in the space with differentoccupancy patterns. In this situation a CO2 sensorshould be placed in each of the major occupied zones

and the outdoor air delivery should be modulatedoff the sensor with the highest reading. Inexpensivetransducers are readily available that are able to takemultiple analog signals and pass thorough the signalthat is highest to the equipment. Figure 4.4 providesadditional guidance.

4 . 1 0 . 1 S E N S O R S E L E C T I O N -

VA V S Y S T E M SIn variable-air-volume systems, CO2 sensors

should be located in each major zone of occupancy. Insome cases this may mean that one CO2 sensor canbe used for multiple VAV boxes if all serve a commonarea with similar occupancy patterns and densities.

20 This proposed change for the standard is called 62n. While not addressed in this handbook, the proposedchanges also alter the recommended cfm/person requirements for most applications. The reader cancheck with ASHRAE or the ASHRAE web site for the latest information on the proposed 62n addendum toANSI/ASHRAE Standard 62.


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5 . 1 O V E RV I EW

DCV involves more than just measuring CO2.The designer must implement an effective controlstrategy to ensure that target cfm-per-person ventila-tion rates are maintained to match occupancy within areasonable lag time. This chapter specifically address-es the steps involved in selecting an appropriate DCVstrategy. These steps include:

1 . Since ventilation control using CO2 is based onan inside/outside differential (discussed in

Chapter 2), the designer must decide how toaccount for outside concentrations.

2 . Determine the equilibrium anchor point for thecontrol strategy that corresponds to the cfm-per-person ventilation requirements for the space.

3 . Select the appropriate control strategy for the

application and type of equipment used.

5.3 S T E P 2 : E S T A B L I S H I N G T H E C O 2 E Q U I L I B R I U M A N C H O R P O I N T

Chapter 2 of this handbook explained the rela-tionship between CO2 levels and ventilation rates. Itdescribed how CO2 concentrations in a space at aknown activity level could be related to the ventilationrate of the space. In this step the CO2 equilibriumanchor point will be established.

It is important to note that the equilibrium anchorpoint is not necessarily a control set point but rather a

reference for establishing the control strategy. Thisanchor point will influence how the control strategywill modulate ventilation. It is not necessarily used asthe control point. The anchor point does become themaximum permissible CO2 level that the DCV controlstrategy will allow.

ASHRAE Standard 62 (Section 6.2.1) states thata differential concentration not greater than 700 ppm

Since DCV is based on determining and control-ling ventilation rates based on an inside/outside differ-ential of CO2, the designer must consider how to inte-grate outside concentrations into the control strategy.

There are two possible options for accounting foroutside concentrations.

The designer can mount a sensor in the outsideair to measure outside concentrations and design

the system to consider real-time inside/outsidedifferential measurement.

The designer can take a measurement or makea conservative assumption of what outside con-centrations will be for that region.

5 . 2 . 1 D I R E C T M E A S U R E M E N T

At first glance, the most obvious approach to con-trol ventilation based on CO2 is to actively measureoutdoor levels and active control based on the realtime CO2 differential measured. In this type of appli-cation a CO2 sensor must be selected that is capableof withstanding the wide range of conditions found in

outside air.This approach is the most expensive of the two

approaches examined and for most applicationsoverkill because globally outdoor levels do not varysignificantly. Outside air sensors are best consideredwhen outside levels at the air-intake are found to varysignificantly due to localized sources of combustion(e.g., loading dock or vehicle idling area).

If high variations of CO2 are measured, it is astrong indication that other harmful combustion relatedcontaminants are also present. Excessive levels ofCO2 in outdoor air may be used as a trigger to closedown air intakes until a localized source is dissipated.

As discussed in Appendix F, Carrier does offer an

outdoor CO2 sensing option for outdoor air, but it

should only be considered if the possibility of significantvariations in outdoor air concentrations is anticipated.

5 . 2 . 2 M E A S U R E M E N T O R A SS UM P T I ON O F O U TS I D EC O2 C O N C E N T R AT I O N S

In most areas designers can utilize a strategywhere outside ambient concentrations are assumedfor the purpose of implementing a control strategy.

The key is to pick a level that represents averageregional concentrations. This level can be determinedby making periodic measurements throughout the dayover a number of days with a hand held CO2 sensor.Alternatively, a safe assumption is that outside levelsare 400 ppm.

It is unlikely that concentrations in any urban areawill be lower than 360 ppm (the lowest level measuredin the middle of the Pacific ocean see Chapter 2). If acontrol approach assumes an outside level of 400 andoutside levels are actually 50 ppm lower, the net resultwill be that cfm/person ventilation rates may be lowerby about 1 cfm per person. This error is generally wellwithin the tolerances of any HVAC control system and

will have a negligible effect on indoor air quality.If CO2 levels tend to be slightly higher, the net

effect is that for every 50 ppm the actual outside levelis over the assumed outside level, the space will beslightly over ventilated by 1 cfm/person. It is highlyunlikely that outside levels will ever be over 500 ppm,unless a localized source of combustion exists. If out-side levels are higher by 100 ppm, the result is overventilation by about 2 cfm per person, again a negligi-ble amount that errors on the side of more ventilation.

If a control strategy is based on an assumedbaseline of 400 ppm or derived from local measure-ments, variation in outside levels will never result in asignificant impact on cfm per person ventilation rates

that might impact indoor air quality.

5.2 S T E P 1 : C O N S I D E R A T I O N O F O U T D O O R C O N C E N T R AT I O N S


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5 . 4 S T E P 3 : C O N T R O L S T R A T E G Y S E L E C T I O N

5 . 4 . 1 THR EE CO NTROL ST RAT E G I E S

There are three recommended control approachesfor utilizing DCV that are dependent on the applicationand type of equipment used in the space. These threeapproaches are detailed below.

1 . Set Point Control2 . Modulated Proportional Control3 . Modulated Proportional-Integral Control

Agraphical representation of the three approachesis provided in Figure 5.2. Each is discussed in greaterdetail in the sections that follow. Detailed sequence ofoperation for these control strategies are also providedin Appendix D of this handbook.

5 . 4 . 2 C O N S I D E R AT I O N O FC O N T RO L RE SP ON SE T I M E

An important consideration in the selection of acontrol strategy is to ensure it is reasonably responsiveto changes in the space so that if occupancy changesthe system will react in a reasonable amount of timeto ensure the target cfm/person ventilation rate is met.

ASHRAE Standard 62-1999 (6.1.3.4) discussesprovisions for leading and lagging ventilation as partof the overall discussion of intermittent and variableoccupancy. It is important to note that this is a recom-

mendation, not a mandatory requirement of thestandard.23 Appendix G of the Standard 62 providesthe rationale applied in ASHRAEs recommendationsfor ventilation lead and lag times. Readers shouldrefer directly to Standard 62 if they require furtherinformation on calculating lag times.

indicates that comfort (odor) criteria related to humanbioeffluents are likely to be satisfied. But, 700 ppm isnot intended as a universal rule of thumb and may notbe appropriate in all circumstances. As described inChapter 2, the 700 ppm differential is considered equiv-alent to 15 cfm per person if the activity level in thespace is similar to an office like environment (1.2 MET).

If local codes require a different ventilation rateother than 15 cfm/person a different CO2 control pointmust be selected. Figure 5.1 provides a reference for

the equilibrium control rate for a range of cfm/personventilation rates. The background to how these con-centrations are established is discussed in Chapter 2.The values in Figure 5.1 are inside/outside differentialvalues and must be added to the outside level todetermine the actual equilibrium anchor point.

If activity levels are significantly different than thatof an office type activity then CO2 production will alsobe different and the resulting CO2 equilibrium controlpoint will be different for a given ventilation rate.

The volume of CO2 expelled from a person, maleand female, is fairly uniform for adults and childrenage 18 and older. The rate of CO2 generation is aproportional function of the metabolic rate, which isdirectly related to a persons physical activity. TheASHRAE 1997 Fundamental Handbook(Chapter 8)contains a detailed discussion of metabolic heatgeneration for various activities.21

If a space is likely to have a higher activity levelthan that of an office space (1.2 MET) the designer canconsider one of two options. The first option is to calcu-late a new set point for the cfm/person target ventilationrate based on the higher activity level. Appendix C ofthis handbook provides details on how to do this.

For spaces with a higher activity level than 1.2MET the second option is to use the equilibrium con-trol point calculated for 1.2 MET. This approach willresult in over ventilation above the target ventilationrate but this could be desirable. The 15 cfm/per per-son ventilation guidelines established in the Standard62 were based on studies of occupant satisfactionrelated to perception of body odor for individuals inoffice type environments.22 CO2 production is oneparameter of metabolic activity. If activity increases

other gases and bioeffluents will be produced inincreasing quantities as well. As a result higher venti-lation rates may be necessary to maintain the satis-faction levels of perceived odors within the space.

F I G U R E 5 . 1D I F F E R E N T I A L E QU I L I B R I U M C ON T R OL P OI N T SFOR VARIO US V E N T I LATIO N RATES (1 .2 MET )

F I G U R E 5 . 2

THR EE DCV CONT ROL STR AT E G I E S

I

Outside CO2Concentration

Inside/Outside Differential CO2 ppm

Base Ventilation Rate

Design Ventilation Rate

100 300 500 700


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Lagging ventilation is acceptable only if thefollowing conditions are met:

Contaminants are dominantly a function of peopleand their activities.

Contaminants do not represent a short-termhealth hazard.

Contaminants are dissipated during unoccupied

times so that the indoor air quality is equal toacceptable outdoor air.

DCV is a perfect example of where lagging venti-

lation would be appropriate because it is an occupancybased ventilation approach.

Designers should note that the base ventilation ratecan satisfy part of the lag requirements but the responseof the sensor to changes in occupancy is also an impor-tant consideration. In the discussion that follows eachcontrol strategy is discussed with regard to the bestapplications to ensure adequate lag time response.

For those interested in modeling and evaluati

26763263 demand controlled ventilation

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