Demand Controlled Ve n t i l a t i o n System De s i g n

Demand Controlled Ve n t i l a t i o n System De s i g n PROVIDING THE RIGHT AMOUNT OF AIR, IN THE RIGHT PLACE, AT THE RIGHT TIME S AVING ENERGY COSTS WHILE OPTIMIZING INDOOR AIR QUA L I T Y

T A B L E O F C O N T E N T S 1. I N T RO D U C T I O N 1.1 Why a Handbook on Ventilation Control...................................1 1.2 CO2 as an Important Control Strategy.....................................1 1.3 Overview...........................................................1 1.4 Summary...........................................................1 2. C O 2 B A S I C 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 2.1 The CO2 DCV Concept...............................................5 2.2 Atmospheric CO2....................................................5 2.3 Indoor CO2 Concentrations.............................................5 2.4 CO2 Differential and Ventilation Rates.....................................6 2.5 CO2 Control Considerations............................................7 2.6 CO2 as a Contaminant................................................7 2.7 DCV Benefits.......................................................7 3. V E N T I L AT I O N, B U I L D I N G C O D E S A N D C O 2 3.1 Ventilation..........................................................9 3.2 The Evolution of Mechanical Ventilation...................................9 3.3 CO2 Control... A New Idea?............................................9 3.4 Ventilation Standards..................................................9 3.5 CO2 and ASHRAE Standard 62.........................................10 3.6 DCV and Building Codes..............................................11 3.7 The Maturation of a Technology........................................11 4. D C V A P P L I C AT I O N G U I D E L I N E S 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 4.1 Overview..........................................................15 4.2 CO2 Control and Standard 62..........................................15 4.3 DCV Design Considerations...........................................15 4.4 Design Steps for DCV................................................15 4.5 Step 1: Is the Space Appropriate for DCV?.................................16 4.6 Step 2: Determining Outdoor Air Ventilation Requirements....................17 4.6.1 DCV or Diversity..............................................17 4.6.2 Constant Volume Systems........................................18 4.6.3 Multiple Zone VAV Systems......................................18 4.7 Step 3: Calculating Base Ventilation Requirements...........................18 4.8 Step 4: Select DCV Control Strategy.....................................20 4.9 Step 5: Locating CO2 Sensors..........................................20 4.9.1 In-Space or Duct Mounted Sensors?................................20 4.10 Sensor Location - Constant Volume Systems................................21 4.10.1 Sensor Selection - VAV Systems....................................21 Use of the information contained in this Manual is voluntary, and reliance on it should only be undertaken after independent review of its accuracy, completeness, and timeliness. Carrier, including its employees and agents, assumes no responsibility for consequences resulting from 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 such information. No guarantees or warranties, including, but not limited to, any express or implied warranties of merchantabilities or fitness for a particular use or purpose, are made by Carrier with respect to such information. 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

5. D C V C O N T RO L S T R AT E G I E S 5.1 Overview..........................................................23 5.2 Step 1: Consideration of Outdoor Concentrations............................23 5.2.1 Direct Measurement............................................23 5.2.2 Measurement or Assumption of Outside CO2 Concentrations.............23 5.3 Step 2: Establishing the CO2 Equilibrium Anchor Point.......................23 5.4 Step 3: Control Strategy Selection........................................24 5.4.1 Three Control Strategies.........................................24 5.4.2 Consideration of Control Response Time.............................25 5.5 Set Point Control....................................................25 5.6 Proportional Control.................................................25 5.7 Proportional-Integral Control...........................................25 5.8 Two Stage Control for Zone Based VAV Control.............................26 5.8.1 Establishing Zone and System Set Points.............................26 5.8.2 Sizing of Zone Heating Coils......................................26 S E C T I O N I I I D e s i g n E x a m p l e s E X A M P L E 1 : S I N G L E ZO N E R E TA I L S PA C E 6.1 Application: Retail Clothing Store.......................................29 6.2 Step 1: Is the Space Appropriate?........................................29 6.3 Step 2: Determine Ventilation Requirements for the Space......................29 6.4 Step 3: Determine Base Ventilation Rate...................................29 6.5 Step 4: Determine Control Strategy......................................30 6.5.1 Considering Outside Concentrations................................30 6.5.2 Determine CO2 Control Points....................................30 6.6 Step 5: Locate Sensors................................................30 6.7 Installation Summary.................................................30 EX A M PL E 2 : S I N G L E A I R H A N DL E R S E RV IN G M U LT I P L E Z O N E S S C H OO L 7.1 Application: Four Classrooms Served By One Air Handler......................31 7.2 Step 1: Is the Space Appropriate?........................................31 7.3 Step 2: Determine Ventilation Requirements for the Space......................31 7.4 Step 3: Determine Base Ventilation Rate...................................31 7.5 Step 4: Determine Control Strategy......................................31 7.5.1 Considering Outside Concentrations................................32 7.5.2 Determine CO2 Control Points....................................32 7.5.3 Selecting the Control Strategy.....................................32 7.6 Step 5: Locate Sensors................................................32 7.7 Installation Summary.................................................32 EX A M P L E 3 : M U LT I PL E ZO N E O F F I C E W I T H VAV 8.1 Application: 14 Zone Office with VAV....................................33 8.2 Step1: Is the Space Appropriate for DCV?..................................33 8.3 Step 2: Determine Ventilation Requirements for the Space......................33 8.3.1 Maximum Airflow..............................................34 8.3.2 Minimum Airflow..............................................34 8.4 Step 3: Determine Base Minimum Ventilation Rate for DCV...................35 8.5 Step 4: Determine CO2 Control Strategy..................................35 8.6 Step 5: Locating CO2 Sensors..........................................36 8.7 Installation Summary.................................................36 A P PEN DIX A: Glossary...........................................................37 A P PEN DIX B: ASHRAE 62 Interpretations IC 62-1999-33................................39 A P P EN DIX C: CO2 Equilibrium Anchor Points for Alternative Activity Levels..................41 A P P EN DIX D: Sequences of Operation...............................................43 A P P EN DIX E: Guide Specifications..................................................45 A P P EN DIX F: Background on CO2 Sensor Technology...................................47 A P P EN DIX G: DCV Compatible Equipment...........................................53 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 W H 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 a strong understanding of a simple but powerful method of 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 a new and an old concept. Windows were the first effective method of ventilation control. Every building built before the 1920s incorporated operable windows, which provided both light and a method of ventilation control that allowed variable control of fresh air on a room-by-room basis. As central mechanical systems for heating and cooling became more common, active zone/room based control of ventilation was lost and replaced by a centrally delivered, fixed ventilation rate intended for the entire building. The result was better temperature control but a loss in flexibility in ventilation control. This passive, fixed ventilation approach was applied with mechanical systems, because until recently there was not an inexpensive method of measuring and controlling ventilation at the zone level. As described in this handbook, indoor CO2 levels have been used as an indicator of outside air ventilation rates for over 90 years. Carbon dioxide ventilation control or demand controlled ventilation (DCV) allows for the measurement and control of outside air ventilation levels to a target cfm/person ventilation rate in the space (i.e., 15 cfm/person) based on the number of people in the space. It is a direct measure of ventilation effectiveness and is a method whereby buildings can regain active and automatic zone level ventilation control, without having to open windows. Recent technical developments in CO2 sensor design and advancement in equipment control now provide an opportunity for this active ventilation control within a competitive market environment. Zone level control of ventilation can also avoid the cost often related to over ventilating a building continually to ensure that one critical zone receives adequate fresh air under all operating conditions. CO2 based ventilation control is a dynamic system that responds to how the building is used and occupied. It is a real time control approach that offers a vast improvement over ventilating a building at a fixed rate based on some pre-construction constant occupancy assumptions. The fixed ventilation approach depends on a set-it-and-forget-it methodology that is completely unresponsive to changes in the way spaces are utilized or how equipment is maintained. This handbook was undertaken because there is no single source that designers can rely on to understand and design HVAC systems incorporating CO2 v e n t i l a t i o n control. This handbook represents the state of the art in the use of CO2 based ventilation control and has drawn on numerous technical papers, codes and standards as well as a wealth of field applied experience. 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 Carrier Corporation believes that ventilation control with CO2 ( C O2 DCV) is an important building control technique that can be applied in to most buildings and types of building occupancies. Carrier believes so strongly in this approach that now every piece of Carrier equipment that provides ventilation has an input and built-in control strategy to utilize CO2 control. It is a proven method to control outdoor air based on actual occupancy. DCV meets the code, it saves money, and it doesn t guess. Carrier is particularly excited about the high degree of comfort that can be provided to multi-zone spaces that utilize VAV systems with active ventilation control with CO2. By integrating zone control of both temperature and ventilation it is possible to measure and control ventilation to ensure that adequate fresh air is actually delivered to all spaces. Previous approaches could only ensure adequate fresh air at the 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 the traditional dependence that ventilation has had on space conditioning load. It is now possible to control fresh air and space conditioning to a zone independently using the same VAV box. The result is that the designer does not have to oversize outside air intake capacity to handle low load conditions. Significant energy can be saved over traditional VAV approaches. 1. 3 O V E R V I E W This Handbook is divided into three sections: Section I which includes Chapters 2 and 3, establishes the background necessary to understand how CO2 DCV works and how it is applied under current codes and standards. Section II which includes Chapters 4 and 5 addresses the application fundamentals necessary to 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 a valuable point of reference to the reader and provides more details on DCV applications for special circumstances, detailed sequence of operation for various types of equipment, and additional details on the sensors and equipment offered by Carrier for DCV. 1. 4 S U M M A R Y This handbook is intended to provide a clear engineering rationale for CO2 based DCV that can be used by the HVAC industry to properly apply this promising technology. Much has been written on the topic of DCV as a good idea. This handbook begins with the conclusion that CO2 DCV is a good idea and then answers the nuts-and-bolts questions of how to apply it. 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

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

I B AC KG RO U N D O N C O 2 A N D V E N T I L AT I O N C O N T RO L 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 3

I 4 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

2. 1 T H E C O 2 D C V C O N C E P T I Carbon Dioxide (CO2) based demand controlled ventilation (DCV) is an economical means of providing outdoor air to occupied spaces at the rates required by local building codes and ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality. Engineers and building owners both lament the high cost of conditioning outdoor air, and the inexact control methodologies that often result in significant over ventilation of spaces. CO2-based DCV offers designers and building owners an ability to monitor both occupancy and ventilation rates in a space to ensure there is adequate ventilation at all times. Ty p i c a l l y, most ventilation systems are set up and adjusted only when they are installed. DCV offers a higher level of control in that it monitors conditions in the space and constantly adjusts the system to respond to real time occupancy variations. The result is that target cfm-per-person rates as established by local codes and standards are maintained based on actual occupancy. Costly over ventilation that typically results from a fixed ventilation strategy (design occupancy X cfm/person) is avoided and energy usage can be reduced. Measurement of CO2 concentrations is an accepted scientific methodology to determine the actual ventilation ra t e in a building. The use of CO2 t o control ventilation rates in buildings is also recognized as a valid control approach in ASHRAE Standard 62 and in model building codes used as reference by most local code bodies. 2. 2 AT M O S P H E R I C C O 2 CO2 is one of the most common compounds in our atmosphere. It is also cited by many as a general indicator of the buildup of greenhouse gases and global warming. Figure 2.1 summarizes data collected from the Mauna Loa Observatory in Hawaii over the past 40 years. 1 The chart shows the gradual increase of CO2 concentrations by 1 to 2 ppm per year. Given its isolated location in the middle of the Pacific Ocean, these concentrations likely represent the lowest concentrations that will be found worldwide. In urban areas outdoor CO2 levels typically range from 360 up to as high as 450 to 500 ppm due to the presence of localized sources of CO2 which can include any combustion device or process. Higher outdoor levels can also be measured when in close proximity to a source of CO2 such as an idling vehicle or a furnace or combustion exhaust. Because of its low molecular weight CO2 will readily diffuse and equalize within an open space. F I G U RE 2. 1 C O2 C O N C E N T R AT IO N S ME A S UR E D I N H AWA I I O V E R 4 0 Y E A R S As a result, outside CO2 levels tend to be ubiquitous and fairly constant over large geographic regions. Because of this consistency, it is possible to use CO2 as a baseline reference for outside air for the purpose of measuring and controlling ventilation. 2. 3 I ND O O R CO 2 C O N C E N T R AT I O N S Indoors in commercial buildings people are the principal source of CO2. Plants, due to their low level of metabolic activity contribute an insignificant amount of CO2 to indoor spaces. Unvented combustion sources can also contribute to indoor CO2 concentrations but are generally not present in commercial buildings. In fact highly elevated levels of CO2 (e.g., 3000 to 5000 ppm) can indicate the presence of potentially dangerous combustion fumes. CO2 is one of the most plentiful byproducts of combustion and can account for 8% to 15% by volume of the content of a combustion exhaust. For ventilation control, it is people as a source of CO2 that we are interested in. People exhale predictable quantities of CO2 in proportion to their degree of physical activity. This relationship is shown in Figure 2.2 and is taken from Appendix D of ANSI/ASHRAE Standard 62-1999, Ventilation For Acceptable Indoor Air Quality. 2 F I G U RE 2.2 C O 2 P RO DU CT IO N A N D A C T I V I T Y L E V E L 1.25 1.00 0.75 0.50 0.25 Activity Level Very Light Moderate Light 40 30 20 10 0 1 2 3 4 5 Physical Activity - MET Units 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 5

I Because CO2 production is so consistent and predictable, it can be used as a good indicator of general occupancy trends. For example, if the number of people in the space is doubled, the amount of CO2 produced will double. If one or two people leave a space the CO2 production will decrease correspondingly. It is important to note that an indoor CO2 m e a s u r e m e n t does not provide enough information to actually count people but it can be used in combination with outside air concentrations to calculate, measure and control ventilation rates. An indoor CO2 measurement is a dynamic measure of the number of people in a space (exhaling CO2) and the amount of outside air at baseline CO2 concentration that is being introduced for dilution via mechanical ventilation and/or infiltration. The result is that it is possible to determine cfm/person ventilation rates 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 S Figure 2.3 shows the typical pattern of buildup of C O2 in a space with office type activity (1.2 MET). T h e chart assumes a steady-state condition where a constant occupancy is present and the ventilation rate is constant. Once people enter a room, CO2 c o n c e n t r a- tions will begin to increase. These levels will continue to increase until the amount of CO2 produced by the space occupants and the dilution air delivered to the space are in balance. This is called the equilibrium point. F I G U R E 2. 3 C O 2 E QU I L I B R IU M L E V EL S A N D PE R P E R S O N V E N T I L AT IO N R AT E S 2,500 2,120 The relationship between indoor/outdoor CO2 d i ff e r e n- tial and ventilation rate is independent of population d e n s i t y. However, population density will affect the time it takes for CO2 to build up to an equilibrium level. This equation only applies when equilibrium conditions exist. This is particularly important when trying to infer space ventilation rates from a spot measurement when non steady-state conditions exist. 4 To make an accurate determination of cfm/person rates one should take C O2 measurements when occupancy has stabilized. Measuring CO2 concentrations that are still in transition to an equilibrium level can result in over estimation of the ventilation rate. Applied properly, spot measurements can be extremely useful in helping to qualify if a space is over or under ventilated. The ANSI/ASHRASE Standard 62 states that: 2,000 1,500 1,000 500 1,050 700 500 350 0 Comfort (odor) criteria with respect to human bioeffluents are likely to be satisfied if the ventilation results in indoor CO2 concentrations less than 700 ppm above the outdoor air concentration. 5 F I GU R E 2. 4 C A RR I E R 70 0 1 H A N D - H EL D C O 2 M O N I TO R Time Note:Assumes office type activity level (1.2 MET) As can be seen from Figure 2.3 the equilibrium point corresponds to a specific ventilation rate per person in the space. The equilibrium level for a particular space can be calculated using a simple mass balance equation found in Appendix D of ANSI/ASHRAE Standard 62-1999. 3 Vo = N/(Cs Co) Where: Vo = outdoor air flow rate per person N = CO2 generation rate per person Cs = CO2 concentration in the space Co = CO2 concentration outside Note that the activity level for the space is a component of calculating N. This will be discussed in greater detail later in this handbook. The equation can also be restated so that the equilibrium level (Ceq) for a particular ventilation rate can be calculated. Ceq = Cs = Co + N/Vo Appendix D of Standard 62 provides an example that shows how this 700 ppm level is derived from the 15 cfm per person minimum ventilation rate established in the standard. The calculation below assumes an activity level of 1.2 MET which would be considered equivalent to office type activity. Average CO2 p r o- duction at this activity level (as provided in Figure 2.2 taken from Appendix D of Standard 62) is 0.30 l/min or 0.0106 cfm. Outside CO2 concentrations are assumed 6 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

to be 400 ppm. If this is the case then the CO2 l e v e l for 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 for 20 cfm per person the inside outside differential would be approximately 500 ppm or an absolute level of 900 ppm (assuming 400 ppm outside). Chapter 5 discusses how this equilibrium level is used as part of a control strategy. I 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 in a building, the use of the equilibrium level is used differently than it would be when using a portable monitor to determine ventilation rates based on a spot measurement. While it is possible to provide a CO2 control strategy where outside air would only be introduced once the equilibrium level is reached, it may not achieve the best results. This is because of the lag time that may result between when people enter a space and when CO2 levels reach the appropriate equilibrium set point (e.g., 700 ppm inside/outside differential). When applying ventilation control with DCV the equilibrium level actually becomes one component or anchor point in a CO2 control algorithm. The actual choice of algorithms will be discussed in Chapter 5 and is based on providing a control strategy that is responsive to changes in occupancy so that the target per person ventilation rate can be provided within a reasonable lag time. 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 threatening contaminant at the levels normally found in buildings (400 3,000 ppm). In fact, in industrial environments OSHA has established an 8-hour exposure level of 5000 ppm and a 15-minute maximum exposure limit of 30,000 ppm. 6 In commercial buildings CO2 is used as an indicator of the per-person ventilation rate in a space, not as a contaminant. So why is it that many people have observed that as CO2 levels rise above 1000 ppm range, increased drowsiness, lethargy and discomfort can occur? This is because CO2 is an indicator of ventilation. As CO2 levels rise above 1100 ppm (a 700 ppm differential between inside and outside) ventilation rates begin to drop below 15 cfm/person. While CO2 is building up other space and people related contaminants are also increasing. It is these other contaminants that are creating the physiological effects. The buildup of CO2 is an indicator of low ventilation rates that generally will result in higher levels of all types of contaminants and a greater level of occupant dissatisfaction. An excerpt from an interpretation to ASHRAE Standard 62-99 provides a good summary of how CO2 is used in the standard: 700 ppm above outdoors is the steady-state carbon dioxide concentration differential corresponding to a constant ventilation rate of 15 cfm/person of outdoor air in a space occupied by sedentary adults. Chamber studies have shown that 15 cfm/person and indoor carbon dioxide concentrations that are about 700 ppm above outdoors correspond to 80% satisfaction of visitors to 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, DCV offers considerable advantages. Carbon Dioxide based DCV does not affect the design ventilation capacity required to serve the space; it just controls the operation of the system to be more in tune with how a building actually operates. Excessive over-ventilation is avoided while still maintaining IAQ and providing the required cfmper-person outside air requirement specified by codes and standards. Operational energy savings of $0.05 to $1.00 per square foot annually can result. This observation has been verified in a recent literature review on CO2 control that sighted numerous studies where energy savings from DCV control approaches ranged from 5% to 80% versus a fixed ventilation strategy. 8 System paybacks can range from a few months to two years and are often substantial enough to help pay for other system or building upgrades. The payback from CO2 DCV will be greatest in higher density spaces that are subject to variable or intermittent occupancy that would have normally used a fixed ventilation strategy (e.g., theaters, schools, retail establishments, meeting and conference areas). In spaces with more static occupancies (e.g., offices) DCV can provide control and verification that adequate ventilation is provided to all spaces. For example a building operator may arbitrarily and accidentally establish a fixed air intake damper position that results in over or under ventilation of all or parts of a space. A CO2 control strategy can ensure the position of the intake air dampers is appropriate for the ventilation needs and occupancy of the space at all times. In some buildings, infiltration air or open windows may be a significant source of outside air. 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 7

I A CO2 sensor will consider the contribution of infiltration in a space and only require the mechanical system to make up what is necessary to meet required ventilation levels. These savings are in addition to those quoted above. CO2 monitoring and control is considered an important part of green building design. It is one of the criteria that can now be used to meet the LEED (Leadership In Energy And Environmental Design) criteria for green building design. 9 When integrated with the appropriate building control strategy, ventilation can be controlled on a zone-by-zone based on actual occupancy. This allows for the use of transfer air from under-occupied zones to be redistributed to areas where more ventilation is required. A control strategy can be used to maintain any per-person ventilation rate. As a result this approach is highly adaptable to changing building uses and any changes that may occur in future recommended ventilation rates. DCV can provide the building owner/manager with valuable information about occupancy trends and the status of equipment operation. This information can be documented and recorded by a digital building control system. C O2 demand control ventilation is a real-time, occupancy based ventilation control approach that can o ffer significant energy savings over traditional fixed ventilation approaches. Properly applied, it allows for the maintenance of target per-person ventilation rates at all times. Even in spaces where occupancy is static, C O2 DCV can be used to ensure that every zone within a space is adequately ventilated for its actual occupancy. Air intake dampers, often subject to maladjustment, or arbitrary adjustments over time can be controlled a u t o m a t i c a l l y, avoiding accidental and costly over or under ventilation. 1 Atmospheric CO2 concentrations (ppm) derived from in situ air samples collected at Mauna Loa Observatory, Hawaii. Source: C.D. Keeling, T.P. Whorf, Scripps Institution of Oceanography University of California, 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 P e r s i l y, 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 To ANSI/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. 8 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

3. 1 V E N T I L AT I O N I In early part of the last century, in the days before central air-conditioning systems, ventilation was a much more natural subject than it is today. Not that ventilation has become unnatural, but prior to World War II, central air-conditioning was rare. Buildings were cooled using natural ventilation and the modern contaminants that today may lead to Sick Building S y n d r o m e were unknown. In the typical prewar off i c e building, odors, tobacco smoke, and combustion byproducts from heating appliances were the principal contaminants of concern. Ventilation was achieved naturally through windows and doors. Infiltration, by design, was often the only source of fresh air in winter months. 3. 2 T H E E V O L U T I O N O F ME C H A N I C A L V E N T I L AT I O N Willis Carrier s research and experimentation in building air conditioning began in 1902 with an idea to use evaporative cooling for humidity control in a printing plant. His scientific engineering process helped to establish further study of the need for devising suitable equipment for carrying out air conditioning processes as well as to identify the need of various industries for maintaining atmospheric conditions, independently of external weather variations (Carrier, 1936). By 1920 Carrier s work had advanced into comfort cooling applications and created a need to define thermal comfort and ventilation requirements. 10 The important technological advance that made mechanical ventilation possible was the development of the electric power industry. But even then, mechanical ventilation was slow to catch on. If you look at buildings constructed in the early part of the 20th century you will notice that most floor plans are generally variations of narrow rectangular areas with the distance from one exterior wall to the opposite exterior wall 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 was to keep all the building occupants within reasonable distance to a window for both ventilation and light. During World War II, ventilation became an important issue. Buildings and factories operating at night during the war had to do so under blackout conditions. Manufacturing facilities for war production were often erected without windows, forcing engineers to consider mechanical ventilation as a source of fresh air and temperature control. Generally, air was provided in sufficient volume to keep the average interior air temperature at about 10ºF (5.6ºC) above the outdoor air temperature. Even as late as 1957, F.W. Dodge Corporation in their architectural record book, Buildings For Industry, discussed ventilation requirements in national defense terms stating: If windows are provided in the building most of the ventilation may be taken care of by them. But in the case of blackout buildings some mechanical means must be provided. 11 3. 3 C O 2 C O N T RO L... A N EW I D E A? The Mechanical Engineers Handbook (Marks, 1916) which featured Willis Carrier as a contributing editor, was one of the first engineering guides to mention CO2 measurement as a reference for ventilation relative to the number of occupants in a space: Air is rendered unwholesome by perspiration, by respiration, excessive heat, humidity, effluvia from the human body and other impurities directly or indirectly imparted by the occupants of a room. The percentage of carbonic acid may be regarded as a measure of the vitiation from respiration and from combustion, but not from the heat and moisture resulting from the same source. Air may be polluted with dust and other harmful matter of which CO2 gives no indication. CO2 tests should be used only for checking the renewal of air and its distribution within the room.the production of this gas can only be assumed as a basis for calculating the air supply where respiration and combustion (gas lights) are the preponderating factors of vitiation; in such cases the CO2 should not exceed 8 or 10 parts in 10,000 [800 to 1,000 ppm]. 1 2 The 1929 New York City Building Code echoed M a r k s Handbook reference to CO2 levels and ventilation:... ventilation consisting of transoms or other similar d evices opening into rooms ventilated directly to the outer air or of other methods capable of maintaining a carbon dioxide content of the air of not more than one part in one thousand [1,000 p p m ]... 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 outdoor air date back to the early 19th century when Thomas Tredgold (1836), an English mining engineer, published an estimate of 4 cfm (1.9 L/sec) per person based on metabolic needs. In 1895, the A m e r i c a n Society of Heating and Ventilating Engineers (ASHVE) adopted a minimum recommendation of 30 cfm (14.2 L/sec). Later, in 1914, ASHVE proposed a model code requiring 30 cfm (14.2 L/sec) per person as the minimum. By 1925, 22 states had adopted the requirement. The first ASHRAE Standard 62 appeared in 1973 titled Standards for Natural and Mechanical Ve n t i l a t i o n. The standard provided minimum and recommended outdoor airflow rates for the preservation 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 9

I of the occupants health, safety and well-being in a variety of different spaces. Standard 62-1973 defined a prescriptive approach, meaning that the airflow rates were prescribed (as rules), and became the basis for most state codes. In 1981 the standard was updated and re-titled as ASHRAE Standard 62-1981, Ventilation for Acceptable Indoor Air Quality. The net effect was a general reduction in outdoor air usage. In the 1989 update to ASHRAE Standard 62 the minimum acceptable ventilation rate increased from 5 cfm (2.4 L/sec) to 15 cfm (7.1 L/sec), which has since been widely accepted. Evolution of the minimum ventilation rate is shown graphically in Figure 3.1. F I GU R E 3. 1 EV O LU T I ON O F M I N I MU M V E N T I L AT I O N R AT E S 4 0 3 5 3 0 2 5 20 1 5 1 0 5 0 1825 1850 1875 1900 1925 1950 1975 2 0 0 0 Year Source: Janssen 1999 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 day standards by establishing that CO2 concentrations should not exceed 1000 parts per million. Appendix D of the standard was also created as a reference to the standard to explain the fundamental relationship between CO2 and ventilation as described in Chapter 2 of this handbook. In 1999 Standard 62 was updated to become Standard 62-99. In this update the provisions for CO2 were changed slightly. The 1000 ppm level was modified to a 700 ppm differential. The exact wording of the standard is as follows: C o m fo rt (odor) cri t e ria with respect to human bioeffluents are likely to be satisfied if the ve n t i l a t i o n results in indoor CO2 c o n c e n t rations less than 700 ppm above the outdoor air concentra t i o n. 13 As discussed in Chapter 2, a 700 ppm differential between inside and outside concentrations is considered equivalent to 15 cfm/person when people are involved in office-like activity (1.2 MET). So where did the original 1000 ppm level come from? Originally the 1000 ppm guideline, which was also used in some of the handbooks and standards in the early 1900 s assumed an outside level of around 300 ppm (300 + 700 = 1000). With the rise of global C O2 levels at about 1 to 2 ppm per year the previously assumed 300 ppm outside level was no longer correct (See Chapter 2 for more information on rising global CO2 levels). Currently a value of 400 to 450 is generally used for outside concentrations. When CO2 was first addressed in Standard 62, some confusion resulted because the Standard was somewhat ambiguous as to whether CO2 was to be considered a contaminant, a surrogate for air quality or a ventilation parameter. To many it was unclear whether F I GU R E 3.2 C O 2 R E L ATE D A SH RA E I N TE R P R E TAT I O N S IC-62-1999-03 Ventilation criteria of the Standard is likely to be satisfied if CO2 levels in EACH space do not exceed 700 PPM above outdoor air C02 levels. The 700 ppm differential noted in the standard is not a time weighted value or a ceiling value. CO2 levels should only be considered during times of occupancy. IC-62-1999-04 The 700 ppm CO2 differential noted in the standard is a guideline based on maintaining adequate ventilation to control perception of human bioeflluents, not a ceiling value for indoor air quality. IC-62-1999-05 If filtration means are used under the Air Quality Procedure to remove human bioeffluents and odors the 700 ppm differential value for CO2 may not be applicable. It is primarily intended for use with the Ventilation Rate Procedure. IC-62-1999-15 CO 2 control cannot be used to reduce ventilation below Table 2 values (CO 2 DCV is normally used to maintain table 2 values based on real-time occupancy using the ventilation rate procedure). CO 2 cannot be used as the sole means of claiming compliance with the Indoor Air Quality Procedure because there are generally other contaminants of concern that must be measured and controlled using this procedure. CO 2 filtration 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 of acceptable outdoor air in Table 2 are provided to EACH space. IC-62-1999-32 Defines the parameters to apply CO2 based demand controlled ventilation. Interpretation 24 above confirms if Table 2 rates are supplied to each space then the multiple space equation is not needed. C O2 sensing and control was to be applied under the Ventilation Rate Procedure (prescriptive) of the standard or the Air Quality Procedure (performance based). The confusion led to several requests for interpretation to ASHRAE 62 committee. A request for 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

interpretation is a procedure where any individual can ask a question to clarify the intent of the Standard. Interpretations are asked as yes or no questions and submitted to the ASHRAE committee responsible for maintaining ASHRAE Standard 62. The use of the CO2 in Standard 62 has been the subject of 6 of the 38 interpretations requested of Standard 62. Figure 3.2 provides a table identifying and briefly describing the various interpretations that have addressed CO2 related issues. All interpretations, including those developed as part of the 1989 standard have now been accepted as part of the relatively new ASHRAE Standard 62-99. All current interpretations are provided with every copy of the ANSI/ASHRAE Standard 62 sold by ASHRAE. All interpretations are also available on ASHRAE s web site (www.ashrae.org). I 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 good practice for HVAC design engineers, local codes ultimately dictate how buildings must be designed. In fact, many codes indirectly draw from ASHRAE Standard 62 to establish ventilation requirements in buildings. ASHRAE Standard 62 is not used directly in codes because it is not written in the language necessary for code enforcement. The majority of local and state code making bodies do not usually have the expertise and resources to write their building codes from scratch. As a result, three model codes have been established that develop standardized building code documents that can be adopted in whole or in part by local jurisdictions. These code bodies are known as BOCA ( B u i l d i n g O fficials and Code Administrators International), ICBO (International Conference of Building Off i c i a l s ) and SBCCI (Southern Building Code Congress International). Recently these three model code bodies have jointly adopted the International Mechanical Code (IMC) which establishes minimum regulations for mechanical systems using prescriptive and performance related provisions. Like the ASHRAE 62 standard, the IMC also provides provisions for modulation of outside air based on occupancy as long at target cfm-per person ventilation rates are maintained. This is addressed in section 403.3.1 of the 2000 International Mechanical Code that states: The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in Ta ble 403.3 and the a c t u a l number of occupants present. [Emphasis added] 15 The IMC has also created a commentary document to provide clarification to the intent of the code. In reference to section 403.3.1, the commentary uses CO2 control as an example of a ventilation system that can provide a specific rate per person based on the actual number of people present. An excerpt from the commentary is provided below. The intent of this section is to allow the rate of ventilation to modulate in proportion to the number of occupants. This can result in significant energy savings. Current technology can permit the design of ventilation systems that are capable of detecting the occupant load of the space and automatically adjusting the ventilation rate accordingly. For ex a m p l e, carbon dioxide (CO2) detectors can be used to sense the level of CO2 c o n c e n t rat i o n s, which are indicative of the number of occup a n t s. People emit predictable quantities of CO2 for any given activity, and this knowledge can be used to estimate the occupant load in a s p a c e. 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 has been a well-understood principle for over 100 years. However its application and the sensor technology price-performance ratio have only evolved over the past few years. DCV is now an attractive alternative to the traditional approach of providing a fixed ventilation rate based on an assumed maximum occupancy. Ventilation control with DCV is a recognized ventilation control approach in ASHRAE Standard 62-99. 1 7 It is a recommended operational approach to control ventilation based on occupancy in the International Mechanical Code and as a result is now being updated into many local building codes. 18 Leading states like California have also integrated DCV as part of the state building code as a method of reducing energy use yet ensuring indoor air quality. 19 Low cost CO2 sensors for DCV first appeared on the market approximately 10 years ago. As any new technology, these first products encountered initial market resistance. In the case of CO2 sensors, price, calibration frequency or their size and appearance were issues. There are still products on the market that have these same problems. Carrier has discovered that these issues can be solved with the right technology. Economical sensors are now available that can self calibrate and offer thermostat-like dependability at thermostatlike prices. The CO2 measurement technology is no longer a barrier to utilizing this promising ventilation control approach. 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

I 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, Ventilation Requirements, 2001. 1 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

II D C V A P P L I C AT I O N F U N D A M E N TA 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 1 3

II 1 4 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

4. 1 O V E RV I E W This chapter provides the framework necessary for integrating DCV into a HVAC system design including evaluating potential for DCV, sizing systems, establishing a base ventilation rate and locating sensors. 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 IC 62-1989-27). A copy of this interpretation is provided in Appendix B to this handbook. This interpretation identified the ground rules for using CO2 as a method of controlling ventilation based on real-time occupancy within a space. 1. The use of CO2 is applied using the Ventilation Rate Procedure of Standard 62, which establishes specific cfm/person ventilation rates for most applications. By definition, ASHRAE Standard 62 says that acceptable indoor air quality is achieved by providing ventilation air of the specified quality and quantity (Table 2 in the Standard) to the space. The standard states: The Ventilation Rate Procedure described in 6.1 is deemed to provide acceptable indoor air quality, ipso facto. Chapter 5 will provide more details on implementing the proper control strategy and Chapters 6-8 will provide examples of DCV designs for three different types of HVAC systems. algorithm should be based on achieving the rates in Table 2. The control strategy should also be developed considering inside/outside CO2 d i ff e r e n t i a l. 3. The control strategy must provide adequate lag time response as required in the Standard. 4. If CO2 control is used, the design ventilation rate may not be reduced to consider peak occupancies of less than 3 hours (often called diversity). In other words, the variable provision of 6.1.3.4 cannot be applied to lower the estimated maximum occupancy for the purpose of reducing the design ventilation rate while using DCV. 5. CO2 filtration or bioeffluents removal methods other than dilution should not be implemented in the space. II 2. CO2 is applied using the provisions of section 6.1.3.4 of the standard that address variable and intermittent occupancy. The CO2 control strategy can be used to modulate ventilation below the design ventilation rate while still maintaining Table 2 ventilation rates (e.g., 15 cfm per person). Sensor location and selection of the control 6. A base ventilation rate should be provided during occupied periods to control for non-occupant related sources. 7. Where applicable the multiple spaces provision of 6.1 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 significant complexity to HVAC design. It is no coincidence that section 403.3.1 of the International Mechanical Code that is relevant to CO2 control is called System Operation. CO2 DCV is primarily an operational parameter that should not significantly affect the design of the system except in the implementation of the control strategies to regulate ventilation levels. DCV is part of an overall control strategy for a building and should be considered complimentary to other building control functions. Such strategies include: Apre-occupancy purge to clear out contaminants that may have built up overnight during system shut-off. Economizer operation to take advantage of times when outside air can be used for free cooling (will override CO2 control). High and low temperature limits used to protect equipment from extreme temperatures that may significantly 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 system is operated, not how it is designed. As a result there are only a few issues to consider when designing a DCV system. There are five simple steps to designing DCV applications: 1. Verify that the application is appropriate for DCV. 2. Estimate the building occupancy and calculate the required outdoor airflow for each space based on ASHRAE Standard 62 or other appropriate (local) code requirement. 3. Determine the appropriate base ventilation rate for non-occupant related sources. This will be the minimum ventilation rate provided during all occupied hours. 4. Determine the appropriate control strategy to use for the application and equipment used. 5. Select type of sensor and determine sensor location. 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 5