Carbon Dioxide (CO2) HVAC Basics

Carbon Dioxide (CO2) HVAC Basics Copyright 1998 Honeywell Inc. All Rights Reserved 1 63-7049 63-7049

BACKGROUND Building managers and owners have become aware that control of temperature and humidity alone is insufficient to meet occupant comfort needs. Air quality affects performance and, in some cases, occupant health. To help maintain proper ventilation rates, Honeywell offers the C7242 line of CO 2 sensors. CO 2 What it is CO 2 is exhaled from us all. Although not normally a health concern, CO 2 levels increase as building occupancy levels increase. Number of occupants, activity level, building tightness, ventilation rate, space volume, plants, and building construction all affect building CO 2 levels. CO 2 levels typically correlate with other bio-effluents such as gases, odors, particulates, bacteria, viruses, and perfumes. Monitoring CO 2 levels helps facilitate ventilation decisions that allow balancing occupant comfort and energy-savings. CO 2 Levels Rural outdoor CO 2 levels are about 350 parts per million (ppm) while urban CO 2 levels are considerably higher. OSHA guidelines allow a CO 2 air contaminant limit of 5,000 ppm for more than 8 hours a. In comparison, submarines can reach as high as 7,000 ppm (and on rare occasions even reach a level of 30,000 ppm). Offices typically are 800-1200 ppm. Tightly packed conference rooms with minimal ventilation can be 2,000 ppm. To learn how stuffy the air feels at higher CO 2 levels, carry a portable C7242 and monitor CO 2 levels throughout your day. This is an excellent way to sell CO 2 sensors. Claims that 1,000 ppm and greater CO 2 levels cause drowsiness, fatigue or headaches are highly subjective and often unfounded. Elevated CO 2 levels do not necessarily cause these symptoms, but can indicate a buildup of other contaminants that do cause such symptoms. Managing ventilation using CO 2 levels can improve occupant comfort because other pollutants are also exhausted with the CO 2. IMPORTANT Do not confuse carbon dioxide (CO 2 ) with carbon monoxide (CO), a highly toxic gas that is also a byproduct of combustion in furnaces, fireplaces and automobiles. Carbon monoxide (CO) can be harmful, even at very low levels (25 to 50 ppm). CO 2 as an IAQ Indicator Claims that CO 2 sensors are indoor air quality (IAQ) sensors are inaccurate. CO 2 levels are one piece of the undefined indoor air quality equation. Low CO 2 levels do not indicate good air quality while high CO 2 levels do not mean that air is harmful. So CO 2 sensors should be presented and sold as sensors of CO 2 levels that provide information useful in determining ventilation rates to control CO 2 levels. C7242 CO 2 sensors should not be used as a primary life safety device or as an IAQ sensor and they are no substitute for an indoor air quality analysis, especially when building air quality is questionable. NOTE: Refer to ASHRAE 62-1989 Standard for details relating CO 2 level to ventilation rate. There are devices marketed as IAQ sensors that output one composite signal. Regardless of what some vendors may say, there are no reliable HVAC market priced indoor air quality sensors because IAQ is a complex problem that includes many variables. To further confuse the issue, some substances are deadly when present in low ppm while others are harmless when present in high thousands of ppm. The list of substances is quite long and not totally defined; see the OSHA web site a. Sensors currently sold on the market cannot solve these issues. However, when one becomes available, Honeywell plans to offer it. It is anticipated that CO 2 sensor use in codes and legislation will improve in the future. The codes will define relationships among ventilation, occupancy and IAQ parameters (including CO 2 ). The new family of C7242 products is designed to meet all CO 2 sensor needs and is also compatible with most controllers. Watch for geographic area code changes that can affect your CO 2 sensor sales. 63-7049 2

CARBON DIOXIDE, DEMAND-CONTROLLED VENTILATION (DCV) QUESTIONS AND ANSWERS IMPORTANT Always refer to state and local codes for details on required and recommended CO 2 levels. Temperature Control and CO 2 Control Temperature controls and CO 2 controls have similarities; both controls have a desired setpoint. The CO 2 setpoint is the equilibrium level necessary to maintain the ventilation rate (cfm per person) for the space. The same factors influencing temperature distribution in a space influence CO 2 distribution. These factors include convection, diffusion, and mechanical air movement. It is important to determine CO 2 sensor placement based on the anticipated loads. For optimum control, place a CO 2 sensor in every zone. If an HVAC system is serving a series of zones with similar occupancy patterns, it may be appropriate to place sensors in return air ducting. It is important to eliminate duct leakage as this can introduce false CO 2 readings. There are also differences between CO 2 and temperature sensing and control. Temperature is usually quite uniform throughout a zone whereas CO 2 levels can vary widely. CO 2 levels within a conference room can reach 2,500 ppm while the level on the other side of the door remains at 800 ppm. Therefore, sensor placement strongly affects the result because the control system attempts to control CO 2 levels at the sensor location. There are a variety of CO 2 control strategies and algorithms used in ventilation control. Proper selection depends on the complexity of the building and its HVAC system. Three common strategies follow. On/Off On/Off control employs an open-closed damper strategy based on CO 2 level. This strategy is the most common approach to CO 2 control. A damper opens at a setpoint and closes when levels drop 50 to 100 ppm below setpoint. This strategy works best in applications with high occupancy densities (20 to 50 people per 1,000 sq ft) and with occupancy varying from zero to full in a very short time. Theaters, conference rooms and some school classrooms are applications where this strategy is used. When CO 2 levels vary around the setpoint, the dampers drive fully open and fully closed, depending on the reading so when other control options are not available, use setpoint control. Proportional The amount of heating or cooling energy provided to a space is the basis for staging many HVAC systems. Ventilation systems with proportional control use a CO 2 sensor signal proportional to CO 2 level. Control begins when indoor levels exceed outdoor levels by a defined difference (100 ppm). Air delivery to the space increases proportionally to provide 100% ventilation rate. Compared to setpoint control, this approach allows faster system reaction to varying occupancy levels. It does not always wait for levels to build to the control point. Applications requiring a higher degree of control use a proportional strategy. Proportional control uses sensor models that allow the installer to scale the output to correspond with the control range desired. For example, a 2 to 10 Vdc signal can be adjusted to match a CO 2 level differential range of 400 to 1,000 ppm. Proportional ventilation control is ideal in spaces with highly variable and unpredictable occupancies such as bars, restaurants, conference rooms, courtrooms, classrooms, and retail spaces. Proportional-Integral-Derivative (PID) Time required to reach equilibrium CO 2 levels in low-density applications creates a potential disadvantage for CO 2 control. Low densities such as six people per 1,000 sq ft can require hours to reach equilibrium CO 2 and staggered or varying occupancy can aggravate these problems. Temperature control of large complex buildings encounters similar problems as a result of unpredictable changes in outdoor temperature, solar gain and internal heat generation. Simple temperature control approaches, such as setpoint or proportional, can be disaster in this type of building. A solution to these complex control problems is PID control. PID control directs system control sequences to examine distance from setpoint, time spent at setpoint, and speed of change from setpoint. PID CO 2 control views trends and CO 2 level change rates. For example, minutes after people enter a building in the morning, the HVAC system reacts to adjust fresh air delivery. This adjustment is based on actual occupancy predicted by the CO 2 level rate of rise. Much like the proportional control system, the PID controller operates based on the linear output signal from a CO 2 sensor. Most DDC and building control systems use PID control algorithms. Stand-alone controllers are available that can translate a linear signal from a CO 2 sensor into a PID signal. However, only experienced control designers and installers should apply PID control approaches. When applied properly, PID control provides fast ventilation rate response to changing occupancy conditions. Remember that PID control integrates best in high-rise, multi-zone buildings with low density, highly variable or unpredictable patterns of occupancy. 3 63-7049

Why is CO 2 Control Important? Most building codes base new building ventilation recommendations on ASHRAE Standard 62-1989, Ventilation for Acceptable Air Quality. This standard has a wide variety of recommended ventilation rates based on space use. Problems related to poor IAQ increase when a serious energy crisis creates sufficient energy savings interest to prompt lowering ventilation standards. A study of 500 IAQ investigations conducted by the National Institute for Occupational Safety and Health (NIOSH) in the 1970s and 1980s indicated that 50% of problems identified resulted from inadequate ventilation. People per Square Foot or cfm per Person The terms cfm per square foot and people per square foot of floor space are often used when defining building ventilation requirements. CO 2 concentration is a function of building volume (not square footage), construction, tightness, ventilation rate and occupant level of activity. One person every three square feet of a large gymnasium does not result in a significant CO 2 level rise. Even if they are all physically exercising, the effects are similar. Apply the same occupant density to a typical room in a multi-story office building, and a dramatic change in CO 2 levels will result (with or without exercise). Recognize that using cfm per person and cfm per square foot CO 2 are attempts to simplify the discussion. Actual measurements more accurately determine CO 2 levels. Always refer to local code and requirements for implementation requirements. High-ventilation Rate Tradeoff While increased ventilation standards decrease the incidence of IAQ complaints, there is a tradeoff for building owners and occupants. Generally, building owners must increase HVAC equipment capacity by 30% to 50% for proper outdoor air preconditioning. So the cost of heating and cooling fresh air can become the most significant cost of HVAC system operation. Extended and flexible work hours, and increased use of schools and public buildings beyond traditional business hours add additional cost to HVAC system operation. Balance Energy Savings and Air Quality Traditionally, building engineers designed the fresh air ventilation system to meet space ventilation requirements based on building design occupancy and application-recommended ventilation rate. This ventilation was maintained during all occupied hours regardless of actual occupancy. However, extended and flexible work hours, work-at-home programs, and other changes in work habits resulted in rarely-met design occupancies and occupancy periods extending beyond traditional routine. So using the traditional approach of ventilation system sizing to design occupancy can result in costly over-ventilation of large spaces. CO 2 control strategy ensures sufficient outdoor air delivery for the occupancy level. Avoiding over-ventilation during low occupancy periods provides attractive savings that a CO 2 DCV system can realize. Can CO 2 Based Ventilation Control Increase Energy Costs? CO 2 used alone to control ventilation can result in increased energy costs. Proper application with ECONOMIZER Logic Modules ensures preventing these increased energy costs. When installed with ECONOMIZER Logic Modules, significant energy savings are possible. Outdoor CO 2 Level Must be Considered A guideline to follow regarding CO 2 and ventilation is that an indoor space is under-ventilated if CO 2 levels exceed 1,000 ppm. This assumes outdoor level of 300 ppm. The indoor level of CO 2 can provide useful guidelines; however, like many guidelines, it is possible to over-simplify. For CO 2, the important ventilation indicator is the difference between the indoor and outdoor levels. Outdoor levels of 300 ppm exist only in areas far from urbanization and industrialization; for example, outdoor levels in Los Angeles or Taipei often exceed 600 ppm. The corrected guideline is: If indoor level exceeds outdoor level by more than 700 ppm, a building space is considered under-ventilated for the number of occupants in the space. Indoor/Outdoor Differential CO 2 Levels Consider outdoor CO 2 levels when implementing CO 2 ventilation control strategy. Calculate ventilation rates from indoor/outdoor differential levels. Using an arbitrary CO 2 control setpoint without considering outdoor levels can result in over-ventilation. There are two approaches used in integrating outdoor CO 2 conditions into a control strategy: 1. Fixed value, premeasured, average CO 2 baseline for the geographic location. 2. Direct CO 2 HVAC system monitoring. NOTE: Use the second approach with extreme variations in outdoor levels (more than 200 to 300 ppm during a day). Extreme variation occurs with building air intakes near sources of pollution (major highways, loading docks, etc.) 63-7049 4

When outdoor air CO 2 levels exceed 800 ppm, investigate the source of CO 2. Levels this high indicate polluted outdoor air that likely contains high levels of other contaminants. When periodic high levels occur, devise a control strategy to close air intakes during unacceptable levels (higher than 600 ppm). Importance of CO 2 Equilibrium An important factor influencing ventilation assessment and control using CO 2 levels is the principle of equilibrium. Consider a number of people entering a space at the beginning of a workday. The CO 2 level in the space would be very low (close to the outdoor level). Then the CO 2 level rises as people enter the room, and levels off eventually. The CO 2 leveling point is the equilibrium. The amount of CO 2 produced by the occupants is in balance with space ventilation. CO 2 levels reach equilibrium as they stabilize to within a 100 ppm range. IMPORTANT The key factor in determining ventilation rate is the difference between indoor and outdoor CO 2 levels. The time required to reach equilibrium depends on number of people, activity level, space volume, and ventilation rate. Poorly-ventilated rooms with low occupant densities can require several hours to reach equilibrium level. Once indoor level exceeds a certain indoor/outdoor differential, the ventilation rate is below acceptable. If a space has high occupant density (school classroom, bar or theater), or poor ventilation, the equilibrium level can be reached rapidly, in 20 minutes or less. IAQ Diagnostic Measurement During IAQ investigations, CO 2 measurements are the first tests performed. When making diagnostic or spot measurements of CO 2 for ventilation assessment, consider the equilibrium effect. Performing a spot measurement before achieving equilibrium results in inaccurate CO 2 levels. For most office spaces, time to reach equilibrium can be two to three hours. In a school classroom, the time can be a few minutes. When the occupancy of the space changes, the potential for misinterpreted spot measurement increases. As a guideline, a high spot measurement an indoor and outdoor ppm difference greater than 700 indicates inadequate ventilation. Low levels with uncertain equilibrium conditions provide inadequate information to evaluate ventilation rate. The use of spot CO 2 measurements has several drawbacks. A more effective diagnostic measurement approach is recording space CO 2 levels for 24 hours. Use a data logger and a continuous CO 2 monitor to record level changes. This provides information about the HVAC system and fresh outdoor air delivery to building zones (in proportion to occupancy levels). For diagnostic purposes, take CO 2 measurements in at least every zone containing a single air handler. Take measurements during typical occupancy patterns. If the HVAC system uses Variable Air Volume (VAV) systems, perform measurements in every VAV zone during full occupancy. If diverse occupancies occur throughout the zone, take separate measurements in each occupied area. It can also be worthwhile to measure CO 2 levels in return air ducting. It is important to take at least one measurement each season. Guidelines for Diagnostic CO 2 Measurement 1. Determine outdoor level. 2. Use trend analysis over at least a day rather than using more difficult spot measurements. 3. Make spot measurements under normal or peak occupancy conditions. An elevated level of CO 2 (over 1,000 ppm or 700 ppm over outdoor levels) generally indicates insufficient ventilation. 4. When using CO 2 to determine ventilation rate, base assessment on equilibrium conditions. (Levels below 1,000 ppm do not necessarily indicate good IAQ.) 5. Periodically check calibration of CO 2 sensor. Demand-Controlled Ventilation (DCV) CO 2 based DCV provides an effective approach for designers, contractors, building owners, and managers. It can apply to new and existing buildings. DCV helps maintain ventilation for acceptable IAQ. Properly applied DCV strategies can reduce energy costs by eliminating over-ventilation during partial or intermittent occupancy. CO 2 Ventilation Control Guidelines 1. Determine equilibrium level for each zone based on design density and ventilation rate. This is the upper setpoint. 2. Actively measure or assume a representative value for outdoor CO 2. 3. Continuously ventilate at a minimum level to control non-occupant related contaminants. 4. Select a control strategy based on occupancy density and variability; see Table 1. NOTE: For proportional or rate of rise (PID) strategy, begin control 100 ppm above outdoor conditions. 5 63-7049

Table 1. Recommended Control Strategy Based on Occupancy Density. Occupancy Density (number of people per 1,000 sq ft) Recommended Control Strategy Highly variable (unpredictable changes greater than 25% occupancy) PID or Proportional Low (7 or fewer) PID or Proportional Medium (7 to 20) Proportional High (20 to 50) Setpoint or Proportional PRODUCT DESCRIPTION SUMMARY C7242 sensors operate on a well-known principle of infrared absorption of radiation called the Non-Dispersive Infrared (NDIR) technique. Simply stated, NDIR relies on the fact that CO 2 molecules have a unique molecular structure. Consequently, they absorb light at a unique frequency. Measuring the amount of light absorbed at that frequency enables CO 2 level determination at the seyéor. When NDIR CO 2 sensors were first introduced to HVAC markets, they used an aluminum tube to guide the light. However, these aluminum tubes corroded due to normal air pollutants. The tube corrosion absorbed energy resulting in false CO 2 indications. The new Honeywell CO 2 sensors are gold-plated to resist corrosion. Therefore, calibration lasts far longer than other CO 2 sensors. No other CO 2 sensor on the market offers gold-plating reliability, or the lasting calibration these sensors offer. Using gold eliminates not only sensor chamber corrosion, but the need for a second sensor to measure drift, and all the problems associated with a second sensor. (Products that incorporate a reference sensor must deal with issues of reference sensor drift, correlation between the sensors, added cost and reliability issues.) Honeywell has been closely tracking CO 2 sensor product developments for years and discovered that many units evaluated did not meet expectations. The new C7242 Sensors use technology unavailable in any other product and they provide the best performance at the lowest service costs while remaining competitively priced. Each unit has two analog outputs. Some models use the second output for other purposes such as a relay, heater control, or low-battery indication. The available analog outputs can use factory defaults for plug and play simplicity, see Table 2. The installer can reconfigure the devices to meet control requirements using the accessory 32002183-001 PC Configuration Software. The analog outputs typically produce a linear output relative to the sensor reading (0-10 Vdc). By customconfiguring the sensor, CO 2, temperature and contact input can be combined to generate custom output curves. The number of output options are too numerous to discuss. Model Two analog outputs One relay output Signal (Vdc) 500 ppm = 0 800 ppm = 2 2,000 ppm = 10 500 ppm = 0 800 ppm = 2 2,000 ppm = 10 Portable 500 ppm = 0 800 ppm = 1 2,000 ppm = 5 Outdoor 500 ppm = 0 800 ppm = 2 2,000 ppm = 10 AN1 Slope (ppm/vdc) Table 2. Factory Sensor Configuration. Integral Time (sec) Signal (Vdc) 150 64 0 ppm = 0 800 ppm = 4 2,000 ppm = 10 AN2 Slope (ppm/vdc) 200 2 Integral Time (sec) 150 64 Switch Point: 800 ppm (SPST contact closes when level rises above switch point) Differential: 100 ppm 300 64 Not Available 150 64 Not Available 63-7049 6

TARGET APPLICATIONS The primary control strategy using CO 2 is outdoor air intake regulation to provide fresh air when required, based on occupancy. This avoids over-ventilation and associated energy costs. ECONOMIZER Logic Module applications, both standalone and DDC based, represent the largest opportunity for CO 2 sensor sales. ECONOMIZER Logic Modules ECONOMIZER Logic Modules provide energy saving decisions by using, when possible, outdoor air in place of mechanically conditioned indoor air. The new line of W6215, W7215 and W7460 Logic Modules takes the control process one step farther. It includes input and control for indoor air and (with B models) outdoor air sensors. The C7242 is the typical sensor that provides these inputs. See Table 3 for CO 2 model details. When indoor CO 2 levels are high, the logic module opens the dampers, providing additional fresh air. On models with outdoor air sensors, the logic module also monitors outdoor air, opening the outdoor air dampers only if outdoor air meets user-defined thresholds. Building DDC Configured In all commercial buildings, code dictates minimum levels of fresh air brought into the building. Using CO 2 sensors and proper DDC controllers, fresh air ventilation rates can provide comfort, efficiency and temperature control. Both the XL10 Constant Volume Air Handler Unit Controller (W7750) and the XL10 Unit Vent Controller (W7753A) accept sensor inputs that indicate indoor air parameters. The C7242 CO 2 Sensor output is one example. The XL10 Remote Input/Output (RIO) module (W7761) provides a CO 2 sensor interface and puts the ppm value on E-Bus. Controllers properly programmed use readings directly from the E-Bus network. By networking multiple RIO modules and a Zone Manager, a C-Bus application sensor bus can be created, potentially reducing installed cost. Refer to the XL10 product literature for operation, connection and configuration information. Residential Systems Houses are subject to the same, if not worse, IAQ issues as buildings. Commercial building codes require a minimum fresh air intake. Houses are now tighter and rarely have to follow codes that require fresh air intake. However, in a tight house, CO 2 levels increase during periods of higher occupancy or during use of unvented gas combustion appliances. Houses equipped with fresh air ventilation devices, such as the Honeywell HR150 or HR200, can use the C7242 to control the ventilation rate based on the indoor CO 2 level. This can result in increased comfort and energy savings. The C7242A1030 and 1048 models are compatible with the PC8900 Perfect Climate Comfort Center, the Honeywell ERV and HRV devices. CO 2 sensor contact closure signals the controller or ventilation device that the CO 2 threshold was reached and the device takes appropriate action. See the device literature for wiring and control options. General Sensor Application This process defines the typical CO 2 sensor application. Specific values are not suggested because it is the installer s responsibility to identify target CO 2 and ventilation levels based on the application and the local code. 1. Determine the sensor location. This is the location that the controller targets to maintain CO 2 levels. NOTE: CO 2 levels can differ widely within the ventilation zone, resulting in over- or under-ventilation. Finer control requires more sensors and greater area-independent ventilation ability. 2. Identify and set the level at which ventilation should increase; for example, an application requires increased ventilation at 800 ppm. Configure the (effective) sensor zero point at 800 ppm. 3. Identify and set the level at which 100 percent ventilation should occur; for example, 100 percent ventilation is required at 2,000 ppm. Configure the sensor to 10 Vdc at 2,000 ppm. NOTE: The CO 2 sensor cannot compensate for equipment that cannot handle the ventilation requirements. 7 63-7049

Table 3. C7242 Models and Accessories. Product Description Display No display Output (quantity) Wall Mount with relay C7242A1030 C7242A1048 Analog (1), Relay (1) Wall Mount C7242A1014 C7242A1022 Analog (2) Comments 7-1/16 in. x 3-15/16 in. x 2-3/16 in. Portable C7242C1002 N/A Analog (1) 9 Vdc NiMH and 120 Vac Charger. Duct Mount C7242B1012 C7242B1020 Analog (2) 9-5/8 in. x 3-11/16 in. x 2-3/16 in. with 1-3/16 in. D x 9-1/16 in. sampling tube. Outside Air N/A C7242E1007 Analog (1) 9-5/8 in. x 3-11/16 in. x 2-3/16 in. Zero Calibration Service Kit 32002181-001 N/A 7-1/2 in. x 7-1/2 in. x 4-1/2 in. Duct Mount Housing only 32002182-001 9-5/8 in. x 3-11/16 in. x 2-3/16 in. with 1-3/16 in. D x 9-1/16 in. sampling tube. PC Configuration Software 32002183-001 Includes RS232 Cable. FOOTNOTES a See http://www.osha-slc.gov/oshstd_data/1910_1000_table_z-1.html 63-7049 8 63-7049 Honeywell Inc. 1998 7-98 Printed in U.S.A. on recycled paper containing at least 10% post-consumer paper fibers.