Demand control ventilation: lessons from the field- how to avoid common problems.
Demand control ventilation (DCV) is a building ventilation control strategy in which the quantity of mechanically supplied outdoor air intake is regulated by some type of occupant density sensing. DCV is intended to save energy by means of sup plying design ventilation air to occupants during periods of high occupancy and supplying the minimum required ventilation to dilute building related contaminants during low occupancy periods. Reducing the amount of outdoor air that needs to be conditioned can save energy. If air side economizers are in use specific outdoor conditions will have an effect on the amount of energy savings. Carbon dioxide ([CO.sub.2]) sensors are the industry standard to determined space occupancy for DCV. It has been shown that [CO.sub.2] levels are a good determination of space occupancy (Turpin, 2001). It should be noted that [CO.sub.2] is not considered an indoor air quality (IAQ) concern at levels found in typical buildings (400-2000 ppm) but is used solely as an indication of occupancy level (Emmerich, 2001).
DCV systems can be incorporated into existing HVAC equipment and often times operate in conjunction with existing economizer controls, sharing the same outdoor air damper (OAD). Savings from DCV systems are achieved by the reduction in outdoor supply air (OSA) that requires conditioning as compared to a fixed OSA flow rate during all occupied hours.
Six spaces were randomly chosen from a list of spaces provided by the funding agency, which supplied incentives for the use of DCV. All study spaces were in ASHRAE climate zone 5B. Table 1 below shows the description of the study spaces. Two control types were encountered in the study and listed in Table 1. The control types were roof top unit (RTU) or control with a building energy management system (EMS). DCV is intended for spaces which have variable occupancy rates, this was the case in the spaces for building codes 04-09 but buildings 01 and 10 had incorrectly located [CO.sub.2] sensors which well be addressed below. All buildings were fully occupied except for building 10 which was estimated by building management to be 70% leased out. Specific occupancy patterns within spaces was not studied.
Table 1 Study Space Building Year Control Type Building Type Building Size, Code Installed SF ([m.sup.2]) 01 2 006 EMS Office 68,000 (6317) 04 2008 RTU Office/Medical 21,104 (1961) 06 2 007 BAS Elementary School 63,400(5890) C 08 2007 RTU High School 65,000 (6039) 09 2 007 RTU High School 102,000 (9476) 10 2 008 RTU Office 15,750 (1463) Building Study Space Description Study Space Size SF Code ([m.sup.2]) 01 2 One third of entire building 22,600 (2099) 04 Break room 300 (28) 06 2 classroom 1,000 (93) 08 Classroom 800 (74) 09 2 Classroom 78 0 (72) 10 2 Half of entire building 7,875 (732)
Functional testing was broken up into three system aspects. First, [CO.sub.2] control signal functional testing was conducted to confirm that the control link between [CO.sub.2] sensors and OA damper positioning was in place. Second, sensor placement functional testing was conducted to confirm that the sensors accurately reported the [CO.sub.2] levels of the controlled zone. Third, the OSA level test was conducted by inspecting the air balance reports to determine the OSA rates and to confirm that the system was balanced in accordance with DCV standards such as presented in ASHRAE 62.1-2007. Only when DCV systems passed all three functional tests could energy savings estimates be carried out.
Functional testing of DCV systems was done in accordance with the basic control type and differed whether it was managed through an energy management system (EMS) or at the roof top unit (RTU). EMS based control systems were tested by manually lowering the [CO.sub.2] set points to be below the current levels within a space and watching (on the EMS screen) for the OA damper to open in that space. RTU based control systems were tested by having one person exhale directly onto the [CO.sub.2] sensor and having another person located at the RTU watch for the damper to respond and for the DCV indicator light to illuminate (if available) on the logic controller. In addition to physical functional testing, trendlog data and other direct measurement data were analyzed. C are was taken when analyzing data to find periods during which only the DCV signals were dictating the OA damper position. Periods of economizer use were avoided. Logged data included air stream temperatures of the outdoor air, mixed air, return air, supply fan state (on/off), fan Amperage, [CO.sub.2] sensor signals, compressor state (on/off), and compressor Amperage.
In this field study, no systems were found to pass the functional tests. Failure modes will be explained below.
All systems studied failed the functional test related to air balancing and DCV standards for OSA rates. From working with local design and TAB professionals the authors believe that a good design specification schedule of a single zone DCV system will include minimum outside air flow rate, the zone [CO.sub.2] concentration at that flow rate, the zone maximum outside air flow rate, and the corresponding [CO.sub.2] concentration at the maximum flow rate. All these values are easily calculated using ASHRAE 62.1-2007. If these exact values are not given other values could be supplied to inform the TAB contractor of these same four setting which are minimum airflow, control signal at minimum, maximum airflow, and control signal at maximum. Table 2 below shows the given system specifications for each system studied. It can be seen from this table that none of the design engineers specified all the information that a TAB professional would need to install and balance a functional DCV system. All drawings noted the use of [CO.sub.2] sensors in the design. Many designs provided notes which described in general terms outside air dampers opening and closing from minimum to maximum positions with regard to zone population or [CO.sub.2] levels. No actual [CO.sub.2] concentration levels, control signal values or outside air flow rates were given as guidance to TAB contractors for the proper balancing of a DCV system. Some interviews with TAB contractors revealed that in some cases minimum OSA levels would be set to zero or some other arbitrary flow which could lead to building pressurization issues if they did not consider exhaust and exfiltration airflows.
Table 2 System specification per design documents Building Code System Total cfm (lps) Min. OSA cfm (lps) 01 28,000 (13,215) 4,000 (1,888) 04 1,500 (708) 300 (142) 04 1,500 (708) 300 (142) 08 2,400 (1,133) 690 (326) 09 6,000 (2,832) 1,500 (708) 10 8,500 (4,012) 1,300 (614)
Table 3 below shows results of the functional testing related to system installation. One RTU failed [CO.sub.2] control signal functional testing. The exact reason for this failure could not be determined but it is suspected that it was caused by a wiring problem given that the physical output from the [CO.sub.2] sensor did not signal damper movement despite functional performance of both the sensor and the damper individually. This failure is assumed to be a workmanship issue and not a systemic problem. Two units failed the sensor placement functional test. This issue is considered systemic and needs to be addressed by the design community. These units had [CO.sub.2] sensors placed in common mixed air returns from multiple zone s. This configuration disallows control of the critical zone. Similar errors have been identified in previous research (Carrier, 2001. Emmerich, 2001). [CO.sub.2] concentrations for sensors placed in this manor tend to be below typical control levels due to this zonal averaging. Figure 1 below illustrates an example from Building 10 where the [CO.sub.2] sensor was located in a common return. It shows [CO.sub.2] levels rising only 100 ppm above ambient levels. This finding not only shows the senor is in an incorrect location, but it also shows that the building is being over ventilated due to the fact that return air [CO.sub.2] levels are so close to ambient levels. It should also be noted that typical sensor accuracy is +/- 50 ppm.
[FIGURE 1 OMITTED]
Table 3 System Installation Test Code Control Type Sensor Placement Control Signal 01 EMS Fail Pass 04 RTU Pass Pass 04 RTU Pass Fail 08 RTU Pass Pass 09 RTU Pass Pass 10 RTU Fail Pass
Intermittent fan operation was noted at two of the sites (both high schools). Intermittent fan operation is a problem in many aspects of HVAC control systems and greatly affects DCV performance. Fan cycling was found to be a problem in 38% of systems studied as part of a RTU field study (Jacob, 2 003). With intermittent fan operation, OSA dampers only function when the supply fan is running, otherwise they move to a fully closed position. When supply fans are set to run only during calls for cooling or heating, [CO.sub.2] levels go unchecked when the fan is not on. This result can be seen in Figure 2 below.
[FIGURE 2 OMITTED]
Fan cycling was also raised as an issue that affects the performance of DCV systems. Fan cycling is perceived to be an energy savings measure by building operators. This is an incorrect perception that requires education to overcome. Operators need to understand that fan cycling reduces indoor air quality, occupant comfort, diminishes accuracy of sensor signals, and reduces DCV functionality.
Table 4 System Fan Operation Code C ontrol Type Fan Operation 01 EM S Continuous 04 RTU Continuous 04 RTU Continuous 08 RTU Intermittent 09 RTU Intermittent 10 R TU Continuous
Best practices for the design and installation of DCV systems are constantly evolving. In addition DCV systems interact with other systems and setpoints such as economizers and building pressurization setpoints. Designers must be knowledgeable of interactions and take them into account when specifying a DCV Systems. As stated above, from working with local design and TAB professionals the authors believe that a good design specification schedule of a single zone DCV system will include minimum outside air flow rate, the zone [CO.sub.2] concentration at that flow rate, the zone maximum outside air flow rate, and the corresponding [CO.sub.2] concentration at the maximum flow rate. The reason the authors believe that [CO.sub.2] levels should be specified is because straight forward equations are available in ASHRAE 62.1-2007 and it is not dependent on the sensor range or control signal type which may not be under the control of the HVAC designer. It could be argued that the use of two [CO.sub.2] sensors, one measuring ambient levels and one in the controlled zone, will provide for better control of ventilation systems due to the fact that [CO.sub.2] levels changed with the seasons or by other factors such as pollution levels. Device accuracy also needs to be considered, many devices currently on the market have an accuracy in the range of 50-100 ppm and only some can be recalibrated in the field. These fact ors need to be considered on a case by case basis. In general [CO.sub.2] sensor are less expensive and more reliable than they were 10 years ago but issues still exist and sensors should be evaluated based on the requirements of the application (Maxwell, 2009).
This study has exposed several issues with regard to the design, installation, and operation of DCV systems that prohibited the association of measured energy savings with the systems monitored. While not looked at in this study, it is important to understand that DCV can sometimes increase energy consumption during cooling periods when not used in conjunction with OSA economizer functionality.
All of the buildings in this study were built under ASHRAE 62-1989. While this standard needs to be considered when asking why systems were installed as they were, code officials interviewed allow newer standards to be used when designers can show reasons for doing so. Confusion about minimum OSA flow rates when using ASHRAE 62-1989 could have led designers to use ASHRAE 62-2004 or 2007, but it appears they did not. Roth (2005) also cited confusion regarding ASHRAE Standard 62 and indicated it was as a primary barrier to implementation of DCV systems. ASHRAE 62-1989 was the first standard that dealt with the concept of DCV and it was not until 1997 that an official interpretation was offered by ASHRAE specifically allowing [CO.sub.2] to be used to modulate out door air intake levels based on occupancy (Apte, 2006). ASHRAE 62-1989 lacks a clear statement of what minimum ventilation levels should be when occupancy is low in spaces. Results from our investigation suggest that mechanical engineers commonly rely up on published tables to determine ventilation rates by occupancy type. This could be for lack of better data in ASHRAE 62-1989, it could be simply out of habit, or perhaps it is to avoid conflicts with code officials. For all DCV systems in this study design occupancy was used to determine ventilation rates as the minimum OSA level, contrary to the theory of DCV.
Interviewing code officials and TAB contractors in the study area revealed low understanding of DCV systems. This is an indicator of low market penetration of this technology. Education of all parties involved would benefit the design, installation, and operation of DCV systems. In addition using an Integrated Design Process and total building commissioning will increase communication among team members and may alleviate some of these design, installation, and operation issues.
ASHRAE Standard 62.1-2007 is much improved over ASHRAE 62-1989 and is written in code enforceable language. It is suggesting that all systems be designed in accordance with the new standard and variance be sought if local codes call for ASHRAE 62-1989.
The authors would like to thank Idaho Power Company for funding this research and the many TAB contractors, engineering firms, building managers and maintenance staff who provided information and time in the process of this research.
Apte, Michael G., (2006), A Review of Dem and Control Ventilation (NBNL-60170). Lawrence Berkeley National Laboratory.
Carrier, (2001). Demand Control Ventilation System Design Guide (Doc. 1001 811-10088). Syracuse, NY.
Emmerich, Steven J. Persily, Andrew K., (2001). State-of-the-Art Review of [CO.sub.2]
Demand Controlled Ventilation Technology and Application National Institute of Standards and Technology (NISTIR 6729).
Jacobs, Pete. (2003). Small HVAC System Design Guide. California Energy Commission (500-03-082-A12)
Maxwell, Gregory. (2009) Product Testing Report: Wall Mounted Carbon Dioxide ([CO.sub.2]) Transmitters. National Building Controls Information Program, Iowa Energy Center.
Roth K W. et al. (2005). Energy Impact of Commercial Building Controls and Performance Diagnostics: Mark et Characterization, Energy Impact of Building Faults and Energy Savings Potential. Prepared for the US Department of Energy Building Technologies Program. TIAX LLC, Cambridge, MA No. D0180.
Turpin, Joanna., (2001) The Dilemma Over Demand Control Ventilation, Engineered Systems Magazine [on-line], Available: www.esmagazine.com
Brad Acker, PE
Kevin Van Den Wymelenberg
Brad Acker is Researcher, Kevin Van Den Wymelenberg is a director and assistant professor at the University of Idaho, Integrated Design Lab, Boise, Idaho
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|Author:||Acker, Brad; Van Den Wymelenberg, Kevin|
|Date:||Jan 1, 2011|
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