Ventilation: Condition Only As Much Outside/Ventilation Air Needed (Demand-Controlled) vs. Prescriptive Code Minimum Based on 100% Occupancy 100% of the Time
Ventilation in commercial buildings controlled by occupancy sensors – usually CO2 monitors – to provide just the required amount of outside air and to avoid over- or under-ventilating a space.
Item ID: 166
HVAC--Sensors & Controls
Technical Advisory Group: 2010 HVAC TAG (#3)
Technical Advisory Group: 2009 HVAC TAG (#2)
Technical Advisory Group: 2015-1 Commercial HVAC TAG (#11)
Average TAG Rating: 3.3 out of 5
TAG Ranking Date: 03/10/2015
TAG Rating Commentary:
- This does not seem like emerging technology to me.
- This is required by code in several applications, and if the heat is gas, the savings will be gas.
- It's commercial, if not ubiquitous. I think the issues are installation complexity, and maybe reliability, but matching ventilation rate to occupancy (and use intensity) seems like a great way to reduce energy waste.
- Needs to have automated controls. I do not think it will perform well with manual controls.
- There are a number of demos in CA showing significant savings
- To achieve cost-effective energy savings, much care has to be taken to choose the correct application.
- BPA should investigate current adoption trends; it might not have to be incentivized to achieve reasonable adoption.
Demand-controlled ventilation (DCV) measures carbon dioxide (CO2) concentrations to determine ventilation needs and then matches the ventilation air delivered to demand. Ventilation is thus reduced when spaces are vacant or operating at lower than peak capacity. Energy savings result from reducing the need to heat, cool, or dehumidify outside air (FEMP, 2014).
Designers have been specifying DCV for over a decade in the more obvious applications – spaces with high but varied occupant density, such as gymnasiums, auditoriums, and large conference rooms. However, for much of that time, the CO2 sensors that were used were unreliable, difficult to calibrate, and required frequent calibration. Now, more reliable and robust CO2 sensors are available, and this strategy is being included in more codes for many spaces. As the technology and experience with it improves, it should be routine in spaces with high-but-varied occupancy, and it may be considered for other spaces that are not as large, dense, or varied in their occupancy, such as classrooms, smaller conference rooms, restaurants, and large open office areas. There are many opportunities to include this technology when retrofitting varied-occupancy spaces and in new construction of the secondary spaces.
The most common method to incorporate DCV into the design of an HVAC system is to adjust the amount of outdoor ventilation based on the level of CO2 in the building air. The CO2 level can be monitored by a sensor located in the occupied zone or in the return airstream. If not already available, an enthalpy-based economizer should be included in any retrofit project (Lawrence, 2004).
DCV energy savings vary considerably, depending on baseline ventilation rate and the use and occupancy patterns of the building. Lawrence Berkeley National Lab shows that, for office buildings in California climate zones, adopting DCV practices can provide savings of 1% to 7% of total building energy use (not HVAC energy use) (Hong, 2009). Savings are much greater for a building with a more variable occupancy pattern (such as a restaurant).
Baseline Description: Outside Air/Ventilation Air Set to Levels Required to Meet 100% Occupancy, 100% of the Time
Baseline Energy Use: 10.5 kWh per year per square foot
The 2009 Commercial Building Stock Assessment gives the actual electrical building energy use index (EUI) for various types of heating and cooling systems (NEEA, 2009 Pg (Table) D-EA5). Office buildings with electric heating and cooling have an EUI of 20.1 kWh/sf/year. Office buildings with no electric heating or cooling use only 8.2 kWh/sf/year (non-HVAC end uses such as lighting and plug load), indicating that the combined HVAC heating and cooling energy use is 11.9 kWh/sf/year. For all commercial buildings, the corresponding EUI values are 19.9 and 9.4 kWh/sf/year, respectively, for a heating and cooling use of 10.5 kWh/sf/year.
Manufacturer's Energy Savings Claims:
This is a strategy; therefore, there is not a manufacturer.
Best Estimate of Energy Savings:
"Typical" Savings: 10%
Low and High Energy Savings: 2% to 40%
Energy Savings Reliability: 5 - Comprehensive Analysis
Calculating energy savings from this measure is challenging because most studies implement DCV along with other HVAC measures. Heating and cooling account for about 53% of the total annual energy use in a commercial building. If the use of DCV results in an assumed 6% reduction in total building energy use (as suggested by Hong), then the savings are approximately 6% x 20.1 kWh/sf/year = 1.2 kWh/sf/year. This is equivalent to a reduction in the heating and cooling energy use of: 1.2 /11.9 = 10%. (Office building savings are taken as representative of the entire commercial building stock.) FEMP claims that DCV has been shown to reduce energy costs by up to 38% of HVAC energy use in an office building. Lawrence Berkeley National Lab shows that, for office buildings in California climate zones, adoption of DCV practices can provide savings of 1% to 7% of total building energy use (Hong, 2009).
DCV energy savings depend on many factors, including:
• The baseline condition of the HVAC system
• Whether or not the building meets code
• Whether or not the building is over- or under-ventilated
• The code ventilation requirements in effect when the building was permitted
• Occupancy patterns and densities, and how they have changed over time
• Weather conditions (affect the amount of time the system is in economizer mode for temperature control versus at minimum outside air for ventilation control)
• Commissioning, calibration, and maintenance of CO2 sensors (affect the persistence and amount of energy savings because designers may adopt conservative C02 setpoints and failure modes to account for sensor calibration and maintenance issues)
• Selected CO2 setpoints and failure modes
Energy Use of Emerging Technology:
9.5 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
Calculations for the areas where this technology may be applied are shown in the table below. These are taken from the preliminary data for the 2013 update to the Northwest Commercial Building Space Assessment (NEEA, 2014). The percentages of the floor area of each type of building to which this technology may apply are rough estimates.
Commercial Building Space in the Northwest where DCV may be Applied
Regional Technical Potential:
|Space Type ||Total Area (sf) || Approx. % could be DCV ||Target Area (sf) |
|Large Office ||283,240,000 || 80% ||226,592,000 |
|Medium Office ||127,610,000 || 80% ||102,088,000 |
|Small Office ||149,740,000 || 80% ||119,792,000 |
|Big Box-Retail ||134,190,000 || 80% ||107,352,000 |
|Small Box-Retail ||247,840,000 || 80% ||198,272,000 |
|High End-Retail ||61,960,000 || 80% ||49,568,000 |
|Anchor-Retail ||119,630,000 || 80% ||95,704,000 |
|K-12 ||251,050,000 || 80% ||200,840,000 |
|University ||128,170,000 || 80% ||102,536,000 |
|Warehouse ||381,740,000 || 80% ||305,392,000 |
|Supermarket ||60,820,000 || 90% ||54,738,000 |
|Restaurant ||59,040,000 || 100% ||59,040,000 |
|Assembly ||238,380,000 || 100% ||238,380,000 |
|Total ||2,243,410,000 || ||1,860,294,000 |
1.95 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
According to several sources, system costs can vary between $1,500 and $5,000, including installation, to retrofit a packaged HVAC system with one of the DCV strategies. Costs for large, built-up systems will vary depending on the required number and location of CO2 sensors.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Because savings are going to vary widely, it is difficult to quantify simple payback. The potential for savings is large, so this technology is expected to be highly cost-effective in buildings with occupancies of over 25 per 1,000 sf.
Ventilation can be one of the top energy uses in a high-occupancy building. Spaces designed for large numbers of people are required by code to have HVAC systems that can provide large amounts of outside air. However, these spaces are frequently only partially occupied or are unoccupied. A table developed by the American Society of Heating, Refrigerating & Air-Conditioning Engineers (ASHRAE) shows typical diversity of occupancy throughout the day (Washington State Building Code Council, 2012 Pg AE-34). Demand-controlled ventilation (DCV) represents an untapped potential source of energy savings for existing buildings and an excellent investment in new buildings.
CO2 sensors are used to optimize the amount of ventilation air provided via the HVAC system. With a CO2 sensor in the return air plenum, ventilation air will typically not need to be supplied until sometime after the room has been occupied. For HVAC systems serving multiple separate spaces, it is better to locate the CO2 sensors in each space, three feet above the floor, and provide ventilation air whenever excessive CO2 is detected in any space. If the control system includes a motion sensor, the supply can be shut off the moment the room becomes unoccupied, regardless of CO2 levels, but this may not be a cost-effective strategy.
DCV strategies typically include:
• Integrated fan cycling to meet ASHRAE 90.1 requirements.
• Variable frequency drive or two-speed fan motors for additional fan energy savings.
• Fleet ventilation systems for large retail applications. Fleet ventilation is a strategy for areas served by more than one system, which allows selected system(s) to provide all of the space ventilation needs and the remaining system(s) to cycle on and off to maintain temperatures.
• Carbon monoxide monitoring systems for parking garages.
• Motorized damper on outside air to modulate based on CO2 levels.
This strategy is used routinely, and may not be considered emerging for new construction of large, variable-occupant-density spaces such as auditoriums, gymnasiums, conference rooms, and performing arts centers. This strategy should be considered for retrofitting these types of spaces, and for other spaces where occupancy fluctuates, such as large open office areas and classrooms.
The standard practice for HVAC ventilation is to provide the code-required minimum outside air, which typically specifies minimum ventilation for the maximum design occupancy at all times. However, it has been documented that many buildings do not meet this code baseline, and other buildings provide many times the required ventilation, especially when not fully occupied, and regularly ventilate unoccupied spaces.
Some corporate chains are already adopting DCV as standard practice, in advance of code requirements.
The option for CO2 control has been included in ASHRAE 62.1 since 1999, and some codes require DCV for larger spaces with high occupant densities. Most codes refer to ASHRAE standards 62.1 and 90.1, which define both the minimum outside air requirements and how DCV can be applied. ASHRAE requirements have changed with every version.
The State of Oregon building codes allow DCV. A proposed code change would require DCV for larger spaces with high occupant densities.
The State of Washington building codes allow DCV via the use of "alternate methods" utilizing ASHRAE 62.1.
The State of Idaho adopted the International Building Code, which permits reduced ventilation during reduced occupancy.
The State of Montana adopted ASHRAE 90.1 for commercial spaces, which allows ventilation to be adjusted for variable occupancy.
CO2 sensors are commercially available, but accuracy and long-term calibration concerns, as well as application and integration issues have prevented widespread adoption. Programming of energy management systems has not been standardized and is a critical element of system operation.
End User Drawbacks:
Potential obstacles include difficulties integrating with an existing energy management system or economizer controller, and determining the appropriate location for the CO2 sensor(s). An effective DCV system requires more careful design than a standard system. For instance, each space being served should have some form of occupant sensing. In some cases, multiple sensors may be needed. For example, a large variable air volume (VAV) system serving multiple spaces would require multiple sensors.
If operators are concerned with the accuracy and tolerance of CO2 sensors, they may adopt conservative set points and failure modes, reducing the effectiveness of the strategy.
Other factors may govern the ventilation system design and operation, such as pressurization between spaces (e.g., between kitchens and dining rooms), release of contaminants that are a health hazard to occupants, and changes to the return and exhaust air controls. Some applications, such as movie theaters or performing arts centers, cannot tolerate fan cycling.
Public schools may be reluctant to implement this technology because it may raise concerns about indoor air quality (IAQ) among parents who are convinced that poor IAQ may be contributing to their child’s poor performance or rashes. This obstacle may be overcome by letting parents know that the school will take extra steps to ensure that the children have a constant supply of adequate ventilation air by monitoring each space 24 hours per day and adjusting the ventilation air supply as needed.
Operations and Maintenance Costs:
No information available.
A 15 year service life based on typical control systems, assuming sensor calibration or replacement at regular maintenance intervals.
If variable frequency drives, energy-efficient motors, duty-cycling of fans, dedicated outdoor air systems (DOAS) and performance-tested HVAC are already in place, the energy savings of DCV systems will be less. However, these technologies can be combined with DCV systems to maximize savings.
Reference and Citations:
Zhang, et. al.,
Energy Savings for Occupancy-Based Control (OBC) of Variable-Air-Volume (VAV) Systems
Report on increased savings achievable in offices with advanced occupancy sensors that can determine how many people are in the occupied space.
Standard Benchmark Energy Utilization Index
Demand-Controlled Ventilation: A Design Guide
Oregon Office of Energy for the Northwest Energy Efficiency Alliance
Total Pacific Northwest Building Stock Based on Preliminary Numbers from the 2013 Update to the CBSA
Northwest Energy Efficiency Alliance
Northwest Commercial Building Stock Assessment (CBSA): Final Report
Prepared by the CADMUS Group for the Northwest Energy Efficiency Alliance
Demand Control Ventilation Using CO2
Demand-Controlled Ventilation Case Study
Oregon Department of Energy
Modern Building Services
Demand Controlled Ventilation System Design
Carbon Dioxide (CO2) Sensors
California Investor-Owned Utilities Partnering for Energy Efficiency Rebates
Goetzler. et. al.,
Energy Savings Potential and Research, Development, & Demonstration Opportunities for Residential Building Heating, Ventilation, and Air Conditioning Systems
Prepared by Navigant Consulting for U.S. DOE Building Technologies Office
Assessment of Energy Savings Potential from the Use of Demand Control Ventilation Systems in General Office Spaces in California
Lawrence Berkeley National Laboratory
Demand-Controlled Ventilation and Sustainability
Washington State Building Code Council,
2012 Washington State Energy Code
Promising Technologies List
U.S. DOE Federal Energy Management Program