In the United States, the two most common energy-efficiency standards are ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, and the International Energy Conservation Code (IECC). These standards play a key role in curbing the energy consumption and greenhouse-gas emissions of new and renovated buildings. Several states have adopted variations of these standards or developed their own standards to address unique energy-efficiency issues. In general, each of these standards includes:
- Requirements concerning building-control capabilities, equipment efficiencies, and installation.
- Compliance paths specifying minimum requirements for building components, allowing limited trade-offs between building components, and demonstrating a proposed design performs at least as efficiently as the baseline design.
According to the U.S. Department of Energy, more than half of the states in the country have adopted energy codes at least as stringent as Standard 90.1-2004/IECC 2006, while more than two-thirds have adopted energy codes at least as stringent as Standard 90.1-1999/IECC 2001.
Appendix G of Standard 90.1-2004 provides a method of rating the energy performance of a project. This method requires the development of energy simulations for a baseline building and a proposed design and provides explicit guidance concerning building-control systems.
Building Energy Codes and Standard Practice
Energy audits of older buildings reveal many instances of inefficient control implementation, such as constant-speed fan control in multizone systems, inadequate outdoor-air-shaft sizes, simultaneous humidification and dehumidification, limited simultaneous heating and cooling, and substantial operation of HVAC and lighting systems during unoccupied periods. This has led to innovation and the deployment of new approaches to building control. In jurisdictions requiring compliance with Standard 90.1 or a similarly stringent energy standard, newer buildings incorporate numerous controls to promote energy-efficient operation:
- On larger air-handling units (AHUs), optimum start/stop controls, which allow minimal operation during unoccupied periods while ensuring comfort conditions are met during occupied periods.
- For HVAC and lighting systems, zone-isolation controls, which allow automated shutoff in specific areas.
- Controls limiting simultaneous humidification and dehumidification to spaces with narrow environmental-control requirements, such as computer rooms.
- Ventilation controls in areas with high occupancy limits for times of partial occupancy.
- For buildings in dry climates, air- or water-economizer controls for free cooling throughout much of the year.
- Hydronic distribution loops designed for variable-flow operation.
- Variable-speed fan controls on air-handling systems serving multiple zones.
- Chilled-water and hot-water-reset controls for lowering building energy consumption during part-load operation.
- On AHUs providing large quantities of outside air, exhaust energy-recovery systems, which transfer heat from the exhaust stream to ventilation supply air.
Building-Energy-Performance Rating Systems
Energy-efficiency codes and standards have helped to transform the building-controls market. This transformation has been aided by energy-efficiency incentive programs and energy and environmental rating systems. For instance:
- The Energy Policy Act of 2005 (EPAct) provides federal tax credits for commercial buildings exceeding Standard 90.1-2001 Appendix G requirements by 50 percent or more. Partial incentives are available for lighting-, envelope-, and/or HVAC- and service-water-heating-system-performance improvements.
- The Leadership in Energy and Environmental Design (LEED) Green Building Rating System uses Standard 90.1 Appendix G performance-rating methodology to rate the energy efficiency of new-construction and major renovation projects under Energy & Atmosphere Credit 1 (EAc1).
- Many utilities provide incentives for projects that exceed minimum local energy-code requirements.
The influence of energy-performance rating methods on the implementation of building controls is limited to controls that must be reflected in baseline-building models and controls that receive credit in proposed-building models.
In accordance with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) performance-rating method, controls modeled in a baseline building include supply-air-temperature reset for multizone systems, variable-speed pumps, and condenser-water wet-bulb reset, while building-control efficiency measures that receive credit within a proposed building include automated fenestration shades, daylight-harvesting controls, occupant-sensor lighting controls, demand-control ventilation (DCV), variable-speed chillers, variable-speed compressors for direct-expansion (DX) equipment, electronically commutated motors, and chilled-water and hot-water reset based on building demand. Controls that generally cannot be modeled for credit in the ASHRAE performance-rating method are those that involve schedule changes, such as night ventilation controls or programmable communicating thermostats, and control-system functions associated with monitoring and verification or fault detection. Appendix G places significant restrictions on the modeling of controls used to facilitate natural ventilation in a natural/mechanical (hybrid) ventilation system.
Rating Systems and the Design Process
Most building environmental rating systems and building energy-efficiency incentive programs encourage the use of integrated design, which typically involves building-energy simulation.
To maximize incentives, energy modelers tend to reference all efficiency measures against the baseline design model they use to document building energy performance. For example, for LEED EAc1, design decisions typically are based on energy and cost savings between Standard 90.1 Appendix G baseline and proposed-case models. Thus, project teams are more likely to choose measures that reflect savings
in the selected rating system over measures for which the rating system does not award credit. For instance, project teams using Standard 90.1 Appendix G generally opt for DCV, for which Appendix G allows full credit, over a hybrid ventilation system, for which Appendix G does not. Therefore, performance-rating methods play a major role in determining the types of efficiency measures that become common in the marketplace.
An effective energy code or standard:
- Raises the baseline of building efficiency, ensuring that the least-efficient newly constructed and substantially remodeled buildings improve upon the average efficiency of older buildings.
- Fosters widespread adoption of building energy-efficiency measures.
- Mandates measures to prevent unnecessary energy use during unoccupied hours.
- Helps to ensure the persistence of energy savings.
- Adapts energy-efficiency requirements based on climate and building function and is widely applicable.
A good performance-rating method also:
- Provides a method of documenting energy performance vs. standard practice.
- Allows documentation of savings associated with efficiency measures that have little acceptance within the marketplace.
- Is free of loopholes that would allow measures with limited energy and cost benefits to receive credit.
- Provides opportunities to demonstrate energy-performance improvement across all climate zones.
Standard 90.1, IECC, and California Title 24 requirements have made tremendous strides in meeting objectives for energy codes and standards, while Standard 90.1 Appendix G and methodologies included in the Savings by Design incentive program (based on Title 24) have provided an excellent basis for performance ratings. However, as the United States faces increasing concerns about dependence on foreign oil, rising global energy costs, decreasing availability of energy, and rising anxieties about climate change, significant room for improvement remains. Specifically, building operation during unoccupied periods and the persistence of energy savings over time require further attention.
Building operation during unoccupied periods. Most standards mandate controls, but not alerts when the controls are not performing as intended. Crediting or mandating such alerts could lead to substantial savings. For example, building operators could restore the functionality of a lighting-shutoff control compromised by an inadvertent change to a lighting-shutoff schedule quickly, if they received an alert stating nightly lighting energy consumption had tripled from the previous week. This sort of monitoring function would have to be widely available to be mandated.
Current energy codes and standards give inadequate attention to unoccupied-period part-load operation of chilled-water systems. Even variable-speed chillers perform inefficiently when their load falls beyond a given level. Add pump and heat-rejection energy, and nightly light loading of chiller plants can noticeably impair the overall energy performance of a building. Consider a building with an efficient chilled-water-plant design occupied from 8 a.m. to 5 p.m. daily. If the chilled-water plant served only telecommunication and small data-center loads during unoccupied hours, energy consumption would far exceed that of air-cooled DX units.
Persistence of energy savings. Many energy-efficiency measures that meet minimum code requirements at the time of installation fail shortly after a building is occupied. Most notably, air-side economizers, particularly in coastal climates, become stuck, which leads to increased heating and cooling loads. Title 24 provides a credit for automated fault-detection diagnostics for air-side economizers, packaged DX units, and zone terminals. Such a credit should be incorporated into other energy codes and rating systems and offered for other equipment.
Recent developments regarding codes, standards, and performance-rating methods have led to the improved energy performance of buildings. Given the energy crisis it faces, however, the United States needs to continue to increase the stringency of energy codes and standards and establish performance-rating methods that advance market development and acceptance of energy-efficient building systems and controls. A focused effort to develop mandatory, prescriptive, and performance controls requirements for the next releases of the nation's building energy codes, standards, and performance-rating methods would help to move the buildings market toward the transformation needed to reverse the upward trend in national energy consumption.
As director of energy services for CTG Energetics Inc., Gail Stranske, PE, is responsible for energy-engineering and sustainability-consulting activities, including building-energy simulation and analysis, energy auditing, commissioning, and Leadership in Energy and Environmental Design (LEED) Green Building Rating System documentation support. She is competent with DOE-2.1E, DOE-2.2, VisualDOE, eQUEST, and EnergyPro software and well-versed in ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings; International Energy Conservation Code; and California Title 24 compliance. She has completed LEED Energy & Atmospheric Credit 1 analysis and documentation for numerous projects and participated in energy audits and commissioning of commercial and institutional buildings totaling more than 6 million sq ft.