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Thermal Distribution Systems in Large Commercial Buildings

Large commercial buildings are known to waste energy through their thermal distribution systems. Previous research by Mark Modera and members of EETD's Energy Performance of Buildings Group has shown that heating, ventilation, and air conditioning (HVAC) distribution systems in commercial buildings suffer from thermal losses, such as those caused by duct air leakage and poor duct location. Because of a lack of metrics and data about the potentially large energy savings from reducing these losses, the building industry has mostly overlooked energy-efficiency improvements in this area.

With support from the California Energy Commission and the U.S. Department of Energy, we conducted research to obtain the technical knowledge needed to properly measure and understand the energy efficiency of thermal distribution systems in commercial buildings. We expect that this new information will assist the California and U.S. building industry in designing better thermal distribution systems for new commercial buildings and in retrofitting existing systems to reduce their energy consumption and peak electrical demand.

There were three technical objectives:

  1. Develop metrics and diagnostics ("yardsticks" and measurement techniques) for determining the efficiencies of commercial thermal distribution systems.
  2. Develop information that the California and U.S. building industry (e.g., HVAC system design engineers and installers) can use to design new thermal distribution systems, estimate energy efficiency, and prevent or reduce the incidence of problems that have been identified in existing commercial systems.
  3. Determine the energy impacts associated with duct leakage airflows in an existing large commercial building that could be mitigated by applying duct retrofit technologies.

The project involved an extensive in situ characterization and duct leakage intervention study at a 25-story office building in Sacramento, California. In particular, we characterized the performance of the variable-air-volume (VAV) duct systems on two floors of the building. One floor was our control floor; the other was the intervention floor, where we could study the effects of duct leakage on system performance.

Metrics and Diagnostics

The most important metric is transport energy—the total energy used to transport air per unit of thermal energy delivered. This metric is useful for comparing the relative performance of various types of thermal distribution systems. We recommend that California's Title 24 (energy code) compliance process for large commercial buildings include quantification of this metric.

Our field tests of diagnostics focused on measurements of duct leakage airflows, fan airflows, and fan power. In particular, of the two duct leakage diagnostics that we tested, only one reliably determined duct leakage airflows. It involves accurately measuring airflows entering and exiting the duct system: the difference is the duct leakage. With further development and testing, we expect this diagnostic will be useful in developing a database that characterizes the distribution of duct leakage airflows in California's large commercial buildings.

Dust System Characterization

Because there has been very little characterization of the actual performance of thermal distribution systems in large commercial buildings, we carried out an extensive characterization of one of these systems. The test building showed every indication of a "tight" thermal distribution system: good application of mastic, metal bands at joints, and overall high quality. To demonstrate duct leakage impacts, we installed temporary calibrated leaks and monitored their effects on system energy consumption and demand.

Measured fan power from the supply air handler (top curves) and induction fans (lower curves) for two floors in the test building. The difference in fan power (25 to 35%) is due to leakage in the duct system.

Figure. Measured fan power from the supply air handler (top curves) and induction fans (lower curves) for two floors in the test building. The difference in fan power (25 to 35%) is due to leakage in the duct system.

Energy Impacts

The principal finding from this project is that duct leakage airflows can have a significant energy impact in large commercial buildings. Our measurements indicate that adding 15% duct leakage at operating conditions leads to an increase in fan power of about 25 to 35% (see Figure). These findings are consistent with the impacts of increased duct leakage air- flows on fan power that have been predicted by previous simulations. The primary benefit from having tight duct systems is reduced electricity use. We estimate that eliminating duct leakage airflows in half of California's existing large commercial buildings could save about 560 to 1,100 gigawatt-hours (GWh) annually (about $60-$110 million per year or the equivalent consumption of about 83,000 to 170,000 typical California houses) and about 100 to 200 MW in peak demand.

We are continuing our investigation, using our new diagnostic techniques to characterize duct leakage in a sample of large commercial buildings. We are also continuing to work with the CEC to introduce improved ways of characterizing energy-efficient ducts in the state energy code. The parallel story in the residential and small commercial sector has shown that it took approximately 10 years to move from the comparable stage in that research to maturity of technology adoption (e.g., commercialization and inclusion in standards). We conclude that a concerted effort will be necessary to make the same—or better—progress for the large commercial sector.

— Rick Diamond and Craig Wray

For more information, contact:

  • Rick Diamond
  • (510) 486-4459; fax (510) 486-6658
  • Craig Wray
  • (510) 486-4021; fax (510) 486-6658

This work was sponsored by the California Energy Commission and the U.S. Department of Energy.

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