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Air leakage and duct wall conduction in forced air distribution systems often waste 20% to 40% of the energy used to condition residences in hot, humid climates. The simulation of these forced air distribution system leakages, their attendant uncontrolled airflows within the building system, and their consequential energy uses may be achieved by treating building spaces as pressure vessels (nodes) that are interconnected with the forced air distribution system, the outdoors, and each other through the basic laws of pressure and airflow. A detailed, hourly building energy simulation program, subjected to rigorous analytical and theoretical evaluation and validated against high-quality, empirical data, is used in this study to simulate these complex interactions in a typical residence located in Miami, Florida. Since energy uses related to forced air distribution systems are dependent on a number of factors, including duct system location and insulation level and the rate and location of the air leakage, a parametric simulation analysis that varies these factors across a range of typical values is used to conduct this investigation. The simulations quantify and disaggregate the typical residential energy uses for a peak summer day in Miami, Florida. The individual components of the predicted energy use that are related to the forced air distribution system include the duct wall conduction, the sensible and latent energy uses resulting from supply and return leaks, and the mechanically induced infiltration caused by the unbalanced forced air distribution system. Delivery and distribution efficiencies for the forced air systems on the peak summer day are calculated for each parametric case as compared with an ideal forced air system that experiences no air leakage and no duct wall conduction. By including accurate pressure and airflow models in the building simulation, the model can provide more accurate prediction. Simulation results show increased energy use in the typical Miami, Florida, residence by between 33% and 42%.

AUTHOR: Lixing Gu, Ph.D., P.E., Philip W. Fairey, Muthusamy V. Swami, Ph.D., Jo Ellen Cummings
CITATION: Thermal Performance of the Exterior Envelopes of Buildings VII
KEYWORDS: December, Florida, 1998
YEAR: 1998