Thermal Energy Distribution (TED) systems provide the critical link between heating and cooling equipment and the conditioned area of buildings. TED systems have also been shown to have a large potential for efficiency improvement, partially in residences. This paper discusses the issues associated with characterizing the performance of TED systems in residential buildings. A possible framework for a universal figure of merit for residential TED systems that takes into account interactions between the TED system and the building envelope, interactions with the heating/cooling equipment, as well as the weather-dependence of TED efficiencies, is proposed. The proposed efficiency characterization framework incorporates the TED /envelope interactions that have been extensively studied in crawlspace and slab-on-grade houses over the past several years, as well as some of the findings of ASHRAE Special Project 43 (SP43), which focused on air distribution systems in basement houses. This framework is designed to allow for compatibility with existing equipment and envelope characterizations (e.g., Annual Fuel Use Efficiency (AFUE), Seasonal Energy Efficiency Ratio (SEER), Heating Seasonal Performance Factor (HSPF), Effective Leakage Area (ELA)), and to allow for comparison of forced-air, hydronic and refrigerant distribution systems, including the impacts of zoning. The proposed backbone for most of the supporting analyses required to develop a practical yardstick is a combined simulation model based upon DOE-2, COMIS (a multi-zone airflow network model) and a combined heat and mass transfer model for duct systems. Efficiency calculations made with this model for a typical attic duct system are included as an example. Two applications identified for this figure of merit are utility DSM programs and building energy codes.
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