This report details the data, assumptions and methodology for end-use forecasting of space conditioning energy use in the U.S. residential sector. Space conditioning end-uses include Heating, Ventilation and Air Conditioning (HVAC). Our analysis uses the modeling framework provided by the HVAC module in the Residential End-Use Energy Planning System (REEPS), which was developed by the Electric Power Research Institute (McMenamin et al. 1992). This modeling framework treats space conditioning separately from appliances such as refrigerators or water heating due to the complex physical and economic interactions that characterize HVAC systems, and because space conditioning is the most significant end-use of residential energy in the United States. Space conditioning accounts for approximately 30% of electricity consumption, 70% of natural gas consumption and 90% of oil consumption in the U.S. residential sector. In terms of primary energy, space conditioning represents over half of all energy consumption in residences (EIA 1993). This report is primarily methodological in nature, taking the reader through the entire process of developing the baseline residential space conditioning end-use models. Analysis steps documented in this report include: defining the thermal shell characteristics, gathering technology and market data for HVAC equipment and systems, developing cost data for the various components of the thermal shell and HVAC systems, and specifying decision models (both the functional form and equation parameters) to forecast future purchase decisions by households. Our implementation of the REEPS 2.1 modeling framework draws on the extensive technology, cost and market data assembled by LBL for the purpose of analyzing federal energy efficiency standards. The resulting residential HVAC forecasting model offers a flexible and accurate tool for analyzing the effect of policies at the national and north/south levels. FOREWORD AND ACKNOWLEDGMENTS This project was undertaken at Lawrence Berkeley Laboratory (LBL) under the direct supervision of Jonathan Koomey, and with the guidance of James E. McMahon and Mark D. Levine. The purpose of this work is to create a well-documented end-use forecasting model for the residential sector, and to use that model to analyze national policies affecting the efficiency and patterns of energy use in the U.S. The work adopts the modeling framework of the Residential End-Use Energy Planning System (REEPS) developed for the Electric Power Research Institute (EPRI). This report is one of a series of three reports documenting the residential sector end-use forecasting project using EPRI-REEPS 2.1, which is the most recent version of the computer software. The first report in the series summarizes the model framework and inputs, and presents the results of the baseline forecast for each end-use (LBL-34044). The other two reports detail the data, assumptions and methodology for the two distinct groups of end-uses in the residential sector. This report (LBL-34045), the second in the se...