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With the advent of the computing age, heat balance based techniques for simulating thermal loads in buildings became a reality for architects and engineers. However, since the 1970s, the capabilities of most of the well-known heat balance based simulation programs have remained fairly stagnant. Much of the reason behind this trend lies with the complexity of the programming required to deal with the fundamental physics encountered in a building and the relative simplicity of the programming languages that were available. With the ever- increasing capabilities of the desktop personal computers and the improved features of the modern programming languages, it is now possible and prudent to revisit the basic heat balance formulation and investigate how its capabilities can be expanded. This paper discusses some of the technical details behind recent advances in heat balance based simulation capabilities achieved by the team of researchers developing the EnergyPlus program for the United States Department of Energy. The EnergyPlus project seeks to combine the best features of the DOE-2 program and the IBLAST program (research version of BLAST). This paper focuses on the marriage of the basic heat balance engine of BLAST with advanced simulation ideas from the IBLAST and DOE-2 programs. This complex task requires careful attention to algorithmic integrity as well as overall program construction and data management. This paper provides the theoretical background for several of the enhancements to the heat balance based simulation technique used in EnergyPlus.