EnergyPlus: New, Capable and Linked Drury B. Crawley Linda K. Lawrie Frederick C. Winkelmann W. F. Buhl Curtis O. Pedersen Richard K. Strand Richard J. Liesen Daniel E. Fisher Michael J. Witte Robert J. Henninger Jason Glazer Don Shirey Abstract For more than two decades, the U. S. government supported development of two hourly building energy simulation programs, BLAST and DOE–2. Designed in the days of mainframe computers, expanding their capabilities further became increasingly difficult, time-consuming, and expensive. For these reasons, a U.S. federal agency began developing a new building energy simulation tool in 1996. This new simulation program, known as EnergyPlus, builds on the capabilities and features of BLAST and DOE-2 and was released in April 2001. EnergyPlus includes a number of innovative simulation features—such as variable time steps, configurable modular systems that are integrated with a heat balance-based zone simulation—and input and output data structures tailored to facilitate third party module and interface development. Other planned simulation capabilities include multizone airflow, and electric power and solar thermal and photovoltaic simulation. EnergyPlus will also be linked to two other simulation programs—TRNSYS (solar thermal, photovoltaic, and complex HVAC systems) and SPARK (complex HVAC systems). As we neared completion of EnergyPlus version 1.0, the development team began working with interface developers so that easy-to-use interfaces would be available soon afterEnergyPlus version 1.0 was released. Beginning in late 1999, a series of five beta versions were released for testing. Version 1.0 of EnergyPlus was released in April 2001. Keywords Energy simulation, building performance, heat balance, daylighting, HVAC Introduction For more than twenty years, the U.S. government supported development of two building energy simulation programs, DOE-2 and BLAST. BLAST (Building System Laboratory 1999), sponsored by the U.S. Department of Defense (DOD), has its origins in the NBSLD program developed at the U.S. National Bureau of Standards (now NIST) in the early 1970s. DOE–2 (Winkelmann et al. 1993), sponsored by the U.S. Department of Energy (DOE), has its origins in the Post Office program written in the late 1960s for the U.S. Post Office. Both programs are widely used throughout the world. The main difference between the programs is load calculation method—DOE–2 uses a room weighting factor approach while BLAST uses a heat balance approach. Each program comprises hundreds of subroutines working together to simulate heat and mass energy flows throughout a building. In some cases, subroutines in DOE–2 were more accurate. In other cases, subroutines in BLAST were more accurate. In both cases, simulation methodologies were often difficult to trace due to decades of development (and multiple authors). Often, this results in ‘spaghetti code’ with data and subroutines for a particular simulation capability spread throughout the program. To modify either program, a developer must have many years experience working within the code, knowledge of code unrelated to their task (because of the spaghetti), and (often for sponsors) an extraordinary investment of time and money. Many people questioned why the U.S. government was supporting two separate (and comparable capability) programs. Discussions on merging the two programs began in earnest in 1994 with a DOD-sponsored workshop. Although nothing concrete resulted from that workshop, DOE eventually took the initiative and began developing a new program named EnergyPlus in 1996. The EnergyPlus team includes U. S. Army Construction Engineering Research Laboratory (CERL), University of Illinois (UI), Lawrence Berkeley National Laboratory (LBNL),