Whole-building simulation with symbolic DAE equations and general purpose solvers

In building simulation, as in several other domains, traditional monolithic simulation codes are still in dominance over simulators based on symbolic equations in a general modeling language. Introduced in 1998, IDA Indoor Climate and Energy has become the first widely spread thermal building performance simulator based on the new technology. Developing a full-fledged dynamic whole-building simulation program is a formidable endeavor in any setting and since the first beta version in 1997 a number of lessons have been learned. The paper shares some of these experiences concerning general program structure, models, performance, integration, GUI design and issues specifically linked to equation-based modeling using a variable timestep differential-algebraic (DAE) solver.