Classical building simulators are typically based on global systems of differential equations that model the physical reality and are numerically solved at runtime. In this paper we propose a new approach. Physical components of buildings, such as walls and spaces, are modeled as computational objects that individually solve the appropriate physical equations at runtime and exchange changes of surface values, such as temperatures, when necessary. Because the object structure is derived directly from the building structure and because generic objects are reused, simulators can be generated with very low effort. Based on object oriented modeling and automatic code generation, new physical effects can be easily integrated. As calculations are triggered by changes, very short real-time responses are achieved if necessary. Also, fast and accurate responses to external events can be guaranteed. Both features are necessary for real-time tests of building automation systems.