This paper describes a methodology whereby a computer program module that models a building's heating, ventilating, and air-conditioning system is automatically generated from a network description of the physical system. This generation is done in a pre-execution step that provides complete generality within a class of network definitions allowed by a set of equipment modules. Graph theory is employed to select a suitable set of equations and develop an algorithm for their solution. This is followed by generation of high-level language instructions directly acceptable to a compiler. The compiled module is downloaded to a microcomputer on which it becomes part of a tool for building energy analysis. Network concepts, program generation, and the microcomputer program are described in the paper.