The paper describes a preliminary design model for air-conditioning systems, (ACS). Developed as an expert system, the model has been constructed to reflect the extensive knowledge, experience and understanding of a specialist air-conditioning design engineer .The paper will explain how the ACS expert system attempts to expand the designer's selection possibilibities, to reduce the potential tediousness of the design process,and to provide a tutoring system for novice design engineers . Though the use of a case study the paper concludes that the ACS package is a useful addition to air-conditioning desgners.