An expert system has been created for the diagnosis of some of the HVAC problems in small commercial buildings (<20,000 square feet floor area). This project successfully explored the feasibility, advantages and limitations of deploying knowledge-based system techniques to diagnose a wide variety of HVAC operations and maintenance problems. The prototype software developed for this project demonstrates that the available diagnostic knowledge can be represented in a systematic fashion to relate the symptoms and causes of most common HVAC operation problems in buildings. The knowledge base contains about 400 rules addressing problems in seven types of HVAC systems. Knowledge from about 20 experts is represented in the rule base; three additional experts reviewed the final software product. A full report on findings including the software and the final knowledge base has been issued.