This article describes a real-time expert system for detecting and diagnosing faults in a 20-kilowatt (kW) microwave transmitter heat exchanger. Expert systems enhance operator productivity by quickly identifying problems and recommending appropriate solutions, or predicting impending problems that have not yet occurred. The heat exchanger expert system has been tested and debugged. Applications and extensions of the expert system to heating, air-conditioning, and ventilating problems are discussed.