In the development of practical applications of expert systems one of the key areas is knowledge acquisition. This is the process whereby knowledge of the subject area possessed by a human expert is transcribed into a set of "if-then" type rules appropriate for the expert system shell.

The process of knowledge acquisition is usually handled by a "knowledge engineer". This is an individual who has understanding of both the subject area and of the expert system shell. There are two key elements in the process° The first is the development of a structure for the knowledge base. The structure must (1) be able to accommodate a growing knowledge base, i.e. the addition of rules must be straightforward, and (2) be suited to the expert system inference engine, i.e., allow efficient problem solving. The second key element is the interviewing. Expert systems may be constructed on a cerebral basis, in which the knowledge engineer is also the expert whose knowledge is to be encoded. However, a stronger expert system can be made by incorporating the knowledge of a number of experts. The interview is the mechanism whereby the expert’s knowledge is elucidated.

In this paper experience gained with the knowledge acquisition process is discussed. This experience is based on the development of expert systems for HVAC diagnostics (both site-specific and site-independent) for aspects of the building energy auditing process and for trouble-shooting solar domestic hot water systems. Over 50 experts, including engineers, technicians, and building managers, have been interviewed. Examples of the development of an expert system and the way in which the structure of the knowledge base and the goals of the interviews evolve are given.