The objective of this research project is the following: Demonstrate the feasibility of applying real-time, on-line diagnostics for HVAC systems. This demonstration should generate significant interest among building owners, consultants, and product developers. Given some data observed in an HVAC system, we wish to infer the state of the system. The possible states include a normal operating state and several or many equipment-fault states. It is assumed the fault diagnosis system cannot effect any changes to the HVAC system, but can only make reports to the equipment operator. Data collected in the normal operating state are available for constructing the diagnosis system, but data collected in fault states is not available. However, engineering knowledge of the fault states is available. Feedback from the equipment operator is, at best, very limited.