The problem of diagnosing faults in air-conditioning systems is of increasing importance. A brief review of heating, ventilating and air-conditioning (HVAC) fault diagnosis schemes is given, the key design issues are identified and the advantages of the fuzzy model-based approach are summarised. The way in which the fuzzy fault diagnosis scheme is able to take account of modelling errors, ambiguous data and sensor bias is explained. A multi-step approach to the fuzzy model-based approach is proposed that makes the results of the diagnosis less ambiguous. A computer simulation study is used to demonstrate that a more precise diagnosis can be obtained using the new scheme. Experimental results are also presented that show that the proposed scheme does not generate false alarms, when it is used to commission an air-handling unit in a commercial office building.
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