Detection and diagnosis of multiple faults in variable air volume (VAV) systems using principal component analysis (PCA) and joint angle analysis (JAA) are presented. Multi-level principal component analysis models including system- and local-level are built to detect the multiple faults occurred in VAV systems simultaneously. As the initial detection, system-level principal component analysis model is used to discover the abnormities in view of the whole systems. And two local-level principal component analysis models are used to further confirm the occurrence of the faults. With the multiple faults separated into different locations, joint angle analysis is used to isolate the faults one by one according to corresponding local models. (c) 2006 Elsevier B.V. All rights reserved.