Abstract: Aimed at faults of variable air volume (VAV) air handling units, a fault detection and diagnosis (FDD) strategy using a hybrid approach is presented for VAV air handling units in this paper. Fault detection and diagnosis software is also developed based on the FDD strategy. Residual-based cumulative sum (CUSUM) control charts are utilized to detect faults in VAV air handing units. The residual-based CUSUM control chart can cope with the dynamic characteristics of the air handing units well. It can also improve the accuracy of fault detection by eliminating the effects of serial correlation on the performance of control charts. Three rule-based fault classifiers, derived from mass and energy balances, are used to find fault sources. The FDD strategy for VAV air handling units was online validated and tested on real VAV air-conditioning systems.