The optimal control strategy proposed for HVAC (heating, ventilation and air conditioning) systems was usually compared with the original one, in which the disparity with perfect operation was seldom discussed. Based on the exergy analysis models developed, this paper presents a CPI (control-perfect index) method to evaluate control of HVAC systems as well as obtain the disparity of candidate strategy with ideal operation. Through minimizing the exergy loss in HVAC system, the limited-ideal operation corresponding to specific operation condition is obtained. With DEA (data envelopment analysis) method, the optimization frontier is trended by sets of limited-ideal operation points and viewed as ideal operation. With the benchmark of ideal operation, the CPI scores of various control strategies can be evaluated. An airport HVAC system is selected as a case study and its simulation is validated using the real operation data. Six control strategies used in the HVAC system are evaluated. The results show that the original strategy has the lowest efficiency. Other five optimal strategies have the higher efficiencies because of their higher CPI scores. The disparity of each strategy with ideal operation is also estimated, which points to the inherent challenge of developing global optimal algorithm. (C) 2015 Elsevier Ltd. All rights reserved.