A methodology for the optimal operation of a HVAC system with a thermal storage water tank has been proposed by one of the authors1). In the present paper improvement of the methodology is explained together with simulation results obtained by using real building data. The main results are as follows. 1) This method is sufficiently robust to be applied to a real HVAC system having time varying air-conditioning (AC) load. 2) Predicted AC load must include some error and it was found that storage operation ignoring the error often experiences shortage of thermal energy and as a result unsatisfied air-conditioning creating room air temperature deviation. To avoid the problem increasing in the load by taking the error level into account should be considered. 3) On peak load days daytime storage operation is normally necessary due to the limitation in tank capacity. The optimal operation method considering this constraint was developed and it was found that the performance is fine.