A calibrated simplified engineering modeling method has been developed to optimize HVAC system operation. This method can be used to optimize operating strategies and control schedules. It has also served to identify malfunctioning components on occasion. This method has been successfully applied to 18 LoanSTAR buildings where over $2.0 million per year in potential savings have been identified, and over $1.4 million per year have been implemented. The method is described in the context of a case study.