Abstract: A building heating/cooling system energy optimization method for a building having a heating/cooling system includes the steps of providing a mathematical model of the heating/cooling system, obtaining real-time weather information, reading the input water temperature (IWT), the output water temperature (OWT) and the supply air temperature (SA) output to the building, periodically transferring the IWT, the OWT and the SA to an optimization system which is operative to analyze the real-time data in coordination with the mathematical model by assigning at least three selected values in a range surrounding and including the current values of each of the IWT, the OWT and the SA and calculating the efficiency profile of the components of the heating/cooling system for each of the selected values, then cooperatively optimizing and selecting those values calculated to provide the highest efficiency profile, then periodically resetting the system values to those selected by the optimization system.