Abstract: A system and method for controlling a building system includes a supply temperature sensor upstream of a thermal load, a return temperature sensor downstream of the thermal load, and a controller. The controller is configured to calculate an actual thermal energy difference from the supply and return temperature sensors. The controller is configured to control a flow rate of a pump such that the measured thermal energy difference is equal to a desired thermal energy difference for the system, and the measured thermal energy difference is constant for a time interval. A system and method for controlling a building system includes determining a desired thermal energy difference for a load, measuring a supply and return temperature of the load, and reducing the system flow rate such that a valve controlling the load flow rate is at an at partially open condition when the return temperature of the load is the desired return temperature.