A heat transfer system (240, 300, 320), comprising: a heat exchanger that defines a first fluid path and a second fluid path; a first variable control pump (102, 122) for providing variable flow of a first circulation medium through the first fluid path of the heat exchanger; a variable flow controlling mechanical device for providing variable flow of a second circulation medium through the second fluid path of the heat exchanger; sensors for detecting variables, the sensors comprising first at least one sensor for sensing at least one variable indicative of the first circulation medium and second at least one sensor for sensing at least one variable indicative of the second circulation medium; and at least one controller configured to control the first variable control pump (102, 122) in a first type of flow control mode, and switch control of the first variable control pump (102, 122) to a second type of flow control mode that is different than the first type of control mode, wherein the first type of flow control mode is a flow control mode in which: i) at least one parameter controlled by the at least one controller maintains a specified fixed ratio of flow of the first fluid path to flow of the second fluid path, ii) at least one parameter controlled by the at least one controller is source-side output temperature of the heat exchanger at a first fixed setpoint, iii) at least one parameter controlled by the at least one controller is output temperature from the heat exchanger to a variable load source-side output temperature of the heat exchanger at a second fixed setpoint, or iv) at least one parameter controlled by the at least one controller maximizes temperature differential across the heat exchanger to the temperature source. a heat exchanger that defines a first fluid path and a second fluid path; a first variable control pump (102, 122) for providing variable flow of a first circulation medium through the first fluid path of the heat exchanger; a variable flow controlling mechanical device for providing variable flow of a second circulation medium through the second fluid path of the heat exchanger; sensors for detecting variables, the sensors comprising first at least one sensor for sensing at least one variable indicative of the first circulation medium and second at least one sensor for sensing at least one variable indicative of the second circulation medium; and at least one controller configured to control the first variable control pump (102, 122) in a first type of flow control mode, and switch control of the first variable control pump (102, 122) to a second type of flow control mode that is different than the first type of control mode, wherein the first type of flow control mode is a flow control mode in which: i) at least one parameter controlled by the at least one controller maintains a specified fixed ratio of flow of the first fluid path to flow of the second fluid path, ii) at least one parameter controlled by the at least one controller is source-side output temperature of the heat exchanger at a first fixed setpoint, iii) at least one parameter controlled by the at least one controller is output temperature from the heat exchanger to a variable load source-side output temperature of the heat exchanger at a second fixed setpoint, or iv) at least one parameter controlled by the at least one controller maximizes temperature differential across the heat exchanger to the temperature source. i) at least one parameter controlled by the at least one controller maintains a specified fixed ratio of flow of the first fluid path to flow of the second fluid path, ii) at least one parameter controlled by the at least one controller is source-side output temperature of the heat exchanger at a first fixed setpoint, iii) at least one parameter controlled by the at least one controller is output temperature from the heat exchanger to a variable load source-side output temperature of the heat exchanger at a second fixed setpoint, or iv) at least one parameter controlled by the at least one controller maximizes temperature differential across the heat exchanger to the temperature source.