A heat transfer system (240, 300, 320) for sourcing a variable load, 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 at least one parameter of the first circulation medium or the second circulation medium by: detecting the variables using the first at least one sensor and the second at least one sensor, and controlling flow of one or both of the first variable control pump (102, 122) or the variable flow controlling mechanical device using a feed forward control loop based on the detected variables of the first circulation medium and the second circulation medium to achieve control of the at least one parameter, characterised in that the 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. 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 at least one parameter of the first circulation medium or the second circulation medium by: detecting the variables using the first at least one sensor and the second at least one sensor, and controlling flow of one or both of the first variable control pump (102, 122) or the variable flow controlling mechanical device using a feed forward control loop based on the detected variables of the first circulation medium and the second circulation medium to achieve control of the at least one parameter, characterised in that the 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. detecting the variables using the first at least one sensor and the second at least one sensor, and controlling flow of one or both of the first variable control pump (102, 122) or the variable flow controlling mechanical device using a feed forward control loop based on the detected variables of the first circulation medium and the second circulation medium to achieve control of the at least one parameter, characterised in that the 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.