A system for changing the temperature of a first fluid to meet a desired temperature comprising: a fluid supply for said first fluid, said fluid supply for said first fluid passing through a heat exchanger, and a temperature sensor for said first fluid to sense a temperature of said first fluid downstream of said heat exchanger; a supply for a second fluid to change a temperature of said first fluid, said supply for said second fluid passing through said heat exchanger; a three-way valve positioned upstream of said heat exchanger on said supply for said second fluid, and controlling a flow rate of said second fluid diverted into a bypass line compared to a flow rate of said second fluid directed through said heat exchanger, with said valve controlled by a control in response to feedback from said temperature sensor; and said valve changes the respective flow rates delivered into said bypass line and through said heat exchanger in a non-linear manner with a change in valve position; wherein said valve includes a rotating spool with a first window delivering said second fluid into a first outlet leading to said bypass line and a second window delivering fluid into a line leading to said heat exchanger and said valve changing position in a circumferential direction to achieve said non-linear manner of changing the respective flow rates, and characterized in that said first and second windows (50, 52) are also designed to achieve a change in pressure drop as said valve spool rotates, such that said valve has a pressure drop profile tailored to balance a pressure drop across said heat exchanger, such as to, in turn, keep a total pressure drop across said system between said valve and said heat exchanger that is relatively close to constant as said valve changes the portion of the second fluid passing through said heat exchanger. a fluid supply for said first fluid, said fluid supply for said first fluid passing through a heat exchanger, and a temperature sensor for said first fluid to sense a temperature of said first fluid downstream of said heat exchanger; a supply for a second fluid to change a temperature of said first fluid, said supply for said second fluid passing through said heat exchanger; a three-way valve positioned upstream of said heat exchanger on said supply for said second fluid, and controlling a flow rate of said second fluid diverted into a bypass line compared to a flow rate of said second fluid directed through said heat exchanger, with said valve controlled by a control in response to feedback from said temperature sensor; and said valve changes the respective flow rates delivered into said bypass line and through said heat exchanger in a non-linear manner with a change in valve position; wherein said valve includes a rotating spool with a first window delivering said second fluid into a first outlet leading to said bypass line and a second window delivering fluid into a line leading to said heat exchanger and said valve changing position in a circumferential direction to achieve said non-linear manner of changing the respective flow rates, and characterized in that said first and second windows (50, 52) are also designed to achieve a change in pressure drop as said valve spool rotates, such that said valve has a pressure drop profile tailored to balance a pressure drop across said heat exchanger, such as to, in turn, keep a total pressure drop across said system between said valve and said heat exchanger that is relatively close to constant as said valve changes the portion of the second fluid passing through said heat exchanger.