Off-design operation problems of a cold plate row and of a space radiator row, encountered in fluid loop control, are solved to find possible inlet/outlet temperatures or to specify a required flow rate. Solutions of the problems are given in the form suitable for numerical computations and are graphically shown in the figures for various combinations of control variables. Mathematical analyses are then made to yield transfer functions of a cold plate and of a space radiator. They are expressed as a second-order delay system, mainly dependent on the heat transfer unit number and the heat capacity rate ratio. The expressions are generalized so as to be applicable to a series of cold plates or space radiators. A further analysis based on such generalized transfer functions is made to develop a method of feedback/feedforward control. Numerical results of the analysis are summarized in the table and also are displayed in the figures. Presented are: 1) the proportional gain and the reset time to actually be used in PI (Proportional-Integral) control and 2) coefficient values of the transfer function proposed for feedforward control. Stability conditions are also discussed and are expressed in inequalities.