A method and apparatus for controlling the circulation of heat transfer fluid for thermal conditioning systems for spaces, in which the temperatures of several at least partially thermally separate spaces are monitored and averaged, and compared with a predetermined set point temperature, for purposes of controlling the operation of a source of thermal conditioning. In addition, the temperature differentials between the several spaces are also monitored, for purposes of controlling the circulation of the heat transfer fluid. The present invention permits the continued circulation of the heat transfer fluid, even if no heating or cooling is required, for purposes of reducing the temperature differentials between the several spaces.