A method for heating and cooling building interiors with a boiler and/or chiller of fluid travelling in a closed loop includes temperature transducers located at the inlet and outlet of the boiler and/or chiller. A Variable Frequency Drive pump and closed loop piping for transporting the fluid is necessarily provided. The temperature transducers at the outlet and inlet of the boiler and/or chiller are electrically connected to a PLC or pump logic control device for receiving information from the transducers and altering the speed and/or frequency of operation of the pump or set of circulating pumps to create an ideal temperature differential of about 20 degrees C. between the outlet and the inlet.