A radiator thermostat (1) to be fitted onto a radiator valve (4) is provided. The radiator thermostat (1) has a transmission element (5, 105, 205) that transmits a compressive force to the valve pin (8) of the radiator valve. An electric motor (12) is provided to drive the transmission element (5, 105, 205). The transmission element (5, 105, 205) is preloaded with a counter force (F2) that acts in opposition to the valve force (F1) using a spring (18; 20; 32). Thus, when moving the transmission element (5, 105, 205), the motor (12) has only to overcome the differential force (df).