In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. Normally, the Stirling convertor provides this cooling. If the convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, and also ending the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) has been designed to allow multiple stops and restarts of the Stirling convertor in an Advanced Stirling Radioisotope Generator (ASRG). When the Stirling convertor is turned off, the VCHP will activate when the temperatures rises 30°C above the set point temperature. A prototype VCHP with sodium as the working fluid was fabricated and tested in both "gravity aided" and "against gravity" conditions for a nominal heater head temperature of 790ºC. The results show very good agreement with the predictions and validate the model. The gas front was located at the exit of the evaporator when heater head temperature was 790ºC while cooling was ON, simulating an operating Advanced Stirling Convertor (ASC). When cooling stopped, the temperature increased by 30°C, allowing the gas front to move past the radiator, which transferred the heat to the case. After resuming the cooling flow, the front returned at the initial location turning OFF the VCHP. The "against gravity" working conditions showed a colder reservoir and faster transients.