The results of design and analysis of a heater system for the photovoltaic radiators (PVR)s on the International Space Station (ISS) is presented. The PVRs are designed to reject heat from batteries being charged by the solar cell arrays. Also, during the early buildup of the ISS, two of the PVRs are dedicated to rejecting waste heat from the man inhabited modules. The PVRs will at times experience environmental conditions that are severe enough to freeze the transport fluid (ammonia) which could cause failure of the coolant delivery lines. The heater system is designed to prevent this situation from occurring in the more vulnerable portions of the PVR. The activities involved in the design process and the thermal analyses used to support the design by aiding in the location of sensor, set point temperatures and failure mode analysis are presented.