Thermal rectification requires that thermal conductivity not be a separable function of position and temperature. Investigators have considered inhomogeneous solids to design thermal rectifiers but manipulations of solid lattices are energy intensive. We propose a thermal logic device based on asymmetric solid-fluid resistances that couples two fluid reservoirs separated by solid-fluid interfaces. It is the thermal analog of a three terminal transistor, the hot reservoir being the emitter, the cold reservoir the output, and smaller input reservoirs as the base. Changing the input temperature alters the transport factor and the flux gain as does the base current in a transistor. (C) 2013 AIP Publishing LLC.