I will consider electric and thermal transport in a junction of three interacting quantum wires. The Wiedemann-Franz law holds for a wide class of fixed points of this system, but I will argue that it can be violated for specific boundary conditions. This happens, in particular, when the wires are connected to an island hosting topological Majorana modes and can be characterized by a universal number. Joint measurement of heat and charge currents can therefore provide a way of detecting the presence of Majorana quasiparticles in this class of systems.