Thermoelectric effects in spintronics1 are gathering increasing attention as a means of managing heat in nanoscale structures and of controlling spin information by using heat flow2, 3, 4, 5, 6, 7, 8, 9, 10. Thermal magnons (spin-wave quanta) are expected to play a major role2, 5, 11, 12; however, little is known about the underlying physical mechanisms involved. The reason is the lack of information about magnon interactions and of reliable methods to obtain it, in particular for electrical conductors because of the intricate influence of electrons12, 13. Here, we demonstrate a conceptually new device that enables us to gather information on magnon–electron scattering and magnon-drag effects. The device resembles a thermopile14 formed by a large number of pairs of ferromagnetic wires placed between a hot and a cold source and connected thermally in parallel and electrically in series. By controlling the relative orientation of the magnetization in pairs of wires, the magnon drag can be studied independently of the electron and phonon-drag thermoelectric effects. Measurements as a function of temperature reveal the effect on magnon drag following a variation of magnon and phonon populations. This information is crucial to understand the physics of electron–magnon interactions, magnon dynamics and thermal spin transport.