  We propose inducing Bose-Einstein condensation of magnons in a magnetic
insulator by a heat flow oriented toward its boundary. At a critical heat flux,
the oversaturated thermal gas of magnons accumulated at the boundary
precipitates the condensate, which then grows gradually as the thermal bias is
dialed up further. The thermal magnons thus pumped by the magnonic bulk (spin)
Seebeck effect must generally overcome both the local Gilbert damping
associated with the coherent magnetic dynamics as well as the radiative
spin-wave losses toward the magnetic bulk, in order to achieve the threshold of
condensation. We quantitatively estimate the requisite bias in the case of the
ferrimagnetic yttrium iron garnet, discuss different physical regimes of
condensation, and contrast it with the competing (so-called Doppler-shift) bulk
instability.
