  We use the Boltzmann transport theory in the relaxation time approximation to
describe the thermal transport of spin waves in a ferromagnet. By treating spin
waves as magnon excitations we are able to compute analytically and numerically
the coefficients of the constitutive thermo-magnetic transport equations. As a
main result, we find that the absolute thermo-magnetic power coefficient
$\epsilon_M$, relating the gradient of the potential of the magnetization
current and the gradient of the temperature, in the limit of low temperature
and low field, is a constant $\epsilon_M = -0.6419 \, k_B/\mu_B$. The theory
correctly describes the low-temperature and magnetic-field dependencies of spin
Seebeck experiments. Furthermore, the theory predicts that in the limit of very
low temperatures the spin Peltier coefficient $\Pi_M$, relating the heat and
the magnetization currents, tends to a finite value which depends on the
amplitude of the magnetic field. This indicates the possibility to exploit the
spin Peltier effect as an efficient cooling mechanism in cryogenics.
