This document discusses how the thermal conductivity K for magnons differs from the textbook formula for fermions. It explains that for magnons, heat flow creates a magnon accumulation and magnetization gradient, which sets up a counter current. This results in a new quasi-equilibrium state with no net magnon current but a modified thermal gradient. The thermal conductivity K is then defined based on the heat current IQ in this quasi-equilibrium state, and takes a form different from the textbook formula using the Lorenz number L22 for fermions.