Heat flows from a region of higher temperature to a region of lower temperature. This direction of heat flow continues until thermal equilibrium is reached between the two systems. Essentially, heat energy moves down the temperature gradient, from hot to cold, following the second law of thermodynamics.

This concept holds for all three modes of heat transfer—conduction, convection, and radiation—and is mathematically described by Fourier's law where the heat flux vector points opposite to the temperature gradient.