The unrecoverable dissipation of thermal energy that flows from a body to a place of lower temperature. The amount of heat flow, or loss, is dependent on the temperature difference of the body to the environment at the lower temperature, the exposed area, and the surface heat transfer coefficient. Heat is lost through the walls of pipes that transport hot fluids. The heat loss can be reduced by surrounding the pipe with an insulating material that has a low thermal conductivity. See insulation.