Heat transfer is the topic centering on the movement and conversion of heat from one system to the next system. In this section, the three modes of heat transfer will first be discussed in order to give a background into the concepts of heat transfer. The three modes of heat transfer are (1) Conduction, (2) Convection and (3) Radiation. Following this discussion, this section will delve into the primary application of heat transfer concepts in the HVAC and Refrigeration field, which is Cooling and Heating Load Calculations. Another important application of heat transfer is heat exchangers, which will be discussed in this section and in the Mechanical Systems section. Finally, determining insulation requirements is discussed. Determining insulation requirements is an important and practical skill and can be used for determining the needed insulation for pipes, ducts, walls and roofs.