Where is the heat drawn from the preheater?

The heat for the preheater is drawn from the flue gas. In many heating systems, the preheater utilizes waste heat from flue gases to enhance the efficiency of the system. By capturing this otherwise lost thermal energy, the preheater can warm up air or fluid before it's used in the heating process. This method not only improves the overall efficiency of the system but also helps in reducing fuel consumption as it makes better use of the energy produced during combustion. The use of flue gas for preheating can result in a more environmentally friendly operation by lowering emissions and making more effective use of the energy available. The other options do not typically provide the necessary heat for a preheater in such systems.