A parallel flow concentric tube heat exchanger is designed to recover exhaust heat by using the exhaust pipe as the tube side and the concentric tube as the shell side. The heat exchanger which is also called the primary heat exchanger is welded concentrically to the intermediate exhaust pipe forming a double pipe heat exchanger. The shell side pipe consists of Cold Water inlet and Hot Water outlet, while the tube side pipe carries the hot exhaust gases. The water flowing in the shell side gets heated due to conduction and convection and is circulated through the Hot Water outlet to produce Hot Water on demand. The same Hot Water is simultaneously circulated to the annular chamber of the secondary heat exchanger, thereby transferring the heat to the Hot Box.