Abstract: The heat exchange system is used for heating a first fluid with a second fluid, and includes a first fluid circuit comprising an upstream end, a downstream end and an intermediate portion therebetween and a second fluid circuit comprising an upstream end, a downstream end and an intermediate portion therebetween. The heat exchange system also includes a flooded heat exchanger unit wherein the intermediate portions of the first and second fluid circuits extend and are in adjacent, thermally-conductive contact for allowing heat transfer from the second fluid to the first fluid. The flooded heat exchanger is capable of being flooded in a determined proportion within the second fluid circuit intermediate portion.