Answer:
Heat exchanger
Explanation:
In a furnace, there is a device that transfers heat from the flame and combustion gases into a heating fluid like water. That particular device is called an heat exchanger.
Primarily, to transfer heat between two or more fluids, what we need is a heat exchanger. These fluids that needs to have their heat exchanged such as the combustion gases and water in this question may or may not be separated by a solid partition. They are usually separated in cases where mixing is not desirable.
The basic underlying principle of a heat exchanger is that it works by transferring the heat only without the fluids being exchanged or transferred
