The answer to 'how does heat exchanger work' is that a heat exchanger is a device used in HVAC systems to transfer heat from one medium to another, usually from a hot gas or liquid to a cooler one. In a furnace, the heat exchanger is the component that separates the hot combustion gases from the air being heated and distributed throughout the home. As the hot gases pass through the heat exchanger, they warm the metal walls, which in turn warm the air being pulled across the exchanger and blown into the ductwork.
Heat exchangers are designed to maximize the surface area between the hot and cold mediums, allowing for efficient heat transfer. They are typically made of metal, such as steel or aluminum, and have a series of fins or tubes to increase the surface area. The hot gases flow through one side of the exchanger, while the cooler air is drawn across the other side, absorbing the heat.