Just as the heat exchanger is the heart of the furnace, it's also the heart of the boiler.
Heat exchangers in boilers have the same function as heat exchangers in furnaces: they transfer the heat from the fire into the heat transfer medium. With furnaces, the heat transfer medium is
. With boilers, the heat transfer medium is
Common materials, include cast iron, steel, alloys and copper.
It is usually located above the burner, and sees the products of combustion coming off the burner. At the top of the heat exchanger, the products of combustion are typically directed into the venting system. The cool water typically enters the bottom of the heat exchanger and the heated water leaves from the top.