Those parts of a boiler which are exposed on one side to only the water being heated and on the other side to the combustion gases, such as the economizer surfaces.

Those parts of a boiler which are exposed on one side to only the steam being heated and on the other side to the combustion gases, such as the superheater surfaces.

Those parts of a boiler which are exposed on one side to the water or steam being heated, and on the other side to the combustion gases.

Those parts of a boiler which are exposed on one side to only the water being heated and on the other side being directly exposed to the furnace flame.