In water tube boilers, the water travels inside the tubes and the heat on the outside, as shown above. Cast iron boilers are similar to water tube boilers, but the water is contained in cast iron sections instead of in tubes.

All steam boilers have four independent systems. The feed water system supplies water to the boiler, the fuel system transports the fuel, the draft system controls air flow for fuel combustion, and the steam system collects and controls the produced steam.