Water tube boilers and fire tube boilers have some key differences. A fire tube boiler relies on having one tube (for one-pass boilers) or several tubes (for two-pass, three-pass, or four-pass designs) to push heated gas from one end of the chamber to the other. The sealed chamber of water pulls the heat through the metal sides of the tubes’ walls using thermal conduction as the heated water creates steam. The fire tube boiler has the water around the heated cylinders and gases.

Water tube boiler design works in the opposite manner where the heated gases inside the chamber surround the water. The firebox generates hot gas that moves into the main rectangular chamber tube of the boiler that runs along its entire length. The hot gases surround the water and create a superheater effect in the steam boiler to heat the water externally. The water tubes are completely sealed as the tubes can withstand higher water pressures. Water boiler systems are safer than fire tube boilers and can help lower how much fuel is used to create the generated heat at the desired level. However, they are less cost-efficient than a high-efficiency fire tube boiler.