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A boiler is a heat exchanger which can be comprised of one, two, three, four, five or more passes; the number of passes is irrelevant.  The critical component is the size (in square feet) of the heat transfer area and whether or not the area is 100% effective (see Wet Back section).

Also important are the efficiency guaranteed by the manufacturer, the flue gas temperature at the exit of the boiler, the boiler fuel consumption at 100% load, and in terms of the useful life of the boiler, both the diameter and heat fatigue coefficient of the furnace.

A four-pass boiler does not necessarily have a larger heat transfer area than a three-pass boiler; nor is it automatically more efficient or consume less fuel.

The number of passes a fire tube boiler has does not provide any intrinsic benefit, but rather simply indicates the arrangement of the tubes within the boiler design.

In the heat exchange (boiler) calculation, the number of passes is not a factor.  The only variables that come into play in calculating the heat transfer are the following:

  • Heat exchange effective area
  • Heat transfer coefficient
  • Logarithmic temperature differential


Calderas Powermaster offers both three- and four-pass boiler designs ranging from 80 to 1000 HP. Both design options provide the same guaranteed efficiencies with the only difference being that the four-pass requires more powerful burner fans – thus increasing its electricity consumption.  For this reason, we recommend our clients opt for the three-pass option unless, because of the geometric constraints of the boiler room, it is necessary that the chimney be at the front of the boiler.