Abstract: A boiler of the type including an elongated firebox extending in a longitudinal horizontal direction and fire tubes arranged in to and fro horizontal passes through a heated fluid chamber and wherein the sets of fire tubes defining the to and fro passes respectively are distinctively arranged in laterally juxtaposed and spaced-apart relationship to obtain nearly the same speed of the combustion gases in the fire tubes of each set. In particular, the tubes of one set are laterally positioned on opposite sides of the tubes of the other set with the latter arranged within the transverse confine defined by a transversely triangular flue chamber at one end thereof.