Abstract: A one-piece tube for use as a watertube within a watertube boiler or heat exchanger has a free end section that is formed to have an integral, outward-extending circumferential ring, or flange, a spaced distance from an adjacent end face of the tube. A boiler is provided that has an elongate header and a plurality of separate watertubes each having an intermediate section and opposite end sections. An end section of each watertube is connected to the header. The intermediate section of each watertube has a substantially constant outer diameter along its full length and is closely spaced to adjacent watertubes within the combustion chamber. At least one of the end sections of each watertube has a transition that reduces the diameter of the watertube as it extends from the intermediate section to an outwardly-extending circumferential flange. This arrangement permits close spacing of the connection sites of the watertubes to the header and permits the connection sites to be provided in a linear array.