(under subclass 51) Devices having two diameters, with that part of the boiler having the greater diameter over the firebox. The great diameter is offset from the main boiler, and the offset portion has short fire tubes passed therethrough, while long fire tubes extend through the whole length of the boiler. The products of combustion pass out of the firebox and return through the short tubes in the offset portion, and then pass in an opposite direction to the other end of the boiler, where the outlet flue is located, or they may return and pass through the upper short tubes and leave the boiler at the outlet flue located at the firebox end.
