a furnace with walls formed by non-orificed vertical tubes having an inlet and outlet;

feed pump means for pressurizing said system to force water into the inlets of the tubes via at least one first conduit for passage to the vertical tubes to transfer heat from the furnace to the water to convert a portion of the water to steam forming a steam-water mixture;

a valveless unbypassed second conduit for transferring the heated steam-water mixture to a separator for separating said steam from said water, wherein the separator comprises a stripper ring and a scrubber;

a third conduit for connecting the separator to the inlets of the vertical tubes for transferring separated saturated water from the separator to the inlets of the tubes for recirculation; and

a on/off control valve along the third conduit and downstream of the separator for controlling boiler circulation mode and passage of water from the separator to the inlets of the vertical tubes,

an unbypassed feed water inlet conduit attached to the third conduit downstream of the control valve, wherein the feed pump means is located along the feed water conduit; and

wherein the system is operable with only one type of circulation system depending on load operation, the separator is unbypassed at all load operations, and the third conduit does not possess a circulation pump.