when the boiler mfr specifies it. For most large boilers, one can design it as either a forced circ or nat circ, for approx the same price. But some mfrs are piramarily forced circ boilermakers, and others are soley nat circ vendors. If you have already selected the boiler mfr, then you have already decided which technology to use.
Large HP boilers of drum design pressure greater than about 2900 psig and fuel heat input greater than approx 3 x 750 MWth would probably need to be forced circ or designed as a once thru ( benson), because it is difficult to configure a nat circ for this size range. Also, there are a few unusual boilers that need a recirc pump ( slagging combustor), but most can use either technology.
