The answer is to reduce the ...

The answer is to reduce the level of dissolved solids in the make-up water to the boiler.  Plain and simple this is a direct, reliable path to achieving your goal.  One could make use of some new water purification equipment:  membrane capacitive deionization, for example, will very efficiently remove hardness from raw water, but further it will reduce other conductive ions to a low level, depending on operating parameters.  Additionally, recovery of water can easily be over 90%, rivaling that of traditional ion-exchange water softening.  No reverse osmosis or other pretreatment is needed, as the membrane CAP DI is very robust, tolerant of all chlorine species to levels not typically found in water supplies.

One can also choose high-efficiency reverse osmosis (HERO), and nearly achieve the same results, but with additional required pretreatment necessary to protect the membranes.