Yes, in some instances for "open" systems (ie systems which requires frequent additions of water due to evaporation or "boil off") we might recommend a portion of the system fill be soft water.  REMEMBER:  the sole purpose of soft water is to decrease the risk of scaling in a system.   In fact, soft water can accelerate mild steel corrosion if used incorrectly.  A good rule to follow:  if you are adding 5% or more of your total system volume each month due to evaporative loss (steam) you are heading for trouble if you do not improve the quality of your "make-up" water, ie soften, etc.  Mineral scale deposits or sludge/sediment will be the result.   NOT a good situation.  Decreased efficiency.  Drastically accelerated corrosion under the deposits/sludge.  Costly clean-up and repair. MAKE SURE your softener is functioning properly and do not allow brine water into your boiler when using soft water.  MAKE SURE you are adding corrosion inhibitor like our Treatment Solution 101 with the soft water fill. We would recommend using double the product dose when using soft water with your fill. Most situations benefit best from a 50% hard water, 50% soft water fill.  With this you get a decrease in scaling potention without a significant increase in mild steel corrosion.  Again, soft water will NOT prevent corrosion.  It may cause it.  Clarifying example:  If your system does not require water additions during the course of a heating season you DO NOT need soft water.