Big problem.  I have seen ...

Big problem.  I have seen this too often.  A big scale build up at the point where the flows mix together.

Wow, don't they teach qualitative analysis in chemistry classes anymore?  High alkalinity from the boiler will precipitate the hardness down for the bottom blowdown valve to remove.  Duh!   High alkalinity from the boiler will precipitate the carbonates, hardness and a lot of other things.    Look up lime soda softeners and learn how they work.   Learn about precipitation and why it happens.  Most of all, get off the net and quit asking questions of people who don't know.  Go get a chemistry text book and read it and forget the computer discussions.  This was written about over 100 years ago.  Good grief kids.