Hard water & choosing an indirect water heater

Dear Readers,
My house has hard water. I plan on upgrading to an indirect water heater (I currently have a hydronic system with tankless coil). Should I choose an indirect water heat that is a tank-in-tank model, or a coil model? Does either type make a real difference in avoiding hard water scaling within the tank and the reduced efficiency it causes?
Any advice appreciated! -Anthony Falcone
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