Salt-free vs. Salt Water Softeners

Traditional Salt-based water softeners reduce hardness minerals from the water by exchanging calcium and magnesium ions for sodium ones. 

Salt-free water softeners (sometimes known as water conditioners or descalers) do not reduce hard water minerals, but instead chemically alter the minerals, making them stick together - reducing scale buildup in your plumbing.  Salt-free water softeners produce water that may still feel hard, cause soap scum, and leave spotting on dishes and surfaces - this is because they are not truly reducing the minerals from the water.