H2O + CO2 + CaCO3 ---> Ca(HCO3)2
Boiling the water reverses the above reaction and the resultant calcium carbonate which is insoluble will settle out. The permanent hardness is from various magnesium salts such as magnesium chloride in the water and they are unaffected by boiling. However, the permanent hardness is usually not a problem for photographers.
Bring the water to a full rolling boil for 5 to 10 minutes in an uncovered container of glass, stainless steel, or agate ware. Cover and allow it to cool overnight. Decant or filter off the softened water.