Water Softener For Water Heater Only . If you know what part of your water softener needs replacing, you can get a better idea of what the water softener cost for repair will be. This just isn’t the case. Water Softener Charlie's Heating and Cooling from charliesheatingcolorado.com A water softener repair cost depends on the severity of the damage and the parts that need fixing. Corrosion inside the tank comes as the greatest cause of a water heater requiring replacement. Energy efficiency isn’t the only issue hard water causes with water heaters.