The resin in a water softener provides soft water ions that replace those which make water hard. The resin consists of negatively charged polystyrene beads that bond with sodium ions. As water passes through the resin, it exchanges the sodium ions for those that make water hard, primary calcium and magnesium. The water softener uses salt to recharge the resin with more sodium.
Hard water is a problem because it makes soaps and detergents less effective at cleaning. The minerals in hard water become deposited in pipes, fixtures and water using appliances, eventually causing them to fail. The ions in soft water remain in solution and do not form deposits.
In most units, the manufacturer designs the resin bed for a 10 to 15 year lifespan; however, if the water source is high in iron or chlorine, these ions can poison the resin, so the unit no longer softens the water without repair or replacement.
When the resin bed fails, the water softener begins using more salt than normal, but does not produce soft water. For water sources high in iron, use of a water softener iron removal product can sometimes restore the resin bed. If the product does not work, replace the resin. As of 2016, having a professional replace it costs about $250, but replacing it yourself saves about $150.