My water softener has a setting for KCl (Potassium) / NaCl (Sodium), what does it do exactly?

Question

I'm having some trouble with a relatively new water softener (<2 years). It seems like it is cycling when it should as the salt level is definitely going down, but the water is still hard.

While troubleshooting I came across a toggle setting that lets you choose KCl (Potassium) or NaCl (Sodium). Honestly, I haven't been paying that much attention to what type of salt I put in there, so theoretically I could have mismatched the setting to the salt that I'm using.

My questions:
1) What does that setting do exactly?
2) Could a mismatch be responsible for it not softening the water?

Edit: For clarification, I understand the difference between NaCl and KCl. My question really boils down to how the softener treats them differently when you toggle the switch and what is the impact of having it set incorrectly?

Answer 1

1. It controls the weight of the salt that is used. Potassium has a higher atomic weight than sodium, but they play the same role in the ion exchange reaction in the softener, so you need a greater weight of KCl to achieve the same level of softening.

   Potassium chloride has a molecular weight of 74.55, whereas Sodium chloride has one of 58.44, so in theory you should need about 27% more KCl. In practice, this article cites a study that suggests that less is needed:

   To achieve the exact equivalent capacity of an NaCl softener, the weight of KCl used would need to be increased by 13 percent at the 4 lb./ft.3 dosage level, but by only 2 percent at the 15 lb./ft.3 level.

2. If your softener is set to use NaCl, but you've added KCl, it'll be less effective than it should be.