I need to start off by mentioning that even as a realist with a strong scientific background, marketing can be a powerful force not to be underestimated - especially concerning aspects that are unfamiliar to oneself.

Water. H₂O. Why do they teach people in primary school that water's chemical formula is H₂O? Only perfectly pure water in a vacuum might approach that composition. Normal water is so much more complicated that trying to understand it is like trying to understand how individual electrons move in a macroscopic object like a person.

Enough rambling. I recently relocated to a small town in Alberta, Canada. The water used to be very soft on the west coast where I came from (TDS = 19ppm). So water treatment was really not needed in any home I have lived in. The water was soft enough to not cause issues with soap, washing machines, refrigerators or boilers, and hard enough not to be corrosive. However it is a different story here on the other side of the Rockies. I measured my TDS with an electronic meter and the result was 260ppm. That is considered very hard water (depending on who you ask). It is enough that I could see buildup of limescale on some of my equipment such as the HVAC unit. The previous owner had a Premier AF-40K water softener installed, however it had no salt in it.

This is where the trouble started. I am not a chemist, I have never owned a home with a water softener so I had no clue how it worked or what it was doing on a technical level. To self educate - I made the grave mistake of entering the void called "The Internet". 80% of all website hits were from manufacturers selling water softeners. Many are pro ion-exchange water softeners, many are against that.

The problem came in when, after reviewing many sites online, I decided to measure the TDS of my water. It indicated to me that the Premier AF-40K was not functioning. So I clearly needed to add salt and start a recharge cycle, which I did. After 3 days (giving the water time to replace the reservoir in the boilers) I tested again - TDS was exactly the same at 260ppm. So I assumed the water softener was not functioning properly. After a very expensive replacement with a Kinetico device, couple of days later I tested my water again - 260ppm. However by this time I was anticipating that result. See - TDS is measured using electrical conductivity as proxy multiplied by some scale factor. Now I do know that an ion exchange water softener works by binding Mg⁺⁺ and Ca⁺⁺ into beads of resin, by exchanging Na⁺. So inflowing water has Mg⁺⁺ and Ca⁺⁺, after contact with the resin it has 2Na⁺ for each ion exchanged I think. In English - it removes Calcium and Magnesium and replaces it with Sodium.

However, from electrical conductivity's point of view, there is no difference between these ions - they all flow the same. So if electrical conductivity is unaffected, then TDS will be too - meaning you cannot use TDS as proxy for the effectiveness of your water softener. What makes things worse is that none of the three technical people I spoke to about my water softener issues corrected me when I mentioned using TDS for measuring.

Now that I realized my mistake I tried the next logical solution - measure the actual amounts of these three ion concentrations in the water both pre and post treatment. This is where things got hairy quickly. There is no affordable kit that I could find that can distinguish between Mg⁺⁺, Ca⁺⁺ and Na⁺. Eventually I remembered that a KH test (for freshwater aquariums) will test the amount of carbonates and bicarbonates whereas a GH test will test the amount of divalent metal ions. Now not being a chemist but understanding basic chemistry, this seemed like a possible proxy. I knew that Mg⁺⁺ and Ca⁺⁺ are divalent whereas Na⁺ is monovalent. So KH should be unaffected as the carbonates and bicarbonates were not lowered, just exchanged. GH should however drop - the multivalent ions should be removed and only the monovalent ions should be left over, which GH will not detect.

I grabbed my stupid API test kit and measured the outside (untreated) tap water and compared that with the water from my kitchen faucet. Lo and behold - the outside water had a GH of 14°GH whereas the treated kitchen faucet water was < 1°GH. However to be absolutely certain I took two samples of water and sent them to a lab for analysis:

Now I have to figure out how to hook up an RO unit to remove the salt for my aquarium...