A demineralizer strips dissolved ions from water using exchange resins, and the thing that surprises most first time buyers is where the cost sits. The unit is the small part. The resin is consumed in proportion to how much was dissolved in the feed water, so the same demineralizer costs very different amounts to run in two different houses.
How capacity is consumed
Every ion removed occupies an exchange site on the resin, and once the sites are full the bed stops working. So capacity is measured as a volume of water at a given feed concentration, and doubling the dissolved solids in the feed roughly halves the volume you get from a cartridge. The USGS classes water as soft at 0 to 60 mg/L as calcium carbonate through to very hard above 180 mg/L, and a demineralizer on very hard water exhausts several times faster than one on soft.
Softening the feed first
This is the standard answer where volume matters. A softener removes calcium and magnesium cheaply using salt, leaving the demineralizer resin to deal with a much smaller load, and it typically extends resin life several fold. On genuinely hard water the salt for a softener costs a small fraction of the resin it saves. For an occasional car rinse the extra equipment is not worth it; for regular use it usually pays for itself quickly.
Knowing when the resin is spent
Exhaustion gives no visible sign whatever, so a conductivity or TDS meter is not optional if the output matters: readings climb as the bed nears the end, and the water stops doing its job before anyone notices. Some cartridges use color changing indicator resin instead, which is convenient and less precise. Running a bed past exhaustion is not merely wasteful, since a fully loaded bed can release previously captured ions back into the flow.
Choosing the format
Disposable cartridges suit low volume use with no handling of chemicals. Refillable vessels take bulk resin at a much lower cost per volume and mean handling loose media. Mixed bed gives the highest purity in one vessel; separate cation and anion beds are used where the feed warrants it. For drinking water none of this is the right route, since ion exchange leaves organics and micro organisms untouched and reverse osmosis is cheaper. Our softener guide covers feed treatment.
Questions people ask about water demineralizer
What determines how long the resin lasts?
The dissolved solids in the feed water. Doubling the concentration roughly halves the volume a cartridge will treat.
Should I soften the feed first?
For regular use, yes. It typically extends resin life several fold, and salt costs a fraction of the resin it saves.
How do I know when it is exhausted?
With a conductivity or TDS meter, or an indicator resin that changes color. There is no other visible sign.
Is a demineralizer suitable for drinking water?
No. Ion exchange leaves organics and micro organisms untouched, and reverse osmosis achieves more for less.