Making deionized water at home is a matter of passing water through the right resin, and the equipment is simple. What decides whether it is practical is the feed water, because the resin is consumed by everything dissolved in what you put through it, and on hard water a cartridge disappears far faster than people expect.
The equipment
A mixed bed cartridge, containing both cation and anion resin, in a housing plumbed into a supply. The cation resin exchanges positive ions such as calcium, magnesium and sodium for hydrogen; the anion resin exchanges negatives such as chloride, sulfate and nitrate for hydroxide; those combine into water, so the ions are removed rather than substituted. That is the whole process, it runs at line pressure, needs no electricity and produces at usable flow.
Feed it the cleanest water you can
This is the decision that determines running cost. Resin is consumed in proportion to the total ionic load it removes, so on hard water a cartridge is used up quickly and on soft water it lasts far longer. Feeding it from a softened line helps. Feeding it from a reverse osmosis membrane helps far more, since the membrane removes the bulk cheaply and the resin only polishes what remains, which is why that combination is the standard professional arrangement.
Knowing when it is spent
Many cartridges include a colour changing indicator resin, which is the simplest guide. Better is a conductivity or dissolved solids meter on the output, since deionized water conducts almost nothing and a rising reading shows the resin approaching exhaustion before the water is visibly affected. Do not run a cartridge past exhaustion: as it fails it can release previously captured ions, so the output can briefly be worse than the feed.
What it is not
Not drinking water. Deionization removes charged species and nothing else, so bacteria, viruses, particles and many organic molecules pass straight through, and the resin bed itself is a wet warm environment that supports growth, particularly in a unit used intermittently. The USGS describes hardness as dissolved calcium and magnesium, and removing them makes water taste flat, but taste is not the issue here: nothing in a technical deionizer was designed to make water safe. Our softener guide covers household hardness.
Questions people ask about how to make deionized water
What equipment makes deionized water?
A mixed bed resin cartridge in a housing, run at line pressure with no electricity needed.
How do I make a cartridge last longer?
Feed it the lowest dissolved load you can, from a softened line or, far better, from a reverse osmosis membrane.
How do I know when the resin is spent?
By a colour changing indicator, or better by a conductivity meter on the output, since a rising reading precedes visible failure.
Can I drink it?
No. Deionization removes charged ions only, and the resin bed itself can support bacterial growth.