A used water treatment unit can look like a bargain on a resale listing, after a home sale, or when a company swaps out its equipment. The price is only part of the picture. A used unit carries history you cannot see from a photo, and some of that history decides whether it will do the job you bought it for.
Get the history in writing
Ask the seller for the make, model, and serial number, and for the reason the unit was removed. Find out whether it came from a single-family home, a rental, or a commercial site, and what water it treated if the seller knows. A unit pulled from a home on a private well and a unit pulled from a city connection may have seen very different loads. Ask for any service records, receipts for media or cartridge replacements, and the approximate age of the unit. If the seller cannot answer these questions, treat that as information.
Check the certification, not just the claim
Look at the product label for the standard it is certified to and for the specific contaminants listed. A certification applies to the model that was tested, not automatically to the single unit you are buying, so ask the seller which standard the performance claim relies on and for the certificate or listing number. You can check that number against the directory kept by the certifying organization. Also ask whether any parts that touch drinking water are certified for that use. A certification for one purpose does not cover another.
Inspect before you pay
Look over the housing or tank for cracks, bulges, and corrosion at fittings. Check the valve or control head: does the display work, does it show a time and a regeneration setting, are there broken parts or brittle plastic? Ask whether replacement parts for that model are still available. If the seller allows it, have the unit connected to a water source and watch every joint for leaks. Ask to see it run through a cycle or have someone show you what each control does. Do not pay until you have seen it work.
Plan for the parts you should not reuse
Some components should be replaced rather than kept. Used cartridges should not be reused, and filter media from another home may need replacing before the unit goes into service. Ask a qualified installer whether the tank or media should be sanitized before use, and whether they will install a unit they did not sell you.
Price the whole job
The real cost of a used unit includes what it needs to work: new cartridges or media, fittings, a licensed plumber for connections, a drain or power connection if the unit needs one, and water testing after installation. Get written quotes for installing the specific unit you plan to buy. Some installers will not connect used equipment or will not stand behind their work on it. Find that out before you buy, not after.
Test the water after installation
Once the unit is connected, test the treated water for the problem you bought it to solve. Compare the results with the same test run on untreated water. If you would rather have a company handle the purchase, setup, and testing, compare providers on the winners list and ask each one the questions above.
Walk away if
The seller cannot identify the model, the certification cannot be verified, the unit leaks under test, the controls do not work, or no installer will connect it. A low price does not make up for any of these.