A used water softener is one of the few pieces of water treatment equipment worth buying secondhand, because the tank and the valve are simple and long lived. It is also easy to buy someone else's failure, since the two parts that wear out, the resin bed and the control valve, are exactly the parts you cannot see from the outside.
Why it is for sale
Ask, and listen carefully. A softener removed during a kitchen remodel, sold with a house move, or replaced because the household upsized is usually fine. A softener sold because it stopped softening is a different proposition: the fault may be a cheap valve part, or it may be a fouled resin bed that costs most of a new unit to replace. Iron staining on the tank or in the brine well is a strong hint at the second case.
The resin is the real question
Resin gradually loses capacity, and chlorine in municipal water shortens its life considerably. Oxidized iron is worse: Penn State Extension notes that softeners can remove dissolved iron when pH is above 6.7, hardness is between 3 and 20 grains per gallon and dissolved iron is under 5 mg/L, but that oxidized iron fouls the resin. A unit that ran on untreated well water with iron may have a bed that no longer exchanges. Budget for a resin replacement and treat it as a bonus if it is not needed.
Inspect the valve and the tanks
Open the brine tank and look for a solid salt bridge, a crusted grid plate and a brine well full of sludge, all of which mean it was not maintained. Check the valve model is still supported and that seals, pistons and injectors can be bought, because an obsolete valve makes the whole unit disposable. Look for cracks in the brine tank, a corroded bypass, and a mineral tank whose threads or jacket are damaged. Ask to see it run a regeneration cycle if it is still connected.
Sizing matters more than the discount
A softener sized for the seller's house may be wrong for yours. Too small and it regenerates constantly, wasting salt and water and delivering hard water at the end of each cycle; too large and it regenerates so rarely that the bed channels and fouls. Work out your own grain requirement from your hardness and household use before you look at a used unit, and walk away if the capacity is wrong. Our water softener installation guide covers the plumbing and drain work you will still need to pay for.
Questions people ask about used water softener
How long does a water softener last?
A well maintained unit often runs for a decade or more, with the resin bed and the control valve seals as the usual wear items.
Can old resin be replaced instead of the whole softener?
Yes. Replacing the resin bed is a routine job if the valve is still supported and the tanks are sound, and it restores lost capacity.
Should I sanitize a used water softener before use?
Yes. A unit that sat idle should be flushed and sanitized, and the brine tank cleaned out, before it is put back into service.