A water softener takes the calcium and magnesium out of hard water and puts sodium or potassium in their place. It does it with a tank of resin beads, a tank of salt water and a valve that decides when to clean one with the other. Here is each part in plain words.
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What makes water hard
The U.S. Geological Survey explains that hardness is caused by compounds of calcium and magnesium, and by a variety of other metals. In hard water, soap reacts with the calcium to form soap scum, so you need more soap or detergent to get things clean. Hard water also leaves deposits on dishes, fixtures and basins and can build up scale on pipes, as water utilities commonly note.
Ion exchange in the resin tank
The tall tank of a softener is full of small resin beads. The beads carry sodium or potassium on their surface. As hard water flows through, the beads take up calcium and magnesium and let go of sodium or potassium in exchange. That swap is called cation exchange, and it is what NSF/ANSI 44, the standard for home softeners, describes: a cation exchange resin regenerated with sodium chloride or potassium chloride. Water utilities put the result simply: sodium is added to the water in exchange for the hardness removed.
Regeneration and the brine tank
The resin can only hold so much hardness. When it is full, the softener regenerates. The shorter tank beside it, the brine tank, holds salt and water, which make a strong salt solution called brine. During regeneration the control head runs that brine through the resin. The flood of sodium or potassium pushes the calcium and magnesium off the beads, and the hardness and used brine are rinsed to the drain. Then the brine tank refills with water, ready to dissolve more salt for next time.
- Backwash: water runs up through the resin to loosen it and flush out sediment.
- Brine draw: brine is drawn from the brine tank through the resin.
- Rinse: fresh water rinses the brine and the hardness to the drain.
- Refill: water goes back into the brine tank to make the next batch of brine.
The control head
The valve on top of the resin tank is the softener’s brain. It sends water through the resin in normal use and runs the regeneration steps. Some control heads regenerate on a timer; others count the water used and regenerate when the resin is due. Either way, they are set for your water’s hardness, which is why a softener has to be programmed after install and checked when your water changes. The maintenance page covers the settings.
What a softener does not do
A softener is built for hardness. It is not a filter for chlorine taste, lead or bacteria, and utilities are clear that it is not a health device: they say hard water is not a health hazard, and that using a softener is a matter of personal preference that may not improve the safety of water. For anything other than hardness, ask what treatment is certified to remove it; the softener versus filter article explains the difference.
There are side effects to know. Salt-based softeners add sodium. Water utilities commonly note that some softening systems can make water more corrosive, which can damage the anode in a tank water heater. And each regeneration sends rinse water to the drain.
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What installing one involves
Knowing how it works explains how it is installed. The softener goes on the main line, before the branch to the water heater, so all the water passes through the resin. It needs a drain, because regeneration sends rinse water away; a power outlet, because the control head runs on electricity; and room for the brine tank and the salt. A bypass valve lets the softener be shut out of the line while the house keeps its water. And the control head is programmed for your hardness. The installation by a plumber page covers each of those steps.
Salt-free systems
Devices sold as salt-free softeners or conditioners do not regenerate with salt, so they work differently from the ion exchange described here. Ask what any of them is certified for. See salt-free softeners.
Questions people ask
What does a water softener do?
It removes calcium and magnesium, the minerals that make water hard, by swapping them for sodium or potassium on resin beads.
Do you need to put water in a water softener?
Usually not. The softener adds water to the brine tank itself as part of its cycle, and you add the salt. The manual says what water level is normal for your model.
