What Is a Water Softener and How Does It Work?
A water softener is a tank of resin beads that pulls calcium and magnesium out of your water and swaps them for sodium or potassium. That exchange stops limescale in pipes, heaters and fixtures. If you already know you want one, our best water softener rankings start with the Aquasana SimplySoft 40K; otherwise, here is how they work.
What hard water is
Rain picks up calcium and magnesium as it soaks through limestone and chalk. Water carrying a lot of those minerals is "hard". The U.S. Geological Survey classes anything above 120 mg/L (about 7 grains per gallon) as hard, and much of the Midwest, Southwest and Texas sits well above that. Heat drives the minerals out of solution, which is why scale builds fastest in water heaters, kettles and dishwashers.
Water hardness chart: gpg, ppm and what to do
How to read this: find your test result in either unit. Classes follow the U.S. Geological Survey scale (mg/L as calcium carbonate); 1 gpg = 17.1 ppm.
| Class | ppm (mg/L) | gpg | What to do |
|---|---|---|---|
| Soft | 0–60 | 0–3.5 | A softener isn't needed for scale. Spend on a filter if taste is the issue. |
| Moderately hard | 61–120 | 3.6–7 | Some scale and spotting. A small softener or a salt-free conditioner fits. |
| Hard | 121–180 | 7.1–10.5 | Visible scale on fixtures and heaters. A salt-based softener is worth it. |
| Very hard | over 180 | over 10.5 | Heavy scale. Use a salt-based softener sized with the calculator; salt-free units usually fall outside their stated limits. |
The difference between hard water and soft water
The difference between hard water and soft water is the amount of dissolved calcium and magnesium, nothing else. Hard water leaves white crust when it dries, makes soap form scum instead of lather, and drops scale inside anything that heats it. Soft water lathers easily, rinses off with a slick feel and leaves few spots. Both are safe to drink; soft water made by a salt softener carries a little extra sodium (about 8 mg per litre for each gpg removed).
How a water softener works: ion exchange
Inside the resin tank are thousands of tiny plastic beads (usually polystyrene cross-linked with divinylbenzene) holding sodium ions. Calcium and magnesium ions carry a stronger charge, so as hard water flows down through the bed they stick to the beads and push sodium off into the water. What comes out the bottom is soft water with a small amount of added sodium. Every hard water softening system that actually removes hardness, from a $500 cabinet unit to a dealer system, works this way.
Regeneration: the brine cycle
Eventually the beads are covered in hardness minerals and can't take more. The control valve then runs a regeneration: it backwashes the bed to loosen it, draws concentrated brine from the salt tank, and lets that flood of sodium push the calcium and magnesium off the beads and down the drain. A rinse follows, and the bed is ready again. A timer valve does this on a fixed schedule; a metered (on-demand) valve counts gallons and regenerates only when needed, which saves salt and water.
Types of water softeners
Water softener types differ in layout more than chemistry. A salt based water softener system comes in one of three layouts; the fourth type isn't a softener at all:
- Two-tank: a tall resin tank with the valve on top, plus a separate brine tank. Standard, serviceable, holds lots of salt.
- Cabinet: resin tank inside the salt cabinet. Smaller footprint, less salt storage.
- Twin-tank: two resin tanks that alternate, so there is soft water during regeneration. Dual tank water softeners suit big households and homes that use water around the clock.
- Salt-free conditioners: not softeners at all; they reduce scale without removing minerals (compare them).
All water softeners in the first three groups use the same chemistry; they differ in footprint, capacity and how easy they are to repair. Our buying guide walks through choosing between them.
What does a water softener look like?
A two-tank softener is a tall, narrow resin tank (usually blue, black or beige fibreglass, around 10 inches across and 4–5 feet high) with a control valve on top, next to a wider plastic brine tank full of salt. A cabinet softener is a single plastic box about the size of a small trash can, with a lid you lift to add salt and the valve built into the top. Both connect to the main line with an in and out pipe, a bypass valve and a small drain hose.
How to soften water at home?
For the whole house, the only way to remove hardness is an ion-exchange softener on the main line. For a single job there are smaller fixes: laundry water-softening additives (washing soda or borax) bind hardness in one load, a vinegar rinse removes spots from glasses, and a portable softener can treat water for an RV or one appliance. Boiling removes only part of the hardness and leaves the scale in the pot.
Do you need one?
Test first. Below about 3.5 gpg your water is soft and a softener won't change much. Between 3.5 and 7 gpg it's a judgment call. Above 7 gpg, most homes see clear benefits: less soap, fewer spots, and longer appliance life (see the benefits of a water softener system). A water softener doesn't remove bacteria, lead, chlorine or most other contaminants; those need filtration.
What it costs to run
Running costs are salt (often one 40 lb bag a month for a family), a few dozen gallons of water per regeneration, and a little electricity for the valve. Our water softener cost guide adds those up, and water softener salt explains which bag to buy.