How Does a Water Softener Work? (Simple Diagram + Types Explained)
Quick answer: a water softener swaps hardness for sodium. Hard water flows through a tank of resin beads that hold sodium. The calcium and magnesium that make water hard stick to the beads, and sodium takes their place. When the beads are full, the softener regenerates: it rinses them with salty brine from the salt tank, which pushes the calcium and magnesium off and down the drain and reloads the beads with sodium. To check that yours is working, test the softened water. Under 1 grain per gallon counts as soft.
The parts of a water softener
A standard salt-based softener has two tanks and a valve. Here’s what each part does:
- Resin tank. A tall tank filled with resin beads, with a riser tube down the middle that carries softened water back up.
- Control valve. Sits on top of the resin tank. It sends water through the resin, runs the regeneration cycles and decides when to regenerate, by a timer or by metering how much water you use.
- Brine tank (salt tank). Holds salt and water, which dissolves into the brine used for regeneration. Inside is a tube called the brinewell, where brine is drawn out and fresh water flows back in.
- Nozzle and venturi. Whirlpool’s manual says this small part creates the suction that moves brine from the brine tank into the resin tank.
- Bypass valve. Lets water skip the softener for repairs. In “service” the water is softened; in “bypass” it isn’t.
- Drain line. Carries regeneration water to a floor drain, laundry tub or standpipe.
Cabinet softeners put the same parts in one box, with the resin tank standing inside the salt tank. See our small water softeners guide for examples.
How a water softener works, step by step
- Hard water comes in. It carries dissolved calcium and magnesium, the minerals that make water hard.
- It flows down through the resin. Each bead holds sodium loosely. North Dakota State University (NDSU) Extension explains that the larger calcium and magnesium ions displace the smaller sodium ions. This swap is called ion exchange.
- Soft water goes out. The softened water rises up the riser tube and flows to the rest of the house, carrying the sodium the beads let go.
- The beads fill up. Bit by bit, the beads trade all their sodium for calcium and magnesium. Before they run out, the softener regenerates.
What is softened water?
Softened water is water that has been through a softener, with its calcium and magnesium swapped for sodium (or potassium). NDSU counts water under 1 grain per gallon (gpg), or 17 mg/L, as soft. Softening adds a little sodium: about 8 mg per liter for every grain per gallon removed, according to NDSU. See is softened water safe to drink? for what that means for drinking.
What does regeneration (regen) mean?
Regeneration, often shortened to “regen” and called a “recharge” in some manuals, is how a softener reloads its resin with sodium. NDSU explains that during regeneration, sodium from the brine replaces the calcium and magnesium on the resin. The softener then flushes them down the drain. A typical regeneration on a Fleck 5600SXT, a common valve on do-it-yourself softeners, runs these steps (default times from one seller’s programming guide):
| Step | What happens | Default time |
|---|---|---|
| 1. Backwash | Water flows up through the resin to loosen and clean the bed | 10 minutes |
| 2. Brine draw and slow rinse | Brine is pulled from the salt tank through the resin, then slowly rinsed through | 60 minutes |
| 3. Rapid rinse | A fast rinse flushes out the leftover brine | 12 minutes |
| 4. Brine refill | Water refills the salt tank, where it dissolves salt for next time | A few minutes, set by softener size |
From Quality Water For Less’s Fleck 5600SXT programming guide. Other brands use the same steps in a different order.
Whirlpool’s softeners, for example, refill the salt tank first, then draw brine, backwash and fast rinse. Either way, a regeneration usually takes about 1.5 to 2 hours; Whirlpool says a manual recharge on its softener takes about two hours. The Fleck guide’s default start time is 2:00 a.m. Overnight is a good time, because a single-tank softener sends hard water to the house while it regenerates. Rheem’s manual advises against using hot water during regeneration, or the water heater fills with hard water.
How a softener decides when to regenerate
- Timer: regenerates every set number of days, whether you’ve used much water or not.
- Metered (demand-initiated): counts the gallons you use and regenerates only when the resin is nearly used up. NDSU notes that many models let you choose either method. Demand-initiated regeneration is one of the requirements for an NSF/ANSI 44 efficiency rating; see our high-efficiency softeners.
Each regeneration uses salt and water. Rheem’s 18,000-grain compact, for example, uses 1.8 to 10.8 lb of salt per regeneration depending on the setting, and up to 26.4 gallons of water. More detail in our guide to water softener regeneration.
What a water softener removes (and what it doesn’t)
| Removes | Doesn’t remove |
|---|---|
| Calcium and magnesium (hardness) | Sediment, such as sand and silt |
| Some dissolved (clear-water) iron and manganese, up to each model’s limit: 3 ppm of iron for Rheem’s compact RHS18, 12 ppm for Whirlpool’s WHES48 | Red-water (ferric) and bacterial iron |
| Barium and radium, on models certified for them (Rheem’s RHS18 is) | Chlorine, bacteria and most chemicals |
From NDSU Extension and Rheem’s and Whirlpool’s manuals.
NDSU says some softeners remove iron and manganese up to 10 ppm, and that higher levels may need a dedicated iron removal system; see our iron guide. For chlorine, chemicals and taste, you need a filter. See water softener vs water filter or our filter and softener combos.
Types of water softeners
| Type | How it works | Softens? | Good for |
|---|---|---|---|
| Salt-based (single tank) | Ion exchange; regenerates with salt brine | Yes | Most homes. See the best water softeners |
| Dual-tank (twin) | Two resin tanks take turns, so one is always in service | Yes | Big households and soft water around the clock. See dual-tank softeners |
| Potassium chloride | The same softener, with potassium chloride instead of salt | Yes | Cutting sodium; NDSU says it costs more and about 10% more is used |
| Portable | A small ion-exchange tank you recharge by hand with salt | Yes | RVs and boats. See portable softeners |
| Salt-free (TAC) | Turns hardness into tiny crystals that are less likely to stick | No | Scale control without salt or a drain. See salt-free softeners |
| Electronic or magnetic descaler | A device on the pipe meant to change how scale forms | No | Weakest evidence of the group (see below) |
| Reverse osmosis | A membrane filter, usually under the kitchen sink | Reduces hardness at one tap | Drinking water. See softener vs RO |
Only the ion-exchange types remove hardness from all the water. Salt-free systems and descalers leave the minerals in, and RO usually treats one tap. In an independent study published by WateReuse (project 08-06), template-assisted crystallization (TAC), the crystal-forming method used by the salt-free systems we recommend, cut scale formation by more than 88%. Electromagnetic and electrically induced devices cut it by about 50%. We didn’t find comparable independent tests of permanent-magnet devices. For a side-by-side, see salt vs salt-free water softeners.
How to tell if your water softener is working
- Test the softened water. Use a hardness test strip on water from a cold tap after the softener. Rheem and Whirlpool include one in the box. Under 1 gpg is soft by NDSU’s scale. For comparison, test your untreated water too; outside faucets are often left on hard water, as Rheem’s manual recommends.
- Watch the salt. The level should drop over a few weeks. Rheem suggests checking every two or three weeks until you know your routine, and adding salt when it’s under a quarter full.
- Look for hard-water signs coming back. NDSU lists soap scum, spotted glassware from the dishwasher and scale buildup as effects of hard water.
- Watch a regeneration. Start a manual recharge and look for water flowing from the drain line. Whirlpool’s manual says that in the brine step, the liquid level in the brinewell should drop noticeably, which may take 15 to 20 minutes.
Is my water softener working? Checker
Answer what you can. The checker uses the troubleshooting steps in Rheem’s and Whirlpool’s manuals.
Answer the questions to check your softener.
How it works: a strip reading under 1 gpg counts as soft, using NDSU’s hardness scale. Each fix comes from the troubleshooting guides in Rheem’s and Whirlpool’s manuals, listed in roughly the order to check them. Your own manual comes first if it says something different.
Water softener stopped working? Common causes
| What you notice | Likely cause | Fix |
|---|---|---|
| Hard water, empty salt tank | No salt, so no brine | Add salt, then start a manual recharge |
| Hard water, blank display | No power: unplugged, tripped breaker or failed power supply | Restore power, reset the clock, start a manual recharge |
| Hard water, salt level not dropping | A salt bridge (a hard crust above the water), or the bypass valve is in bypass | Break the bridge gently with a broom handle; move the valve to service |
| Salt tank full of water | The softener isn’t drawing brine: a plugged nozzle and venturi, a kinked drain hose or low water pressure | Clean the nozzle and venturi; straighten the drain hose |
| Water hard only sometimes | Wrong clock or hardness setting, hot water used during regeneration, or leaking faucets and toilets | Reset the settings; fix leaks |
| Still hard after all that | Worn or iron-fouled resin | Clean or replace the resin |
From the troubleshooting guides in Rheem’s RHS18 and Whirlpool’s WHES48 manuals, except the last row.
Rheem warns never to use a sharp tool on a salt bridge or pound on the tank, which can damage it. For the last row, see our guides to resin replacement and water softener cleaners, and check your hardness setting.
Frequently asked questions
What does regen mean on a water softener?
Regen is short for regeneration: the cycle in which the softener rinses its resin with salt brine to reload it with sodium and flushes the calcium and magnesium down the drain. Some manuals call it a recharge.
How long does a water softener take to regenerate?
Usually about 1.5 to 2 hours. A Fleck 5600SXT’s default steps add up to about an hour and a half, and Whirlpool says a manual recharge on its softener takes about two hours.
What happens if I unplug my water softener?
Water still flows through the softener, and the resin keeps softening it until it’s used up, but the softener can’t regenerate without power. Rheem’s manual says its controller keeps the correct time for several hours after a power cut, and the hardness setting doesn’t need re-entering. If the time is flashing when power returns, reset it, or regenerations may happen at the wrong time of day.
Does a water softener need electricity?
Most automatic softeners do, to run the control valve. Rheem’s, for example, plugs into a 120-volt outlet. Portable softeners for RVs need none; you recharge them by hand.
Do salt-free water softeners work the same way?
No. They don’t remove calcium and magnesium; they change them into crystals that are less likely to form scale, so the water won’t test as soft. See salt vs salt-free.
Related guides
- Best water softeners (all types)
- What size water softener do I need? (calculator)
- Water softener regeneration explained
- Salt vs salt-free water softeners
- Best dual-tank water softeners
Sources
- NDSU Extension, Water Softening (Ion Exchange), WQ1031 (revised May 2022): how ion exchange works, the hardness scale, sodium added, potassium chloride and the effects of hard water.
- Rheem, RHS18 installation and operation manual (rev. G, 2025): specifications, power outages, salt, salt bridges and troubleshooting.
- Whirlpool, WHES48 installation and operation manual (2023): nozzle and venturi, regeneration checks and troubleshooting.
- Quality Water For Less, Fleck 5600SXT programming guide: regeneration steps and default times.
- WateReuse, Evaluation of Alternatives to Domestic Ion Exchange Water Softeners (project 08-06, Arizona State University, 2014).