What Is a Water Softener and How Does It Work? (+ System Diagram)

This page may contain affiliate links. If you buy a product or service through such a link we earn a commission at no extra cost to you. Learn more.

What is a water softener? How does a water softener work? And what’s inside of it?

These are questions commonly asked by people who want to know if and how a technology really works before they spend their hard-earned money on it.

Well, it’s time to shed some light into the dark…

Key Takeaways

  • A water softener is a type of water treatment system that makes hard water soft by removing hardness minerals.
  • Hard water comes in from the main supply line and enters the softener’s head valve. It is directed into the resin tank where it comes in contact with small resin beads that remove the hardness minerals in a process called ion exchange (mainly calcium and magnesium are replaced with sodium or potassium).
  • A brine tank is used to store salt and mix a highly concentrated saltwater solution called “brine” that is required for regenerating the resin bed.
  • The softener’s head valve operates the entire system, putting it through its various cycles, controlling water flow direction and rate.

What Is a Water Softener?

Salt-based water softeners are simple things, really. They all work the same way having the purpose to turn hard water into soft water. And yes, they do so very effectively.

The overarching goal is to reduce scale buildup to extend the lifespan of your home appliances and plumbing system which in turn lowers maintenance and repair costs.

Other benefits include healthier looking hair and skin, less staining on fixtures, sinks, toilets, glassware – you name it – and better lathering soaps and detergents.

That being said, what are the parts a water softener is made of?

Parts (+Diagram)

A water softener system consists of 3 main components:

  1. The resin tank – The tall resin tank is where the actual softening takes place. It contains a bed of microbeads, the resin, covered with sodium (or potassium) ions.
  2. The brine tank – The shorter brine tank stores salt – up to 300 pounds and more. It is used to mix a highly concentrated saltwater solution called “brine” that is required for regeneration (more on this below).
  3. The head valve – The head or control valve usually sits on top of the resin tank. It is the brain of every softener and operates the entire system, putting it through its various cycles and controlling water flow direction and rate.

Please note: Cabinet-style softeners have the resin and brine tank combined in a single cabinet. These are smaller-capacity models for the most part.

Water Softener Parts Diagram

Water Softener System Parts

Other important components are:

  • Built-in bypass: Turns off water supply to softener for regeneration, maintenance and repair; house is supplied with unsoftened water while valve is in bypass position; does not come with every model
  • Valve motor: Rotates rotor valve to perform different operation modes such as softening and regeneration
  • Flow meter: Measures the amount of water passing through softener; does not come with every model
  • Softening resin media: Synthetic or natural, sand-like material; usually polystyrene compound (plastic)
  • Riser tube: Riser tube a.k.a. distribution tube located in the center of resin tank; channels water out of resin tank to head valve; has a basket on the bottom which prevents resin from entering your home’s plumbing system
  • Salt: Mixes with water to form highly concentrated sodium chloride solution
  • Brine float assembly: Safety float shuts off water supply when it gets up to a certain level to prevent overfilling
  • Fill tube: Connects brine tank to head valve; used to draw brine during regeneration and also to refill brine tank
  • Brine injector/venturi valve: Suck brine into resin tank; make use of a mesh net or another type of screen to trap dirt
  • Grid plate: Placed inside brine tank; height of salt grid sets volume of water in brine tank for a measured quantity of saturated salt brine containing a fixed amount of dissolved salt per gallon
  • Pre-filter (optional): Traps larger particles like sand and silt to prevent premature failure of the softener; especially important for well supplies

By the way, don’t miss out on our Top 10 Favorite Water Softeners!

What Does a Water Softener Do?

You now know about the different parts a water softener contains. But how exactly do they work together so that soft water comes out of the system in the end?

It all starts with hard water being distributed to your home. It’s hard because it contains so called “hardness minerals“. These are primarily calcium (Ca2+) and magnesium (Mg2+) ions, and to some extent iron (Fe2+ or Fe3+) and manganese (Mn2+ or Mn3+) among others.

The hard water comes in from the main supply line, passes the bypass and enters the softener’s head valve. From there it is directed into the resin tank all the way down to the distribution tube.

On its way to the tube, the hard water gets in contact with the resin beads that are supersaturated with sodium ions (Na+) or in rare cases potassium ions (K+). These small beads are responsible for removing the hardness minerals from the water by a process called “ion exchange” – cation exchange to be more precise.

By the time the water reaches the bottom of the tank, it’s soft.

On a side note: Potassium chloride can be used as a regenerant instead of sodium chloride if you are on a salt restricted diet. It also has the benefit that it is easier to manage for sewage treatment plants. In certain circumstances it may also be more environmentally friendly. The downside is that potassium chloride is more costly than its counterpart.

A Little Bit of Chemistry

The ion exchange process is fairly simple:

The hardness ions which are positively charged attach to the negatively charged resin beads that act like a magnet. Thereby, they substitute the loosely held Na+. This is because calcium, magnesium and the like carry a stronger positive charge. Basically all hardness minerals are removed from the water and the sodium goes into solution.

Thus, we end up with soft water with slightly increased sodium levels. It comes up the riser tube, flows back into the head valve and is finally distributed throughout your house.

water softener service mode diagram

Softening Process Diagram

YouTube video illustrating the ion exchange process:


Some (older) water softener systems may use fewer steps to regenerate.

After some time, usually a couple of days, the majority of resin beads will be covered with hardness ions unable to soften any more water. This is when a softener needs to regenerate which can be initiated by a timer, a meter or manually, depending on the type of system.

You might be wondering, What happens with all the calcium and magnesium that has accumulated in the conditioning tank? It must be flushed out. Above that, the resin needs to recharge its softening capacity. Here the brine tank comes into play.

The brine tank stores the salt used to replenish the resin beads which are now depleted of all sodium. Mixed with water, the salt forms a highly concentrated brine solution (about 10% sodium chloride solution).

But first things first. Before any brine is used, in stage 1 of the regeneration cycle the resin tank is backwashed.

Stage 1: Backwash

The softener switches to bypass so that temporarily only H2O is available to the home (this is why regen cycles are usually triggered at times of low usage, e.g. 2:00 am in the morning).

Then the flow direction inside the softener is reversed and water gets pumped into the resin tank from the bottom up. It flows through the resin and out the drain.

Backwashing is important to loosen and expand the resin and remove any sediment, debris or precipitated iron that might have accumulated in the bed.

water softener backwashing diagram

Backwashing Diagram

Stage 2: Brining

During the brining phase, more fresh water flushes through the resin tank and goes down the drain. The difference, however, is that the flow direction is back to normal. What’s more, the flow creates a siphon effect sucking saltwater from the brine tank through the fill tube into the resin tank.

The brine washes over the resin beads and releases the accumulated hardness ions replacing them with a fresh layer of sodium.

But didn’t you say that sodium ions carry a weaker positive charge?

You are right, but the sheer amount of sodium in the brine solution causes the calcium and magnesium to give way.

The brining ends once all brine has been drawn out of the brine tank. At this point, most of the hardness minerals have been washed out.

water softener brining cycle diagram

Brining Diagram

Stage 3: Slow Rinse

During the slow rinse, brine and any remaining hardness minerals are removed.

water softener slow rinse diagram

Slow Rinse Diagram

Stage 4: Fast Rinse + Refill

Lastly, a fast rinse sends any excess salt to the drain. The high flow also packs the resin bed back into its initial state.

On top of that, the salt tank is refilled to prepare brine for the next regeneration cycle (only with post-fill systems).

water softener refilling diagram

Refill Diagram

After about 1 or 2 hours the regeneration is complete and the water softener resumes normal operation.

All in all, periodic regeneration cycles allow a system to continuously soften water for years on end.

Frequently Asked Questions (FAQ)

1. What does a water softener remove?

A softening unit can remove nearly all calcium and magnesium from source water and to some extent iron (5-10 ppm), manganese (5-10 ppm) as well as other metals and salts. (Source)

However, it does not filter water in a general sense. For this you need a carbon or reverse osmosis filter system for example. They will help you to get rid of all kinds of contaminants such as chlorine, lead and bacteria.

You can learn more about the difference between water filtration and softening here.

2. How does a commercial water softener work?

A commercial water softener works in exactly the same way that a conventional whole house system does: It softens water by exchanging hardness ions with sodium or potassium – but on a scale.

If you have any thoughts or questions about how a water softener works please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
Learn more about .


Information provided on BOS is for educational purposes only. The products and services we review may not be right for your individual circumstances.
We adhere to strict editorial guidelines. Rest assured, the opinions expressed have not been provided, reviewed, or otherwise endorsed by our partners – they are unbiased, independent, and the author’s alone. Our licensed experts fact-check all content for accuracy. It is accurate as of the date posted and to the best of our knowledge.

Leave a Comment:

John says April 21, 2022

This is by far the best explanation I’ve seen, thanks. Does the final rinse leave some of the sodium from the brine on the beads?

    Gene says June 15, 2022

    Hi John, thanks for your kind words. It does!

Harry Sutherland says November 29, 2022

Are water softener systems used in the cold water or just in the Hot water or both?

    Gene says December 5, 2022

    They come before the water heater, so they are used for both.

Add Your Reply