Water passes through a bed of resin beads loaded with sodium. Calcium and magnesium bind to the resin and sodium is released into the water. When the resin is exhausted, brine is drawn through it to reverse the exchange, flushing the hardness to drain and reloading the resin.
The exchange
Inside a softener is a cylinder packed with small resin beads. Each bead carries fixed negative charges on its surface, and clinging to those charges are positively charged sodium ions.
Calcium and magnesium – the ions that make water hard – carry a double positive charge, and the resin holds them more strongly than it holds sodium. So when hard water passes through the bed, the calcium and magnesium displace the sodium: they bind to the resin, and sodium ions go into the water.
What comes out has no calcium or magnesium in it. The hardness has not been neutralised or conditioned – it has been physically removed from the water and is now sitting on the resin. That is why softening genuinely changes how the water behaves in every respect, and conditioning does not.
The exchange is stoichiometric: the amount of sodium added is proportional to the amount of hardness removed. Harder water means more sodium in the softened output, which is one reason the drinking supply is kept unsoftened.
Regeneration
The resin has a finite capacity. Once the beads are loaded with calcium and magnesium, no further exchange can happen and hard water begins to pass straight through – this is called breakthrough.
Before that point, the unit regenerates:
- BackwashWater is passed up through the bed, lifting and separating the beads, flushing out any accumulated sediment and re-settling the bed evenly.
- Brine drawA strong salt solution is drawn from the brine tank and passed slowly through the resin. The sheer concentration of sodium ions overwhelms the resin’s preference for calcium, and the exchange reverses.
- Slow rinseThe displaced calcium and magnesium, along with the excess brine, are flushed to drain.
- Fast rinseClears remaining brine from the bed so the softened water does not taste salty.
- Brine refillWater is returned to the salt reservoir to dissolve more salt ready for next time.
The whole cycle takes a while and the unit is typically not softening during it – which is why single-cylinder machines usually regenerate overnight, and why twin-cylinder machines exist.
Timer, metered, and twin-cylinder
| Type | How it decides to regenerate | Consequence |
|---|---|---|
| Timer | On a clock – every so many days | Regenerates whether or not the resin is exhausted. Wastes salt and water, and can break through if usage is high |
| Metered | Measures water used and regenerates when capacity is reached | Substantially more efficient on salt and water. The sensible default |
| Twin cylinder | One cylinder softens while the other regenerates | Continuous softened water with no gap. More expensive, and useful where demand is high or where regeneration timing is awkward |
Most modern domestic units are metered, and if you are choosing one, that is the feature that matters most for running cost. It is also why the unit has to be set up with your actual hardness figure and household size – a metered unit configured from a guess is calculating its capacity from the wrong number.
Salt
The salt does one job: it provides the concentrated sodium solution that reverses the exchange. It never enters the softened water supply directly – the brine is flushed to drain, and the fast rinse clears the residue.
- Block salt is clean to handle, fits compact units, and generally costs more per kilogram.
- Tablet salt is used by a wider range of machines and is generally cheaper per kilogram, but comes in heavier bags.
- Granular salt suits some larger machines.
- Never use ordinary table or cooking salt. It contains anti-caking agents and impurities that will foul the resin.
More on the practical differences in block salt or tablet salt.
In a tablet or granular unit, salt can form a hard crust across the top of the brine tank with a void beneath it. The tank looks full, but the water below is not in contact with salt, so no brine is made and regeneration achieves nothing. The unit stops softening silently. Breaking the crust up occasionally, and not letting the tank get damp, prevents it.
What a softener does not do
- It does not filter. Sediment, chlorine taste and particulates are unaffected. That is a filtration job – see filtration, softening and conditioning.
- It does not disinfect. It has no effect on bacteria.
- It does not remove nitrates, iron, or most other dissolved substances, though it will remove some low levels of iron and manganese, which can then foul the resin.
- It does not soften the drinking tap, because by regulation that tap is taken off before the softener.
- It does not remove existing scale instantly, though softened water will gradually dissolve deposits over months.
Keeping one working properly
- Keep the salt topped up. Do not let it run empty – a unit with no salt regenerates with nothing and stops softening.
- Check for salt bridging occasionally, particularly in a damp garage.
- Have the settings checked against your actual hardness and household size, rather than leaving factory defaults.
- Keep the bypass valve accessible and know how to use it, so the softener can be isolated without shutting the house off.
- Check the drain line runs freely and that the air gap is intact.
- Have it serviced periodically. Resin has a long but finite life, and valves and seals wear.
- Test occasionally. A simple hardness test on a softened outlet confirms it is actually working – a unit can fail silently.
Softener not seeming to work? Check for a salt bridge first – it is the most common cause and it costs nothing to look.
Questions we get asked about this
Is softened water salty?
No. The brine is flushed to drain and the rinse clears the residue. What remains is a small amount of dissolved sodium proportional to the original hardness, which is not detectable as saltiness but is the reason the drinking tap stays hard.
How much salt will it use?
It depends on your hardness, your water use, and whether the unit is metered. A metered unit regenerates only when the resin is actually exhausted, which uses considerably less than a timer-based one.
How long does the resin last?
Many years in normal use – resin has a long life, though it degrades slowly and can be fouled by iron, chlorine or sediment. It is replaceable when it does eventually need it.
Why has my softener stopped working?
Most commonly a salt bridge – a crust across the brine tank with a void beneath, so no brine is being made. Also: salt run out, settings wrong, bypass valve left open, drain line blocked, or resin fouled.
Can I drink softened water?
The regulations require an unsoftened supply for drinking and food preparation, which is why a hard tap is retained. It matters particularly for anyone on a sodium-restricted diet and for making up infant formula.
Does it need electricity?
Metered electronic units do. Some non-electric units use water flow to drive the metering and valve operation, which can be useful where there is no convenient socket.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.