Water softener or scale inhibitor?
These two get compared as though they were competing versions of the same thing. They are not. A water softener removes hardness from your water. A scale inhibitor leaves the hardness in and stops it forming scale on hot surfaces downstream of where it is fitted.
That difference determines everything else — what you notice, what it costs, where it goes and whether it is worth it.
The short version
Fit a scale inhibitor if your only concern is protecting the boiler, or if a manufacturer's warranty requires scale protection. It costs a fraction as much, fits in an hour, and needs no salt, drain or power.
Fit a softener if you want to stop cleaning limescale off everything, use less detergent, and protect every appliance rather than one. It costs considerably more, needs space and a drain, and you will be buying salt for as long as you own it.
Fit neither if your water is soft. In Scotland, Wales, the North West, the North East and most of the South West peninsula, both products are solving a problem you do not have.
Side by side
| Water softener | Scale inhibitor | |
|---|---|---|
| What it does | Removes calcium and magnesium entirely | Leaves them in; stops them bonding as scale |
| Where it works | The whole property | Downstream of the fitting point only |
| Limescale on taps and screens | Gone | Unchanged |
| Soap and detergent use | Substantially reduced | Unchanged |
| Protects the boiler | Yes | Yes — that is its job |
| Needs a drain | Yes | No |
| Needs power | Usually | Usually not |
| Ongoing consumable | Salt, roughly monthly | Phosphate cartridge annually, or none |
| Space required | Substantial — under-counter unit | Small — fits by the boiler |
| Typical fitted cost | Higher, and installation is a real job | Much lower, often done during a boiler service |
| Suitable for renters | No | Sometimes, with permission |
The question that actually decides it
Not 'how hard is my water' but 'what is annoying me'.
If the answer is the boiler, or a warranty condition, an inhibitor does the job for a tenth of the cost and there is no good reason to spend more.
If the answer is that you are descaling the kettle every fortnight, scrubbing the shower screen, going through shampoo, and replacing appliances sooner than you think you should — an inhibitor will not touch any of that. It protects equipment downstream; it does not change the water you wash in. Only a softener does that.
This is the single most common mis-sale in the field: an inhibitor sold to someone whose complaint was limescale on the shower screen. It will not help, and they will be told six months later that they need a softener after all.
Where hardness comes in
Hardness decides whether either is worth it, not which one to choose.
Below roughly 150 mg/l CaCO₃, neither. Between 150 and 200, an inhibitor on the boiler is reasonable insurance and a softener is hard to justify. Between 200 and 250 both are defensible and it comes down to what is bothering you. Above 250 — London, Kent, Surrey, Essex, Hertfordshire, Cambridgeshire, most of the South East and East Anglia — a softener is genuinely worth costing out, and most households who fit one in that band are glad they did.
Check your own figure before deciding anything. Your water company publishes it for your postcode, and a test strip will confirm what is actually at your tap.
Can you have both?
Yes, and it is occasionally sensible — for instance a softener for the property with the drinking tap left hard, plus an inhibitor on a separate unsoftened branch feeding something that needs protecting.
For most households it is unnecessary. A correctly installed softener already protects the boiler, so the inhibitor has nothing left to do.