Basically, the water softener does both, the scale inhibitor doesn't.
Found this on a site, may be of some help to you.
'1. Do you want to eradicate scum and have clear washing water that lathers easily and washes away cleanly?
2. Do you want water that feels soft and silky on your skin without bath oils or bubble baths?
3. Do you want to eradicate unsightly tidemarks round the bath?
4. Do you want to reduce the time it takes to clean the bath/basins/showers and sinks?
5. Do you want to realise savings of more than 50% on soaps, detergents and shampoos?
6. Do you want to make your hair soft and silky and easy to manage without the aid of expensive shampoos?
7. Do you want to improve the efficiency of your pipes and heating system and extend the life of your domestic appliances by removing all the scale build up in your system?
8. Do you want to make your laundry brighter and your glasses and dishes sparkle?
9. Do you want to make car washing easier, with the minimum streaking and spotting?
10. Do you want a low-cost way of preventing more scale build up in your pipes and hot water system and nothing else?
If the answer to any of these questions from 1-9 is yes, and the answer to question 10 is no, you need an ion-exchange water softener.
If the answer to all the questions 1-9 is no and the answer to question 10 is yes, you need a scale inhibitor.'
Hope this helps, I don't think you were thinking of your soft hair or silky skin (might interest the missus though ) it will protect your pipework and appliances.
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