by Cecile Cinco
What is hard water? Point-blankedly, my daughter answered: ice! 
Kidding-aside, it is the most common water we have that comes out of the tap. It’s also what comes out of the well and even rain. It is very good for cleaning and watering (plants and trees) but it makes your kettle form lime. Life expectancy gets shorter for washing machines, dishwashers and water heaters. It is so primarily because of calcium and magnesium.
A water softener is needed to make your laundry use less soap. It lessens the soap scum too. And it makes water potable. This is because of the ion exchange done by the water softener, which is basically sodium based.
People with certain ailments who need to cut on sodium should be careful. Three considerations are available to lessen sodium in soft water: reverse osmosis, distillation and deionization.
Learn about it before you decide what is right for you.
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I think it works like this.
On the one hand there is the water which goes around the boiler, radiators, etc. This is called the primary circuit and more or less sealed (i.e. it’s the same water that just just gets circulated) and it should contain an inhibitor. This is the circuit that gets really hot (up to about 90 deg C). I’ve read somewhere that it’s best to fill this with unsoftened water which is easy enough to do if you leave one tap untreated (as most people do) and it only really applies to the initial fill. Loss from this circuit is negligeable so it’s probably OK to have any further topping up done with softened water.
This water goes through a coil in the hot water tank, which in turn heats the domestic water (but only to about 60 degrees C) so it’s fine to use softened water here.
I’m not so sure what happens in a combi boiler though. I suspect the domestic hot water is heated directly by the boiler from the incoming main. In this case, I think the heat exchanger would have to be stainless steel in any case, if so, it’s no problem.
Thinking about it, I’ve never heard of anyone living in a soft water area (there are a few about) having problems with boilers because the water is too soft.