Just trying to understand your thought process here - did you think that the parent thought that the filter would somehow remove lime out of thin air? I'm honestly amazed how you didn't immediately come to the conclusion that it's just for filtering lime out before pouring.
I have been in homes with hard water, but its not my day-to-day life.
I usually use a $15 kettle, and no lime precipitates when I typically use a boiler. I can see that lime eventually builds up in the kettle, but as described earlier: a bit of vinegar removes those deposits without much issue.
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Honestly, I can't say that I've ever seen water so hard that it'd precipitate visible amounts of lime in a kettle. I usually see that sort of stuff on humidifiers or distillers. Not really a kettle.
I mean, everything eventually has lime buildup ("soft water" isn't free of lime/calcium, it just has 1/2 the calcium of hard water). Knowing how to clean that stuff off is important.
I had no idea how bad hard water could get until I moved to London.
I get limescale on everything. I descale my kettle every week or two (almost daily if not using a Brita filter), shower head monthly. I pour citric acid down the toilet weekly. My taps actually form little calcium stalactites over a few months. Thermostatic shower cartridges are an annual replacement item because the limescale gets them too.
I keep demineralised water around for sensitive equipment like humidifiers.
