Boiling precipitates calcium bicarbonate out of solution, which is why kettles scale.
What happens
Temporary hardness — calcium and magnesium bicarbonate — decomposes on heating into insoluble carbonate, carbon dioxide and water.
The carbonate falls out as limescale on the kettle element.
Permanent hardness — sulfates and chlorides — is unaffected. It stays dissolved.
What that means for coffee
Boiling reduces alkalinity somewhat by removing bicarbonate. That is genuinely beneficial.
It does not produce controlled or clean water. The reduction is partial, uncontrolled and dependent on how long you boil.
It does not remove chlorine reliably — some evaporates, and chloramine, used in many supplies now, does not.
It concentrates everything else. Water lost as steam leaves the remaining minerals more concentrated.
Repeatedly boiled water
The old advice against re-boiling is mostly overstated for safety, and the taste point is real: repeatedly boiled water is progressively more concentrated in whatever did not precipitate.
Fill the kettle fresh.
The practical position
Boiling is not a water treatment. Use a filter, bottled water or a recipe.
The one thing boiling does tell you
How much scale your kettle produces is a free hardness test. A kettle that furs up in a month is telling you to filter — for the coffee and for the machine.
What people want to know
How do I descale a kettle?
Fill it with a citric acid solution, about a tablespoon per liter (quart) of water, bring it to the boil, leave it for an hour, then rinse and boil fresh water once. Repeat if scale remains.
Last reviewed 30 October 2026