Limescale in a kettle is calcium carbonate that came out of solution when the water was heated.
The mechanism
Calcium bicarbonate is soluble in cold water. Heating decomposes it into insoluble calcium carbonate, carbon dioxide and water.
The carbonate deposits on the hottest surface — the element — and builds up.
What the speed tells you
Furring within a month — hard water, likely 250 ppm or more. Your espresso machine is scaling at the same rate.
Furring in six months — moderate.
Barely any in a year — soft water.
This is a free and reasonably accurate hardness test.
Why it matters beyond the kettle
Scale insulates the heating element, so the machine takes longer to heat and brews cooler.
Then it blocks narrow passages — solenoid valves, group heads, flow meters.
An espresso machine boiler can be destroyed by it. A kettle is a warning.
Descaling a kettle
Citric acid — a spoonful in a full kettle, boiled, left an hour, rinsed thoroughly. Cheap and effective.
Vinegar works too and is acceptable here, since there are no seals or warranty to worry about.
The real fix
Filter the water. A jug filter reduces hardness enough to slow scale substantially, and it improves the coffee at the same time.
The one thing not to do
Ignore it and hope. Scale is cumulative and it does not stop.
Last reviewed 8 August 2026