What is Hard Water-Hard water is a kind of water that contains calcium or magnesium mineral ions that often in the form of carbonate-These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits. How to Descale/Clean the Kettle?1. Filling the kettle with 1 cup of vinegar or lemon juice and 2 cups of water.andnbsp;2. Boiling the liquid and leave it for 2 hours, then use a cloth to clean and wipe.andnbsp;3. Rinse the kettle with clean water, and dry with a soft, clean cloth.andnbsp;4. Repeat as needed. Notes:1. It's normal if your kettle exposes spots or rust on the stainless steel bottom after the first use if the water quality in your area is so hard.2. When you choose a preset with Keep Warm Mode on, and then lift the kettle during heating; the CO117-DK will continue the heating process if the kettle is returned to the base within 1 minute.