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.