A kettle’s innards tend to become ‘furred up’ with lime scale, and because an encrusted kettle takes longer to boil and uses more energy, it’s advisable to keep it as scale-free as possible.
To minimize the amount of scale that is deposited, the most important thing is not to leave water sitting in the kettle. a traditional way of reducing the amount of scale in a kettle is to place a small, rough seashell into it, which is seen to attract the scale, thereby preventing it from being deposited over the inside of the kettle.
You can remove deposits that have already formed by pouring a solution consisting equal parts vinegar and water into the kettle and boiling it up. Don’t fill the kettle more than half full because the vinegar will expand as it heats. Leave the solution to become cold and then empty the kettle and rinse it out.
Although this treatment will usually cope with a light deposit, to remove a heavy one you’ll need to keep bringing the solution to the boil and rubbing the inside of the kettle as the solution cools. The vinegar will have softened the deposit, making it easier to remove.