Using hard water in kettles produces calcium carbonate scale or fur. In metal kettles this sticks to the side and in plastic kettles, which have smoother sides, it appears as a film on top of the water.
This scale does not increase the time it takes to boil the water or the amount of fuel you need. However, we do recommend you remove it regularly.
Advice
Only boil the water you need, not only does this help with water efficiency, it will help prevent kettle limescale by not boiling the same water over and over again.
You can de-scale your kettle by half filling it with water and adding two tablespoons of white vinegar. Leave it for four hours, then empty the kettle and remove the loosened scale. Rinse and repeat as necessary. As an alternative you can buy a scale remover, and follow the instructions on the packet.