I used to manage a coffee shop where we had to use expensive chemical descalers to clean our hefty coffee machines and boilers. I wouldn't consider that option at my house even if it was free. At home, to clean my kettle thoroughly, I add equal parts water and white vinegar inside and let it sit for an hour. I then bring it to a boil and pour it out. I fill the kettle back up with water and bring it to another boil, then pour it out. The kettle is no longer dingy.
As a side note, when I say I pour out the two sets of waters, I actually use them to clean other things: the floors, the bath. Nothing goes unused at mine!
