Use a water softener. We had one installed on our last property and it worked a treat with our radiator pipework and immersion. Do not use for your kettle though...it might clear the limescale but softens the water leaving a horrible taste and also bad for your heart.
No depending on the size though. We needed to buy about 5 bags salt a year, about �5 per bag. You can operate the water softener as often as you wish...ours came on about twice a week. so it depends how often you run the hot water tap.