With the advent of high input and larger storage tanks in both commercial and residential heaters, deliming has become a necessity of modern maintenance. Lime (CaCO3), is the most notable factor when discussing water hardness. Lime is present in every water system to some degree across the entire United States. Since lime is inversely soluble (the more you heat, the more lime comes out), higher usage, excessive hardness, and an increased heating surface can lead to a high incidence of "limed-up" heaters.
Symptoms often include a popping of water trapped under lime deposits or the sizzling of water trapped next to elements, boiling it to steam.
Treatment of a "limed-up" heater is relatively simple. Since CaCO3 is a base, the easiest way to dissolve it so it can be flushed from the heater is with an acid. The most commonly used is phosphoric acid at a food-grade level.
Two common treatments marketed by A.O. Smith are Mag-Erad and Un-Lime. The directions on the product should be followed explicitly.