All water supplies contain varying quantities of impurities, with calcium and limestone being the most prominent. When water is heated, these minerals “fall out” of the water and are deposited on the interior surfaces of the tank as scale. The formation of scale increases with the temperature of the stored water. Scale forms on all surfaces including the element itself. The scale builds up an insulating layer which impedes the efficient transfer of heat which may allow the element to glow red hot in specific localities along its surface.
When this occurs, the scale that has built up on the element surface will flake off. When water comes back in contact with the hot element surface, the water will flash into steam. This alternate heating and cooling of the element shortens its life span. This process also creates a layer of debris at the bottom of the tank which can build to considerable depths, even to the point where the lower element is covered, affecting the proper operation of the tank.