limescale

thumb|Limescale build-up inside a pipe reduces both liquid flow through the pipe and thermal conduction from the liquid to the outer pipe shell. Both effects will reduce the pipe's overall thermal efficiency when used as a heat exchanger.

Limescale is a hard, chalky deposit, consisting mainly of calcium carbonate (CaCO3). It often builds up inside kettles, boilers, and pipework, especially those used for hot water. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where hard water has flowed. Limescale also forms as travertine or tufa in hard water springs.