Hard water is caused by the chalk/limestone (Calcium Carbonate) deposits present in the ground dissolving into the rain water which, when it has been treated, eventually comes into your house.
Hard water is effectively a solution of Calcium Carbonate (CaCO3). When hard water is heated up in your house or when the water pressure changes as it does when leaving a tap or suddenly changing direction in an L-bend pipe, the Calcium Carbonate comes out of solution and reverts back to chalk once again. This chalk is of course now called limescale but is still hard deposits of chalk.
The problems you can see are where scale has deposited in various appliances and around taps and showerheads but what you cannot see is the steady build up of limescale around the heating coils and immersion heater or the heat exchangers in your boiler. The results are not only the serious reduction in heating efficiency (see graph) but also the eventual breakdown of components in showers, washing machines, dishwashers and of course the immersion elements themselves.
Our Little Plumber range of Electronic Water Conditioners: