Limescale deposits can sometimes occur very quickly though. Once some scale takes hold on the smooth surface of the heating chamber, it becomes much easier for more scale to attach because the surface is rougher but also because it has a greater surface area.
It could explain the sudden loss of heating power.
It can be a very effective thermal insulator too!
One way to avoid it is to use an ion-exchange filter on the water intake of the house. Pricey enough, but it can protect the pipework for years.
All European dishwashers have such a filter as part of their standard design. It reduces the need for large doses of phosphates / other water softeners.
[This message has been edited by djk (edited 01-07-2007).]
