Scale build-up from hard water is often cited as a cause of energy waste in hot-water systems (I am talking here about ‘domestic hot water’ supply, not closed loops within central heating systems). Actually though, contrary to claims for some water treatment devices, it is not necessarily the case that energy waste will be significant. Indeed with an electric immersion heater on a 24-hour service, all the supplied energy still gets into the water; there is no loss. Of course the rate at which the water temperature recovers will be reduced, and the heating element will fail prematurely, but those are service and reliability issues not energy waste.
The story is a little different on intermittent hot water storage of any kind. Here, because scaling will retard temperature recovery, users may extend preheat times and that will result in a marginal increase in standing heat loss. If the heat supply is from a primary (boiler-fed) water loop, the primary return temperature will be higher because scale impedes heat transfer, and this also will increase standing losses although in reality not to a significant extent in the grand scheme of things. If hot-water recovery times deteriorate markedly, users may of course dispense with time control altogether and in those circumstances avoidable standing heat loss might become significant if thermal insulation is poor.
Preventing scale build-up
Simplifying the story somewhat, the main constituent of scale is calcium carbonate, which starts to form above about 35°C through breakdown of the more soluble calcium hydrogen carbonate that is present to varying degrees in the public water supply, with ‘hard’ water containing higher concentrations of it. Calcium carbonate crystals of the normal ‘calcite’ form stick to surfaces and each other, and that is what constitutes limescale.
One way to deal with this is softening which (in its strict sense) involves a chemical process to turn calcium carbonate into sodium carbonate which does not precipitate as crystals but stays in solution. The process is costly in terms of chemicals; a waste product, calcium chloride, needs to be flushed away periodically; and the softened water is unsuitable for drinking and cooking because of its high sodium content.
The alternative to chemical treatment is physical conditioning. Various proprietary methods are available. Some involve electric or magnetic fields which are supposed to affect the calcite crystals in some way (for example giving them an electric charge so that they repel each other, or in some other manner inhibiting their tendency to agglomerate).
Another class of conditioner is electrolytic. Electrolytic devices release of minute quantities of zinc or iron into the water, which change the calcium carbonate to its ‘aragonite’ form which, unlike calcite don’t stick together, so they stay in suspension and do not contribute to scale formation.
With the exception of electrolytic devices, there is no scientific explanation of how or why most of these physical conditioners work, and there are no accepted tests of efficacy. There is only anecdotal evidence, but if it works, it works.
The one method of physical condition which is definitely effective (and I can vouch for it personally) is polysilicate-polyphosphate dosing. This has a dual action. It modifies the carbonate crystals to stop them sticking to each other, and it coats the inner surfaces of pipework and appliances to inhibit scale formation.
For anybody wanting further references, this note from WRc commissioned by Southern Water is what I currently regard as the most authoritative advice on the subject of water treatment techniques.