The two main functions of a boiler are to transfer heat to water to produce steam under pressure and to separate steam from the water in the boiler. The boiler can only carry out these two functions effectively if the qualities of the feed water and the boiler water are properly controlled.

Impurities introduced into the boiler with the feed water can produce scale and other deposits, which impede the transfer of heat and may restrict the flow of water. In addition to reducing boiler efficiency either effect leads to insufficient cooling of the metal heat transfer surfaces, which may become so hot that it is no longer strong enough to withstand the operating pressure, causing metal failure. Deposits may also lead to corrosion, by shielding the underlying metals from the protective conditions in the water or steam.

When steam is produced from water containing dissolved solids, the solids remain and concentrate in the boiler water. As the steam separates from the surface of the water in the boiler, it tends to carry with it small droplets of the boiler water. Contamination of the steam is increased by such factors as increasing concentrations of dissolved and suspended solids in the boiler water.