This hard shell is called scale and is often found on the outside of the fire tubes. Scale insulates the heating surfaces recducing the ability o fthe fire tubes to transfer heat from the hot combustion to the boiler water. High stack temperatures or ruptured fire tubes are common problems related to scale build up. Boiler water also contains dissolved gases such as oxygen or carbon dioxide. These gases in the presence of water and metal can cause corrosion.
Corrosion eats away the metal effecting the durability of the boiler.
The amount of make up water and the level of naturally occurring impurities in the water will determine the type of water treatment required. Boiler heating systems that have very few leaks will require simple water treatment programs. Your boiler inspector or water treatment professional can assist you in developing a water treatment program. Steam and hot water boilers typically require different water treatment programs. Scale producing dissolved minerals accumulate in the boiler water when mineral rich make up water replaces mineral free steam and condensate leaks.