Chemical treatment of water inside the boiler is essential whether or not the water has been pretreated. Internal treatment, therefore, compliments external treatment by taking care of any impurities entering the boiler with the feedwater (hardness, oxygen, silica, iron regardless of whether the quantity is large or small. In some cases external treatment of the water supply is not necessary and the water can be treated by internal methods alone. Internal treatment can constitute the sole treatment when boilers operate at low pressure, much of the condensate is returned and the raw water is of good quality. However, in moderate and high pressure boilers, external pretreatment of the make-up water is mandatory for good results. With today’s higher heat transfer rates, even a small deposit can cause tube failures or wasted fuel.