The nature and extent of the deposits which occur on the heating surface of a boiler unit are variable in the extreme. The method of firing and the rate of combustion are open to choice. The completeness of combustion can be controlled to some degree, but the quantity and chemical constituents of the ash can seldom be forecast, and may vary widely.The form of fouling, and the effect on the different portions of the heating surface of the boiler unit, are examined.Development of the application of soot blowers from 1919 is discussed, and particularly the use of the single-nozzle blower. The design of the boiler to enable full use of the different types of soot blowers available is referred to, and mention is made of the effect on furnace walls, boiler tubes, and superheaters, economizer and air-heater surfaces. The future design and rating of boilers depend to a great extent on the development of the means of cleaning the heating surface.Reference is made to cleaning by water lancing and other means for, special cases.The importance of the availability of plant is emphasized.The essential features of power-operated mechanical soot blowers, with special reference to automatic operation, are outlined.The scope of use of the three main types of soot blowers is set out, as well as the essential features of their design.Points for consideration in the periods between individual soot-blowing operations and the extent of each blowing, are considered.A short history of the development of automatic soot blowers is drawn up, including a reference to a Patent Specification in 1920, and to a number of early designs of blowers and installations.The types of automatic soot blowers on the market are briefly described in Appendix (14.4), and a general indication of the trend of modern design of blowers, both electrically and hydraulically operated, is given.The object of the paper is to provide up-to-date information on a subject of general interest to users of large boiler units.