Boiler No. 4 was installed in 1923 as an additional source of steam for the Hathorn Davey engines. The original boilers burnt household rubbish, but when the number of rubbish collections decreased in the 1920s, the coke-burning boiler No. 4 was installed as a back up. During the Second World War, the amount of rubbish collected dropped significantly and boiler No. 4 became the main boiler.
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