Hot water storage vessels, or cylinders have a primary or back up electrical heating element source called an "Immersion". Standard Immersions are threaded and screw into the connection point of the vessel or cylinder. As well as having to be screwed in very tight when new, in a lot of cases immersions become very difficult to remove if they have been fitted for a very long time. The Rothenberger Box Immersion Spanner has a very strong one piece construction and is supplied with a tommy bar which is used to deliver maximum purchase when turning.