walls are in mechanical contact with a space between them 17 which is vented outside of tank fitting 6 hence if liquid leaks through the outer wall 16 it will flow into space 17 and out the top of fitting 6 this protects the heating elements 28 32 34 and 36 inside tube 18 from contact with water 30 the heating elements 28 32 34 and 36 inside tube 18 are sealed with an atmospheric moisture seal 19 which may be made using a potting resin the heating elements 28 32 34 and 36 inside tube 18 may be encased in insulating ceramics or ceramic powder 34 or immersed in mineral spirits or a similar insulating non flammable fluid a single wall isolation immersion heater 39 is shown in fig2 it is threaded into the hot water tank port 24 so that water can enter or exit the tank 26 via a plumbing fitting 6 the heater element is encased in one wall 18 hence if liquid leaks through the wall 18 it will flow into the space occupied by the heating elements 28 32 34 and 36 inside tube 18 and the unit will need to be replaced the heating elements.