the tubing outer wall 80 is preferably formed from a non metallic material such as high density polyethylene and may take the form of a co extruded coating the tubing outer wall 80 preferably has a thickness t 2 of approximately 0 020 inches and the outside diameter d 5 of the tubing outer wall is preferably about 0 665 inches a strip or wire not shown may optionally be placed between the tubing inner and outer walls 78 and 80 of the heat exchange tubing 70 referring to fig1 a wire 100 is positioned between inner and outer walls 78 and 80 alternatively fig 11 illustrates the use of a strip 102 between inner and outer walls 78 and 80 such a strip or wire may be used to provide a path along which trapped potable or non potable water can travel out of the system the strip or wire can be applied axially or spirally to provide an axial or spiral water passage fig8 provides a cross sectional view of detail 34 a 34 of the combined water heating system 50 shown in fig4 the dielectric fitting assembly 72 provides a termination for the heat exchange tube 70 at both the tank inlet port 62 and tank outlet port 64 in the water storage tank top 58 shown in fig4.