EFI 86 1/2" Fusible Plug Outside Fireside Fusible Plugs - Outside Outside Fireside - 250 PSI Fusible Plugs are installed between the fireside and the waterside of either the plate, flue or tube of the boiler and below the water level. Those installed from the waterside are called waterside or inside type fusible plugs; those installed from the fireside are called fireside or outside fusible plugs. EFI Fusible Plugs are manufactured to conform to ASME standards and meet U.S. Coast Guard requirements. All Fusible Plugs are filled with pure tin having a melting point between 445_ and 450__F. WHEN ORDERING SPECIFY: Inside Waterside or Outside Fireside.