Theres a small box inside my furnace that connects the house wiring (from the switch on the side of the unit) to the furnace wiring. Inside the box there is a green ground wire from the furnace that is not connected to anything. I have continuity between the furnace and my grounded metal conduit, continuity between the duct work and the conduit, continuity between the gas piping etc. There is also continuity between that green wire that's not connected and the metal of conduit, furnace etc. However, with that ground wire not connected, does that mean the electronics are not grounded? Should that wire be connected to a ground wire and jumpered to the switch box on the side of the unit? Do I need to go to the trouble of having a separate ground wire installed?

We just bought a house that has had a small electrical fire. I plan on calling an electrician to do the work but im looking to get a little more information before I go that route. The wall has been exposed where the fire has started. Im assuming partially due to the fire itself and when the fire department came and inspected. It took place on a step landing going to the second story. Will I just need that line rewired? Im not sure of code but i was told that I can still have the older wiring as long as its in good condition. Obviously some of it is not. Would i need the whole house rewired? and at a rough guess how much is that going to cost?