I need to install a GFCI receptacle in my unfinished basement for the new power-vented hot water heater that I had installed. The plumber said I could use the existing furnace circuit. I mapped out the circuit but am unsure as to how I should wire this up. Can someone take a look at the attached drawing and tell me how the new receptacle should be wired? Ideally, the switch would in the middle of one of the two 12/2 runs at the top of the diagram, because this is what I have easiest access to. (It would be much more difficult for me to mess around with the junction box because it is in a cramped location. Thanks.
Note that the switch at the top right is the "Emergency furnace shut off" switch in the kitchen, and the switch on the bottom is the one that is attached to the side of the furnace.