Please Help -- Wiring for Nest Thermostat (add Common Wire?)
I'm looking to replace my current thermostat with a Nest. Unfortunately, it seems that I don't have a "Common" wire at my thermostat (and I would really like a Common wire feed so that I don't have to rely on the Nest's internal battery). My thermostat is wired with four wires like this (RC/RH, G, Y, W):
I thought I would have to get a VenStar Add-A-Wire, but when looking at the wires outside my furnace, I'm presented with this:
To make it easier to interpret, I drew out a wiring diagram for the mess of wires outside my furnace:
I found it interesting that it appears that an orange-coloured wire is presumably running to my thermostat, but is cut off outside my furnace. Is there any opportunity for me to use that orange wire to add a "Common Wire" to the thermostat? I did try to pull out the wire at my thermostat to see if there was an orange one tucked in the wall, but something was preventing me from pulling the wire out of the wall further than in the photo below. I was hoping to pull the wire out to the point that I would see the brown wire casing and hopefully an unconnected orange wire. If you think that I likely have an orange wire and that it can be used as a "Common Wire", then I could drill a larger hole behind my thermostat to try and find it.
Thanks for any help!