I've upgraded a few thermostats before, and they have been quite straightforward; i.e. take pictures and connect the wires as the the original. However, this is my first attempt at a "smart" WiFi-enabled thermostat and as I've read, I'd need a common connection to power the device (I was expecting battery-powered). So that's my very first obstacle as I don't have a 'C' wire coming from the wall. Then when I head down to the furnace and opened it up hoping to see a 'C' wire that might've just be hidden behind the wall, I encountered a non-typical connection (see pictures).

Furnace Terminals 2 Diagram

As you can see, there are no connections to the 'Y' terminal, there's no yellow wire at the wall or the furnace. The blue wire is capped with a red wire that I haven't been able to trace to see where it goes (some A/C connection I suppose).

Hence, I'm baffled and don't know where to go from here. Should I just run a C wire down to the furnace or replace the whole bundle with a 5-wire? Even if I do I'm not really sure what that capped blue wire is suppose to do. I'd truly appreciate any help anyone out here can offer.

  • 2
    The blue nutted to red which you've labeled ?? almost certainly runs outside to your compressor. So it looks like you're going to need an extra wire running to the thermostat one way or another.
    – brhans
    Jul 15, 2017 at 21:30
  • Thanks @brhans! Would you happen to know why the Y terminal on the furnace is not connected? From what I've read, that's the cooling unit. So what exactly is supposed to happen for a typical connection when a yellow wire connects to the Y terminals on both the thermostat and the furnace? I'm trying to understand the purpose of that extra connection to the supposed compressor. Thanks.
    – David H.
    Jul 15, 2017 at 22:11

1 Answer 1


After doing some research and physically tracing the wires, I was able to confirm that the white cable labeled "??" in the diagram was indeed going to the outside compressor unit. Hence it looks like the Blue wire was being used in place of the missing Yellow wire. As a result, my solution is to rewire the Blue to the "C" terminals on both end and then run a yellow 18 gauge wire to where the Blue one used to be connected. That way, the wires are connected as they're colored.

I'm guessing the Y terminal on the furnace is probably to control the fan speed for cooling mode, but since the A/C works now I'll leave it as a separate question.

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