I'm looking to upgrade my programmable thermostat to a wifi thermostat. I purchased a basic Honeywell wifi thermostat but upon installation could not find a 'C' wire. I started to investigate more closely to see if I had an extra wire or if I had to run a new one.

From the pictures attached, I investigated further and at the furnace there is a 'C' wire hookup with a wire, but I can't seem to tell where it ends on the thermostat. if the 'red' 'C' wire from the furnace is the same 'red' wire down at the furnace that is split with the 'white' wire, you'll see that it connects:

Thermostat: red -> blue white -> black

Its a bit confusing since I would think the red wire is the 'C' wire but I don't know if I should disconnect it or run another wire to hook up to the thermostat.


Furnace 1 (note that yellow and white are sharing and that there are two additional cables (red/white) and (black)

Furnace hook up

Thermostat hook up

Thermostat wiring

2 Answers 2


I'm not sure exactly what's going on with the wiring, since in the one photo there's a giant ball of wires and cables, and in another there's extra wires hooked up. Here's what I see in the photos you've posted.

enter image description here

In this photo, you'll notice the cable with a red and white wire. That cable should go between the furnace and the A/C unit. The white wire from the cable is connected to Y, while the red wire is connected to C.

The other cable I've labeled should go to the thermostat. The wires from that cable should be connected as follows:

  • White to W
  • Green to G
  • Yellow to Y
  • Red to R

It's difficult to tell if that's what's going on, due to the big mess of wires and cables in the middle of the photo.

Also take note of the blue and black wires, that are wrapped around the thermostat cable.

enter image description here

In this photo, the cable coming from the furnace looks completely normal. However, there seems to be an extra cable that comes from who-knows-where. It appears that the wires from this mysterious cable, are connected to the blue and black wires that at the furnace are wrapped around the thermostat cable. Which if that's the case, they're doing nothing.

It's possible that you might be able to attach the blue wire that is wrapped around the thermostat cable, to the C terminal at the furnace. Then attach the blue wire at the thermostat, to the C terminal on the thermostat. Though without knowing what the mysterious cable at the thermostat is, there's no way to be sure.

I wasn't able to decipher the diagram on the wall behind the thermostat, so I'm not sure what that's all about. I'm sure it meant something to whoever drew it, but it's not helpful to me.

I'm not sure if you have multiple A/C units, and the extra cable is used to control that. You'll have to investigate further, to determine what exactly is going on with that.

  • Thank you! To answer some of your questions here is what I've deduced: At the furnace, the ball of mess is mostly extra wire. Essentially there are 4 cables: 1. normal 18/5 2. 18/2 R/W and 3. black #1 going off to the right 4. black #2 going off to the right Commented Apr 16, 2015 at 0:25

If I had to guess I would say that the red/white pair go off to a separate (maybe replacement) transformer was added. Kind of like the transformer you often see sitting around in an attic or crawlspace that powers a doorbell. If you meter across those two wires, I'm guessing you will find around 24 volts AC.

Such a transformer is usually built into the furnace unit, but maybe that one went bad and somebody just made a MacGuyver style repair.

Here is some "supporting info" on C wire I wrote a while back. Maybe it will help you sort things out...

(note that someone seems to have partially ignored the conventional wire coloring in your installation...fortunately, electrons are color blind)

The lowdown on C wire

This is my attempt to "demystify" the whole "C" wire thing for non-technical folks. This explanation intentionally ignores lots of details that are not relevant to a basic understanding of the concept. An old-timey thermostat is mechanical in nature. You can think of it very much like 3 separate mechanical on/off switches. (the "switches" in this case are mechanically actuated by temperature using springs and tilting glass bulbs full of mercury and other old-timey mechanical mechanisms) The circuit is actually VERY simple. The old-timey thermostat switches on and off the "juice" which comes in on the RED wire...separately on and off to each of the other 3 wires (often green for the fan, white for the heat, and yellow for the AC).

That's all the old-timey thermostat does. Hook or not hook the red wire to each of the other three as needed.

Modern thermostats control your HVAC system with the same connections. They do the same thing...hook the red wire up to the proper green/white/yellow to turn various parts of you HVAC on and off as needed.

Since the old-timey thermostats were mechanical in nature, they didn't need any juice of their own to operate. If you think of a modern thermostat as an old-timey thermostat plus a smart phone stuck to the wall right next to it...you need something to power the smartphone. (some "kind of modern" thermostats have batteries in them for this purpose...to power the "smarts")

The red wire brings juice into the thermostat...but there is no "ground wire". Our "smartphone stuck on the wall next to the old-timey thermostat" needs both a "hot wire" and a "ground" (or "common"). (again, this is simplified so as to not get lost in irrelevant details and confuse the non-technical readers this was written for)

The "C Wire" (often blue) is this additional "ground wire" needed in order to complete a circuit for the electronics (which old-timey thermostats didn't have). The "other end" of the C wire is hooked to the other side of the transformer that supplies juice to the R or red wire. It is the other wire needed by the electronics in the newer thermostats in order to power up their "smarts". Hope this helps a few people understand what the C wire is about.

  • Any input on this diagram I did based on what I found? !diagram Commented Aug 24, 2015 at 19:43
  • All possible disclaimers apply. It is clear that your wiring has been "messed with" several times by different people. Despite all the disclaimers, my "guess" would be that the "red" wire in the "red/white pair" could be used as a C wire. Before trying that I would put a meter between the actual RED wire for the thermostat, and that red in the red/white pair and see if it is about 24V ac. If it is, then run the system through all of its paces (heat cool fan etc.) and verify that it never changes that voltage significantly. If you see that, it will probably work fine for you. Commented Aug 25, 2015 at 12:40

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