New to me house and I am looking to update thermostat. Previous homes all had C wire. When I take current thermostat off the wall only 4 wires are connected but there are 4 more in the wall. In the attic, from the furnace it looks like there are 5 wires wired up and spliced in to 2 other sets of the same wiring that then leads down to my thermostat. Yes, j know that in the photo the yellows need to be reconnected. My question is this, would I be able to simply connect the blue wire in the wall to the C line on my new thermostat? The first photo is the splice coming out of the furnace. The second picture is that line split and the third photo is what is coming out of my wall. I am trying to figure out if I need the power adapter or whatever it is called for the new unit.

The unit im taking out had batteries so no real need for c wire i reckon. New unit does not. Makes me think they just didn't utilize the wire.

If you look at the control board in the furnace, you can trace where that blue (white) wire goes back and connects. It's likely a C wire, but worth checking.

Assuming it's hooked up to the furnace control board, just fish it out of the wall and wire it up to the thermostat.

hurricanejake02 said:

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If you look at the control board in the furnace, you can trace where that blue (white) wire goes back and connects. It's likely a C wire, but worth checking.

Assuming it's hooked up to the furnace control board, just fish it out of the wall and wire it up to the thermostat.

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Ok, I just couldn't believe it could be that simple, but exactly what I was hoping.

I got similarly lucky - it wasn't hooked up to the furnace board, but they used 8 conductor wire and only hooked up 4. I was able to just hook up the blue wire on each end.

The unit I was working on was installed with an old mercury thermostat... C wires weren't necessary at the time.

So the yellow isn't actually connected ti anything in the furnace but the blue is connected. Wtf.