First let me say that I'm a newbie and have zero electrical experience. I'm an IT Guy and thought "Hey, its electronic, so I can probably figure it out."


I appreciate any feedback/suggestions you folks may offer.

I have installed a $20 Honeywell thermostat and the air blows, but not cold. The AC Unit wont kick on outside. This only happened after I installed this new STAT. I have a 5 wire bundle and an additional 2 wire bundle.

5 Wire Colors:

G - Fan R - Return W - Heating Y - Cooling B - Common

2 Wire Colors:

R - ?? W - ??

As you can see from my photos, I have connected 4 of the 5 wires, based on the obvious W to W, R to R, etc etc markings. I do not know if I need to hook up the black. I am almost sure that the reason I do not have the AC Unit kicking on, is due to the 2 wire red and white not being connected.

My question is, where do the 2 wire red and white wires connect on the Thermostat?

Again, I really appreciate you folks taking the time to reply


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  • Were those wires attached to the old thermostat? Based on your photos, everything looks right to me. But without knowing where all the wires go, there's no way to be sure.
    – Tester101
    Apr 12, 2015 at 21:34
  • Looks like maybe the system was updated, a new cable was installed, and the old cable was abandoned in place.
    – Tester101
    Apr 12, 2015 at 21:56
  • Yeah the wiring looks correct, other than the red/white in the wall. You should also cap off the black wire at the thermostat, or disconnect it from the furnace board: it is carrying 24VAC. and could short out against one of the other wires.
    – gregmac
    Apr 13, 2015 at 1:40
  • 1
    C and R on the control board are 24VAC. The red/white wires connected to the control board are the control wires for the outside condenser: when you put 24VAC on them, they power up. Your thermostat connects R to Y to turn this on, completing the circuit. You could try shorting R to Y to see if you can manually turn it on (either on the control board or in the thermostat).. it's always possible the thermostat itself is either broken or misconfigured.
    – gregmac
    Apr 13, 2015 at 1:45

2 Answers 2


Everything at the furnace looks good.

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There seems to be an extra cable at the thermostat, which might be an old abandoned two wire thermostat cable.

You'll also notice the C wire just hanging out, which should be either used or capped off.

besides that, everything looks good at the thermostat too.

I'd investigate where that abandoned cable goes. I'd start by looking in/near the furnace. But based on what you've shown, I wouldn't suspect the problem is with the wiring.


You'll need that C wire if you hook up a smart thermostat! I don't have the C Wire and need to use an adapter.

The other wire might have been for a humidifier or something in the past. Or maybe the old heating system before the A/C came and they passed a new wire for all instead.

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