I just bought the HoneyWell Wifi thermostat and so wanna use it. However my old Thermostat was battery powered and does not use the "C" wire. I opened up my panel and found that there were three unused wire. a brown color, an orange color and a blue color wire. One by one I plugged each into the "C" terminal of the honey well wifi thermostat (R, W, G are already plugged) but none of them worked. I tried plugging G into C but it didn't work (my cooling isn't gas or propane powered but is electric. Maybe that's why). The thermostat doesn't turn on. What should I do? Should I buy a 24V power adaptor and plug the power chord into C. I don't wanna do it as there would be a wire hanging outside thermostat. Do I have any other option?

P.S I don't have a cooling circuit board. All I have is an on / off circuit breaker.

  • If you search the site you will find many answers to this
    – Steven
    Commented May 4, 2014 at 17:40
  • You'll likely find the answer if you look at other thermostat-c-wire questions. To get a specific answer, you'll have to take a picture of both ends of the wire, showing the labelled terminals on the furnace/air handler as well as the thermostat itself. Wire colors are not actually standardized and so you can't make assumptions, and usually HVAC installers don't connect the C-wire at the furnace unless it's actually needed.
    – gregmac
    Commented May 5, 2014 at 16:34

5 Answers 5


I just got off the phone with Honeywell. If you only have a 4-wire older system, you'll only be able to use the fan in the "auto" position. It won't let you use the fan-only selection. You need a 5th wire for power for the wi-fi side of the thermostat. Call Honeywell and they'll walk you through it for free.


You need to call an air conditioner company and have someone hook it up for you. You need to go inside the air handler to connect the wire to the proper spot. Doing it wrong may destroy the circuit board and require replacement. That will be expensive. Have them clean and check your system while they're there.


The C wire is the common wire...ground wire. You already have 24v ac going to the red wire. If you have an extra wire connect it to the C at the thermostat and ground it at the transformer box in the AC unit. To check, just put a voltmeter and put the red lead on the R and the black lead on the C. You should get 24 to 29 VAC. There should be no voltage on the C wire.


You have to connect the other end of one of the extra wires, to the C terminal in the heating/cooling unit. See this answer for more information. Without knowing exactly what type of equipment you have, it's difficult to provide a more detailed answer.


It sounds like you are lucky to have extra wires not connected on either the thermostat side and the HVAC side. The real question is: What voltage is the thermostat looking for at the "C" terminal? I assume it needs to be always on so you can either connect the blue wire to the "C" terminal on the thermostat and the other end of the blue wire to the positive terminal of the 24volt transformer output, the "C" terminal (if there is one) or add the appropriate power supply at the HVAC system location which you can connect to the blue wire to power the thermostat. I wish the Thermostat suppliers like Honeywell would just say what the voltage needs to be at the "C" terminal so you know exactly what to do. It doesn't even say it in the manual. It simply says the "C" wire is needed to power the Wifi thermostat. I think it is 24 volts (not 12V, 5v, etc) but it would be great if they would simply confirm this so you know what to do with the other end of the wire which is not connected to anything at the HVAC unit.

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