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Problem: How do I wire up my new thermostat? What do the wire colors mean?

Problem: I purchased a new programmable thermostat. My thermostat wiring only has only three wires. The thermostat instructions call for a red power wire, a yellow, green and white wire. My three wire colors are White, Yellow and Green. How do I wire this thermostat up?

Answer: Please always take a picture or mark the wires with tape or labels as to which terminal they were connected to on the old thermostat. I would be able to help you easily if I had a picture. Do you have both heating and cooling? Most thermostats have a Red wire that is the power wire that usually connects to R & RC terminals of the thermostat, a Green wire that energizes the Fan relay, connected to the “G” terminal on the thermostat, a Yellow wire that energizes the outdoor unit’s contactor, (if you have air conditioning) and a white wire that energizes the gas valve, electric heat relay or oil burner when you need heat. The white wire is usually connected to the “W” terminal. The thermostat acts like a switch, like a light switch. The Red to white switch turns on the heat. Red to Green turns on the fan, Red to Yellow turns on the air conditioner. The main question would be do you have air conditioning? One of the wires has to be the hot wire coming from the power side of your low voltage transformer. This power wire is usually the Red wire. The color might not be red, but maybe they, the installer, used another color for the hot wire. You would need to look at your low voltage transformer, trace the wire down that is coming out of the transformer to determine the color. Below we have a very informative Youtube Video made by grayfurnaceman which explains thermostat wire color codes. We also have another real good video made by David Jones which shows how to wire up a thermostat.  If you have any other questions please feel free to email us: or comment below. We would love to help you out and have your business!

25 thoughts on “Problem: How do I wire up my new thermostat? What do the wire colors mean?

  1. Hi Steve
    I am replacing an old mercury analog thermostat with a new analog thermostat( no mercury) There are only 2 wires, one Red and one Black. the old thermostat has a R and Y hook up and so does the new one. I unfortunetly did not take a picture of how it was hooked up before taking it apart. I hooked up the new one with Red to R and Black to Y. But when I turn up the heat nothing is happening. Is it possible that the Black should be hooked up to the R ? I live in a condo that has Hot water heat. The heating store that I bought the thermostat at checked on the computer to be sure I was replacing with the proper model. Honeywell T822L

    1. Hi Carmel! On the new thermostat, the “W” connection controls the heat. So I would try Red to R and Black to W. “Y” on most thermostats controls the cooling and should have nothing to do with the heat. I hope you get it working right soon! Steve

  2. Hello Steve,
    I am trying to replace my Honewell TH9421C1004 with and RTH7600 by Honewell. The control box is a THM5421C and the wiring is set up as a 3 wire control which is what is required on the TH9421C1004 and it is marked 1,2,3. The RTH7600 has more wires and in the wiring diagrams in the book shows no 1,2,3 and is set up with the multi wire system.

    Can the RTH7600 be used as a 3 wire or do I need to wire the disconnected wires back up in the THM5421C so that they are energized?

    Thanks in advance

    1. Hi Chris! Thanks for asking this question, but I am sorry, I do not know the answer because I am not familiar with the two thermostats. I would suggest that you contact Honeywell technical support and ask them your question. So sorry that I can not help you out! Steve

  3. Hey Steve – question for you – I upgraded to a nest, hooked it up without a C wire to see if it would work, it didn’t do well so I took it off and put my old honeywell thermostat back on. I took a picture and hooked it up the same way, but now the AC isn’t blowing air out the vents. All the fans inside and outside are running fine though. My thermostat does heat/AC and I have a white, blue, green & red wire; the blue was hooked up to the ‘Y’ spot on my thermostat originally. Any suggestions? Nice of you to keep up with this post and help people out, I’m sure everyone appreciates it.

    1. Hi Conor! Thank you so very much for your kind words! I would suggest that you test the control board wires with a volt meter set to “Volts AC” with the thermostat calling for cooling and the furnace safety switch door taped down so you can get power to the furnace or air handler. You should be getting 24 volts between R and C, G and C, and Y and C. R and C shows the power coming from your low voltage transformer, G and C sends 24 volt power to the fan relay on your control board to turn the blower on and Y and C sends power to your outdoor unit’s contactor to make it turn on. If you are not getting 24 volts between those wires then you have either a thermostat problem or a thermostat wire problem. If you want to by-pass the thermostat then, you could turn the power off to your furnace. Take all the wires loose from your thermostat. Tie or twist the R, Y and G wires together, turn the power back on to your furnace and the air conditioner and furnace blower should blow. If not then you may have damaged the fan relay on your control board. I hope you can find and fix the problem soon. If you have any questions please let me know. Steve

  4. hello i have the yellow wire connected together with the W terminal
    can you give me a little help with

    1. Hi! I would suggest that you try to trace down where the yellow and white wires terminate in your furnace, air conditioner or air handler. Since you say the yellow and white wire are on the same W terminal then this would indicate that who ever wired the thermostat wanted the heat and the outdoor unit to come on at the same time. Usually the W (white wire) controls the (electric heat) or gas valve and the Y wire (yellow) usually controls the outdoor heat pump or air conditioning unit. If you have a heat pump then maybe your thermostat is wired to cut the electric heat on with the outdoor heat pump unit. I strongly suggest that you trace and find out where the yellow and white wires are going to see what they are controlling. I hope you can easily find and solve the problem. Steve

  5. I would like to replace my a Honeywell TH8000 with an Ecobee or similar wireless Thermostat. I have only three wires (R, G and W) which are connected to R/RC, G and W on the thermostat. The thermostat controls gas heater and separate A/C unit so I believe C is wired at the fan relay.
    Can I use any replacement thermostat

    1. Hi Russ! I am sorry, but I do not understand how 3 wires can control both a furnace and an air conditioner. To me you would need at least 4 wires for a thermostat that controls heating and cooling. As in the post. R is the hot wire from the hot side of the low voltage transformer. G attaches to the fan relay to turn the fan on when the air conditioner is on W white turns the gas valve on in heating mode and Y yellow would turn the out door air conditioning unit on. You would need a 5 wire thermostat cable to run the “C” common wire to the thermostat. I am sorry that I do not have any suggestions because to me you do not have enough low voltage thermostat wires. Sorry that I can not be of any help.

  6. Hi. I am installing a new smart thermostat. Currently I only have an 18/3 wire in place (R, W, G). I am planning to fish through a new 18/5 wire to add a common for the new thermostat. I don’t have AC in the house, and was wondering if I should connect the fifth wire up to my thermostat and furnace, or if I should leave it unconnected. I have enough terminals on the new tstat, and the furnace control board has unused terminals marked W2, Y1, and Y/Y2?

    1. Hi Andrew! Most electronic thermostats do not require a 5th wire because most you can use the batteries to run the thermostat. On many of these thermostats if you want the thermostat to light all the time then it will need the Common wire. If the 5th wire is not connected to anything in the furnace then I would recommend leaving it disconnected. So you probably have 4 wires connected: R (red the power wire from the thermostat) W (white wire to control the heat or attached to the gas valve, G (green wire controls the fan relay to turn the fan off and on) and B (blue wire connected to the C common terminal on the control board) I hope this helps you out. Hope you have a great day! Steve

  7. My old thermostat is techy. The blue and green wires were wrapped up and not used. The red and white were screwed connected to the back panel. Additional red and black wires were screwed into panel and plastic connector at other end plugged them into what looks like a motherboard. Replacing it with a Honeywell RHT2300. As instructed for a heat pump without auxiliary/backup heat I connected the red, white, green and blue wires with a jump between yellow and white and left the jump between the R and Rc. I moved the switch from gas or oil to electric or heat pump. Didn’t work. Any suggestions? I used a paperclip for the jump wire. Oh, and then I tried to put the old one back on and it didn’t work! I’m replacing it because if you set it above 68, it loses communication with the furnace and does not shut off furnace. Hope you have idea!

    1. Hi! Sorry to hear you are having trouble wiring up your new thermostat. I sounds like that you are saying that only the red and white wires were used on your old thermostat which usually means that your old thermostat was a heating only thermostat. The red wire is the power wire from your low voltage transformer and the white wire controls the gas valve to start the heating cycle. Sounds like the other wires might not be hooked up. You will need to trace each of the thermostat wires down to see what each color wire controls. I hope the following post and Youtube videos on the page will help you out: If you can not get the old thermostat to work after reinstalling it then you might have a blown fuse on the control board. I would suggest checking the fuse. I hope you can easily find and fix the problem. Sorry, I do not know about wiring a Honeywell RHT2300 thermostat. Sorry that I can not be much help. Steve

  8. I was about to replace my thermostat and found that the yellow wire was connected to the “y” post on the thermostat, but it is connected to the “c” post on the control board on the furnace. Nothing is connected to the “y” post on the control board. My extra “c” wire is wrapped around on both ends and not being used. For the new thermostat, I need to use the “c” but I’m afraid to connect it to the “c” terminal on the control board because the yellow is already connected there. Ever seen anything like this?

    1. Hi Mark! You would need to trace the wires down and make sure the thermostat is wired right or it could burn up many of the low voltage controls if not wired properly. Sometimes when installers connect the air conditioner up they do not join the wires at the control board, but join them away from the control board with wire nuts. I would suggest that you find out which two wires go to your outdoor AC unit. The AC is controlled by the Y terminal connection on the thermostat and the other wire that leads out to your AC unit has to be connected to the C (com) terminal in order to get 24 volts to make the contactor engage. Rule of thumb colors for the wires are: R (red) power wire from the hot side of the furnace’s transformer. C (blue) comes from the common side of the furnace’s transformer. G (green) usually goes to the fan relay “G” connection on the control board. W (white) goes to the W connection on the control board and controls the furnace’s gas valve. Y (yellow) goes to the outdoor AC unit’s contactor to turn the AC off and on. I hope you can get the wiring figured out and corrected. Steve

      1. Thanks very much for your response!

  9. I just bought a Honeywell thermostat to replace the old one. The old one had only three wires connected….green, red, and white. The white was connected to the yellow terminal. There was a fourth wire, another green one, the wasn’t attached. Now for the problem. I attached the wires exactly the way they came off but only got cold air. I switched the white to the white terminal and got heat. How do I connect my compressor now?

    1. Hi! I would suggest going to the furnace and see how they have connected the thermostat wires to the control board. I do not know how to advise you because in every thermostat that is a gas furnace and has air conditioning the thermostat would need 4 wires. Red (hot low voltage wire) Green (the fan relay wire) White (the heat or gas valve wire) and Y (the air conditioner contactor wire). Sorry that I can not give you any advice. I hope you can figure it out by watching the videos we have at the bottom of this post. Steve

  10. Just bought a Honeywell replacing the old Honeywell. The new one doesn’t have a W for the white wire?

    1. I have never heard of the thermostat not having a “W” connection. All the thermostats that I have installed over the years have had a W, W1 or W2 connections. Here is a link to a good Honeywell thermostat installation YouTube video: I hope you can figure it out.

  11. I just bought a Honeywell thermostat but my wire has red yellow green and black, how do I wire my black wire. Thank you so much.

    1. More than likely the black wire is the heat (W) connection, but I would make sure by going down to the furnace and see where the black wire connects on the control board. In a normal thermostat set up the red is the R power wire, Green is the G fan wire and yellow is the Y outdoor air conditioner contactor wire. So the only one left would be the W (usually white) gas valve wire. I would check to make sure down at your furnace’s control board. It is always a good idea to take a picture before removing wires from a thermostat or a furnace. I do this to make sure I get things wired back up right. I learned to do this from my mistakes! Thanks for asking this question. Steve

  12. Mine only has three also but the colors are white red n blue what color goes to what

    1. Hi Silvia!
      I would suggest that you take the furnace or air handler blower door off and see where the wires are attached on the control board. Most of the time Red (R) is the hot wire from the low voltage transformer. White (W) controls the heat or gas valve and you would have to see where Blue is attached on the control board. It could go to (G) green which would control the fan or it could be a (C) common wire which attaches to the common on the transformer or (Y) which attaches to the contactor on the air conditioner if you have air conditioning. If you have air conditioning then more than likely the Blue wire is attached to the Y connection which turns the AC off and on. I hope I have answered your question. Steve

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