Thermostat Wire Colors Code | HVAC Control 5/5 (1)

What you will learn in this article:
1) What thermostat wire color is likely to go to which terminal on the thermostat
2) The function of each wire terminal on the average thermostat highlighting some of the differences in those functions from manufacturer to manufacturer

Sometimes the thermostat is not the problem. Please check this article for a checklist of things to check before calling a heating and cooling repair service.

Thermostat Wire Colors Code | HVAC Control

Thermostat Wire Colors CodeThermostat Wire Colors Code – Always follow the thermostat manufacturers instructions whenever changing the thermostat. Always turn the power off at the air handler and the condenser and make sure there is no voltage at the transformer before proceeding. The thermostat wire colors below are the most likely colors used for most installations however your thermostat wire colors may be different than what is indicated here. There is no official standard for thermostat wire colors. Thermostat wiring colors are not standardized but these are the most wiring methods used based on my years of experience in the HVAC field where I successfully wired a lot of thermostats.
Honeywell Thermostats Available Here

For instructions on how to wire a thermostat please see our how to wire a thermostat page.

Thermostat Wire Colors Code – High Performance HVAC Thermostat Terminal Designations

Before Proceeding it is highly advised that you turn the power off at the circuit breaker for both the condenser and the air handler/furnace. It is possible you could cause a dead short in the control circuit and that could potentially cost you a service call for troubleshooting and repair.

Thermostat Wiring and Wire Color Chart – Thermostat Wire Colors Code

Tstat Terminal Designation Color of Wire and Termination
R – The R terminal is the power. This comes from the transformer usually located in the air handler for split systems but you may find the transformer in the condensing unit. For this reason, it is a good idea to kill the power at the condenser and the air handler before changing or working on the wiring at the tstat. If you have a package unit then the transformer is in the package unit. Red for the R terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
RC – The RC terminal is designated for the power for cooling. Some HVAC systems use two transformers. A transformer for cooling and a transformer for heating. In this case the power from the transformer in the air conditioning system would go to the thermostat terminal. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. Red for RC terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
RH – The RH terminal is designated for the power for heating. See RC above for an explanation. It should be noted that a jumper can be installed between RC and RH for a heating and cooling system equipped with a single transformer. Red for RH terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
Y – This is the terminal for cooling or air conditioning and goes to the compressor relay. Typically a thermostat wire pull is made to the air handler on split systems and then this wire is spliced for the separate wire pull which is made to the condenser. Some manufacturers put a terminal board strip near the control board in the air handler so a splice is not needed. Yellow for Y Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
Y2 – This is the terminal for cooling second stage if your system is so equipped. Many systems only have a single compressor but if you have two compressors which should only operate off of one thermostat then you need the Y2 thermostat terminal for second stage cooling. *The most common color I’ve seen used for this terminal and wire designation is light blue but this varies and is completely up to the installer what color to use.
W – This is the terminal for heating. This wire should go directly to the heating source whether it be a gas or oil furnace, electric furnace, or boiler or auxiliary heating for a heat pump. White for W Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
W2 – This is the terminal used for second stage heat. There are gas furnaces with low fire and high fire and some depend on control from a two-stage heating thermostat with a W2 terminal. Heat Pumps use staging for auxiliary heat and need a W2 terminal. *The most common color I’ve seen used for this terminal and wire designation is brown but this varies and is completely up to the installer what color to use.
G – This is the terminal used for the fan relay to energize the indoor blower fan. On a split system the blower fan is in the air handler while with a package unit the blower fan is in the outdoor package unit. Green for G Terminal. *Although be aware that this may have changed especially if the person who wired the thermostat didn’t use conventional color coding.
C – This is the terminal which originates from the transformer and is necessary to complete the 24 volts power circuit in the thermostat but only if the thermostat consumes electricity for power. Many digital thermostats require 24 volts for power so the common wire is necessary. C stands for common and there is no universal color used for this terminal although black is the most common color I’ve seen.
O or B – These terminals are for heat pumps and the B tstat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Most other manufacturers of heat pumps will utilize the reversing valve for cooling and the O thermostat terminal will be utilized for this purpose. This wire goes to outside heat pump condenser where the reversing valve is located. Orange for O and Dark Blue for B depending on the installer of the heat pump and the manufacturer. If you have a Trane, Carrier, Goodman, Lennox, Ducane, Heil, Fedders, Amana, Janitrol, or any other manufacturer other than Rheem or Ruud you will be utilizing the orange wire for reversing valve. Rheem and Ruud will usually utilize the blue wire for reversing valve.
E – This terminal is for heat pumps and stands for Emergency Heating. If for whatever reason the heat pump condenser fails and it is necessary to run the heat there is an option on heat pump thermostats for emergency heating. Basically this simply utilizes the back-up heat source many heat pumps have to heat the home without sending a signal to the condenser to run for heat. E – There is no universal color used for this  terminal designation but this should be wired directly to the heating relay or the E terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.
X or Aux – This terminal is for back-up on a heat pump and allows for auxiliary heating from the back-up heat source usually located in the air handler. X or Aux – There is no universal color used for this terminal designation but this should be wired directly to the heating relay or the Aux terminal on a terminal strip board in the air handler or package unit if you have a heat pump package unit.
S1 & S2 or Outdoor 1 and Outdoor 2 – Some tstats have this terminal and it used for an outdoor temperature sensor. The wire uses for this should be special shielded wire and completely separate form the other thermostat wires. Using shielded wire prevents electromagnetic forces generated from other wires from interfering with the signal inside the shielded wire. A remote temperature sensor is a solid state device and the signal needed to get an accurate temperature is sensitive to electromagnetic forces from other wiring inside the structure.

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Thermostat Wire Colors Code

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  1. I really like the descriptions on this page, especially the “Y” designation. Rushing through the first installation of our thermostat, a 4 wire system, I hooked the blue wire to “B”…which was incorrect. Heat would work but not shut off, A/C wouldn’t engage, but the central fan would. I had to trace back from the furnace to find that the Yellow went to white out to the return, then red back into the house, then finally to a blue wire…which needed connected to the Y post 🙂 9 month pregnant wife wasn’t so happy with the landlord for not getting out here to check what we thought was a stuck relay 🙂 Thanks for posting this!!! Oh, and check your main wiring schematic before hiring someone to hook up your thermostat.

  2. Thank you. You helped me for my ex-wife and my daughter’s furnace. Now it works really good. Again thank you

  3. with my thermostat in the heat position the furnace and air comes on but the air out the ducts is cold

    1. it is possible you have a problem with your heating system. Best to call for HVAC service.

  4. Hi, As a handyman, I change thermostats to the new ones regularly. However, I just came upon a Carrier system that has a thermostat with A,B,C,D,S1 & S2 as the wiring designations at the thermostat as well as at the heater itself. Can you possible tell me how these relate to the normal designations?

    1. It looks like this is one of those smart home thermostats with the automated home. Can’t say about any of those designations except the S1 and S2 (used for a remote temperature sensor typically an outside air temperature sensor).

  5. I found that A & B are “Data” and C & D are 24vac. So I don’t see any way I can change it to a normal thermo…

    1. We are so happy you found out that information for us. I have always known these Smart Automated electronics in these newer homes are proprietary which makes things difficult if there is no technical support, available technicians, or even if the manufacturer goes out of business. It often leaves the homeowner/property owner with no decent or cost efficient alternatives. What seemed like a good idea when they purchased the home, with all the gadgets and technology, ended up costing them more money in the long run. Thank you for your diligence in looking this up George. You are a stand up guy!!! And there is a way to convert the home over to a normal home you just have to be able to identify the proper designations and know what it controls.

  6. I have a question about my own new HVAC system which is using too much propane, in my opinion. My system is a Nordyne Heat Pump with a 93& efficiency propane furnace as the Aux Heat. No toasters installed. When the temp outside goes below 30, the Aux Heat is on as well as when the Heat Pump is in defrost mode, the Aux Heat is on. My question is can I install an SPST switch next the to the thermostat that interrupts the Aux wire so that I can control when the Aux heat comes on? And, can I change the settings so that the Aux Heat comes on below 20 instead of 30?

    1. You can certainly do that. I have done it on my own system in a different way although 20 degrees seems to be a bit cold. And I only recommend someone who really knows controls and wiring controls do this.

  7. should there be any continuity between the red thermostat wire and the green, i have a short in the low voltage side, i have checked all the wiring and it is fine ( nothing going to ground ) except at the thermostat, i have continuity between the red and the green

    1. No there should not be any continuity between any of the wires. To do a proper continuity test all wires should be disconnected between the equipment and the thermostat. Provided none of the wires are touching you should get infinity on all colors. If you do not then you definitely have a problem with the thermostat wire. Most likely it is old wire and the insulation has worn off and the copper is touching from one color to the next or someone drove a nail in the wall and hit the thermostat wire creating a connection where the nail is.

      1. Thanks Richard, i’ll isolate all conductors and see if it clears, right now the only thing i can get the thermostat to operate is the is the aux heat on the air handling unit which is driving my power bill through the roof !! i can manually push in the contactor on the heat pump and it will run, so it’s obviuosly not getting the 24 volts to the coil, my initial problem started with the low voltage fuse fuse blowing and i had to bypass it in order for the relays to pull in for the aux heat so i dont know if it is a motherboard prob or thermostat, there are no nails through the wire as i can see the wires to all units…any other ideas ??

        1. beyond that and without looking at it myself I cannot offer you any other suggestions.

  8. I bought my first home 8 years ago from a little old lady who had lived in the home since it was built (1966). The kids/Realtor said they had finally installed a new A/C the previous summer and were really excited about it (they hadn’t removed the swamp cooler, yet, and the A/C compressor outside looked well-worn, so old A/C may have gone out years earlier – swamp cooler looked newer than compressor). Trane A/C system is attached to Bryant gas heating system. Thermostat was new, but a non-programmable digital. Recently bought a smart thermostat, and found the installer used an Add-a-Wire system to change the 2-wire in the wall to a 4-wire on the Trane A/C system. There’s a jumper between RC & RH on the old thermostat and another jumper between G and Y on the control board, with the Add-a-Wire system inserted in between (diode at t-stat, box mounted next to control board). I’m running new 5-wire line today to replace the old 2-wire/Add-a-Wire setup. Can I simply match new wire colors from unit to new t-stat and remove Add-a-Wire setup? What about the control board & t-stat jumpers – can those be removed, too, since the newly-run wires will cover all 5 spots on the control board (G, Com24V, W, Y, R) and will have their own terminals on the new smart t-stat? Thanks!

    1. the jumper at the t-stat should remain if you have an RC/RH terminal on the new thermostat. RC/RH are the 24 volt hot terminals for systems with separate transformers or control power supplies. From what you are describing your system only has one 24 volt power source or one single transformer for both the heat and the cooling. It is likely you have a simple R terminal at the thermostat and that is where the red wire should be terminated. The jumper at the control board can be removed from G to Y. That would simply make the blower come on at the same time as the cooling whereas this will now be controlled from the thermostat and you should see a little efficiency in the system from that change. Everything else is correct as you describe it. Good luck!

      1. Thanks so much for your help! I ran new all wire between t-stat and furnace & removed old wiring & Add-a-Wire setup from control board. Matched all terminals on board to terminals on t-stat with jumper between RC and RH on t-stat. Works beautifully, thanks so much! Now I just need to find a new forum to get insight on installing new ducts (I’m finishing my basement myself – lots of remodeling experience as a teenager), possible zoning of existing system (separate upstairs & downstairs), and possible whole house fan to move cold basement air upstairs during Summer, and warm air down to basement in Winter (rambler style house). I just want to collect information before I have a professional come and try to overcharge me or give me wrong information – as a single woman, I get that a lot, unfortunately. Until I show them I’m not completely ignorant – then they’re usually pretty friendly toward me. Know a good forum for that info?

        1. Glad to hear everything worked out for you. Duct work is a different animal and something I have shy’d away from for my career – at least the installation part. Good duct work can make a difference and my only advice is this – make sure it is sized properly for the CFM load, make sure all the joints have been properly attached and secured with screws or panduit straps, make sure all the joints are sealed with duct sealer (goes on like a paste but drys and seals any leaks), and then make sure everything is insulated (including the boots) and that the insulation is sealed. That is important because if the insulation does not have a vapor barrier the inside of the insulation will end up sweating in the summer and the insulation gets soggy and falls off. So the insulation really serves two purposes…………one it is an insulator and two it is supposed to provided a vapor barrier from outside air moisture which as you may know will the moisture in the air will condense on anything below the dew point and the duct work is commonly below the dew point on the summer. Many times the old duct work is still good it just needs to be resealed and re-insulated. The best type of duct work is round hard sheet metal duct work. Installed properly it provides the best air flow with the lowest turbulence (inside the duct). lastly, make sure the duct work is supported properly. Good luck!

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