Table of Contents
Thermostat Wiring Colors Code Page Navigation
- Essential Thermostat Wiring Color Guide Including Manufacturers Instructions
- Thermostat Terminal Designations Quick Guide - Detail Wiring Color Guide Below
- Red-Wire - connects to the R terminal with the red wire
- RC-Wire - connects to the RC terminal RC red
- RH-Wire - connects to RH terminal RH heat
- Y-Wire - connects to the Y terminal and is for cooling stage 1 with the yellow wire
- Y2-Wire - connects to Y2 terminal and is for cooling stage 2
- W-Wire - connects to W terminal and for heat stage 1 with the white wire
- W2-Wire - connects to W2 terminal and for heat stage 2
- G-Wire - connects to G terminal and for fan operation with the green wire
- C-Wire - connects to C terminal (common 24v power) with the C wire
- O or B-Wire - connects to O or B terminal and is for reversing valve operation
- E-Wire - connects to E terminal and is for Emergency heat
- X or Aux-Wire - connects to X terminal and is for auxiliary
- S1 and S2 Wires - connects to S1/S2 terminals and is for outdoor temperature sensor
- Thermostat Wire Color Code Chart
- Tracing the Wire to the Source
- Additional Resources
What you will learn in the Thermostat Wiring Colors Code article:- First, what thermostat wire colors is likely to go to which terminal on the thermostat in your heating and cooling system
- Secondly, the function of each wire terminal on the average thermostat highlighting some of the differences in those HVAC system functions from manufacturer to manufacturer as recommended for thermostat wire color
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 Wiring Colors Code | HVAC Control
Always follow the thermostat manufacturer’s 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.
For instructions on how to wire a thermostat, please see how to wire a thermostat page. Refer back to this page for thermostat wire colors.
Thermostat Wiring Colors Code - High Performance HVAC Thermostat Terminal Designations
Before proceeding, we highly advise that you turn the power off at the circuit breaker. Furthermore, the power for both the condenser and the air handler/furnace. You may cause a dead short in the control circuit. Additionally, that could potentially cost you a service call for troubleshooting and repair.
Thermostat Wire Colors Code | HVAC Control
Thermostat Wiring and Wire Color Chart – Thermostat Wiring Colors Code | |
|---|---|
| Tstat Terminal Designation | Color of Wire and Termination |
| R – The R terminal is the power. It is a red wire and 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, kill the power at the condenser and the air handler before changing or working on the wiring at the t-stat. Lastly, if you have a package unit, then the transformer is in the package unit. | Red Wire for the R terminal - *You should 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. Furthermore, 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 Wire 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. Finally, this is only for heating and cooling systems equipped with a single transformer. | Red Wire 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. This wire is then spliced for the separate wire pull which is made to the condenser. Furthermore, some manufacturers put a terminal board strip near the control board in the air handler. Therefore, a splice is not needed. | Yellow Wire 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. Finally, many systems only have a single compressor but if you have two compressors (or a two stage compressor) 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 entirely up to the installer what color to use. Furthermore, for the thermostat, wiring colors code for this terminal (if equipped) consult with the installer. Finally, if that is not possible, then trace the wire out to the source. |
| 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 Wire for W Terminal. *You should 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 a brown wire, but this varies and is entirely up to the installer what color to use. Furthermore, for the thermostat, wiring colors code for this terminal (if equipped) consult with the installer or trace the wire out to the source. |
| G – This is the terminal used for the fan relay to energize the indoor blower fan. Furthermore, on a split system the blower fan is in the air handler. A package unit the blower fan is in the outdoor package unit. | Green Wire for G Terminal. The fan wire. Furthermore, *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. Finally, 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. For the thermostat, wiring colors code for this terminal (if equipped) consult with the installer. Finally, if that is not possible, then trace the wire out to the source. |
| O or B – These terminals are for heat pumps and the B t-stat terminal is used on for Rheem or Ruud and any manufacturer that energizes the reversing valve in heating mode for the heat pump. Furthermore, other manufacturers of heat pumps utilize the reversing valve for cooling. The O thermostat terminal is for this purpose. This wire goes to the outside heat pump condenser, for reversing valve control. Finally, this is for heat pump thermostats. | Orange Wire for O and Dark Blue Wire 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. Lastly, Rheem and Ruud will usually use the blue wire for the reversing valve. |
| E – This terminal is for heat pumps and stands for Emergency Heating. Furthermore, 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. The E terminal activates the back-up heat source. | 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 stripboard 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. However, this should be wired directly to the heating relay or the Aux terminal on a terminal stripboard. It is terminated 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. Furthermore, these are for an outdoor temperature sensor. Special shielded wire is used for this run and completely separate from the other thermostat wires. Finally, some manufacturers will show this the T terminals on their thermostat. | 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. Furthermore, the signal needed to get an accurate temperature is sensitive to electromagnetic forces from another wire inside the structure. Finally, this type of wire is different from the typical thermostat wire and a separate wire altogether. |
Wiring Color Chart for Air Conditioners and Heat Pumps
Tracing a Wire to the Source
If you find the colors are not matching to the convention described here, you can always trace to the source. Furthermore, only if you are confident in your skills. Replacing an old thermostat with a new thermostat can be daunting so make sure to follow all thermostat wiring instructions. Lastly, follow the instructions on our How To Wire a Thermostat page for many great tips such as: Take a picture of the old thermostat wiring before you unwire it.
- First, as mentioned, there is high voltage in the equipment and can be hazardous to your health.
- Lastly, if you wire it wrong or accidentally touch the wrong wires together, you can blow the transformer.
Furthermore, either reason can be harmful.
Additional Resources
Also, check our thermostat category for other detailed articles on thermostats. Additionally, it includes other detailed pages about the thermostat wiring color code.
Thermostat Wiring Colors Code
I have a Tempstar air handler. I’m trying to install a new “smart” thermostat, but can’t figure out which term is the common. The handler has W, G, B/C, R, and X terminals. The X is coated, indicating they don’t want you to use it, I assume. The B/C has the Y wire attached. There are no other terminals. In this config, which would be used as the common? Also, would it be normal for there to be 2 thermostat wires connected to the same terminal, as each already has a wire, except the X?
you need to find the transformer and then trace the wires to the terminal strip. It should look like this:
There are two sides to the transformer, the line side with high voltage one one side of the transformer and the control side which are the wires that provide the control voltage for the system. This should be 24 volts give or take a fraction or two. If you follow the wire from the R-terminal it should go directly back to the transformer. That R wire is the hot side. The other wire coming out of the same side of the transformer is the common side of the transformer. This is the wire you are looking for. Tap into that and you have solved your problem.
If you are not comfortable doing this then I advise you call a professional.
Wow, I can’t thank you enough. That saved me a service call. After hours on the phone with the manufacturer’s tech people, YOUR reply was the piece of the puzzle that let me fix the installation issue.
The Trane model number is: OAUA-152-A
Also, in my old configuration, the blue wire is tapped off. When I install a new modern thermostat, do I need to connect the blue wire if there is a slot for it?
Thanks.
Rabbit
Here’s my problem: I want my new upstairs radiant floor heat to be controlled by my downstairs nest, which currently controls my forced air. The upstairs radiant heat currently has a Danfoss with N, L, and S wires, nest doesn’t have any of these in its diagrams. My guess is, I don’t need to power nest, it’s already powered, so I can ignore the N and L wires. I also suspect I can run the S, signal, into the nest as the AUX wire, then Nest will see this as auxiliary and let me control it? Think this will work? Will I blow anything up? Ruin my nest? Thanks!
Here’s a little more info: I have the Nibco RHP-1 and nest 2nd gen. This is a dual fuel system, per nest standards, and I think I should use the S (Signal) wire coming from my radiant heat panel and plug that into the O or B or O/B terminal at my nest.. However, nest says to call a professional because I can cause damage. Any advice here? The RHP instructions says all the thermostat does is close the dry leads to active the heat panel, can just the O or B wires do this?
Hi I am trying to replace an existing thermostat for water/ baseboard heating. It is a 2 wire system, the old one has R and Y terminals only, but the new thermostat only has terminals for R and W. So when I connect the old Y to the new W, turn the heat setting up, the heat system turns off. I understand that Y is normally for cool cycle, so the switch is reversed it appears. Is there anyway to change the wiring configuration at the baseboard value or thermostat so that the old Y wire can be activated by the new W terminal? Thank you 🙂
not 100% sure exactly what you have there or why they used a cool only thermostat for heating??? Usually a thermostat for a baseboard heating system will energize a circulator pump or open a zone valve that then energizes the circulator pump. Seems to me not to be wired conventionally. In this case I recommend you call an HVAC service company to look at it as this not normal what you have explained here.
wiring colorcode old Trane heatpumps:
red is 24v ac
green indoor blower
yellow compressor
white reversing solenoid for cooling.
why don’t the thermostats come with a wiring diagram,so no technician will have a problem anymore.
most new thermostats do come with a wiring diagram and include instructions for properly wiring a new thermostat.
I moved into a newly built home and need to swap out the thermostat for one that will work with my alarm. I have a thermostat that has the flowing connections:
C – Blue, Y1 – Yellow and Black (2 wires), G – Green, W2 – White, Y2/O – Orange, Rh – Red
My Goodman air handler has the following connections:
C – Blue, Y1 – Yellow, Y2 – Black, G – Green, W1 – White, O – Orange, R – Red
The new thermostat has separate terminals for everything. Do I just match the air handler and separate yellow and black and move the white to W1, or make the same connections from the old thermostat to the new one?
I don’t understand why there are two wires on one terminal. To be absolutely sure you need to trace out the wires to see exactly where they go in the air handler and the condenser. Y2 would be for 2 stage cooling and while it is possible you have two-stage cooling it is unlikely so I would not use that terminal unless you have determined you do have two-stage cooling. Except for that black wire everything else looks correct from the information you provided. You need to find out what that black wire is for.
I have a nibco radiant heat panel (rhp) where the instructions advise to connect wires to the A(rc), B(rh), and 3(on/w), and to then connect the corresponding wires to the rhp. The rhp has connections for C, R, and W. I connected the 3 to the C, the R to the A and the W to the B, but it didn’t work. Any suggestions?
Not sure without seeing a schematic diagram. I did look on their website and didn’t see any specific wiring diagram for what you are talking about here. They do have a tech hotline for to call and several videos on their website. Not sure that helps but again with a specific schematic diagram I really can’t help.
I have RHP-1 and want to tie it into my nest.. similar problem. Do you get a Danfoss FH-WS with your Nibco, too? the panel works great, I just want to control it with my phone.. is it working out for you?
Is it required to connect the green “Fan” wire? Obviously this wire is used to run the fan in air handler when the tstat is put in fan mode. However, can the heat pump operate normally if the fan wire is not connected? In other words, does the air handler control electronics activate the fan itself, or does the tstat always run the fan motor in every mode? Normal operation, except without the green connected you can’t run fan only, of course.
Depends on what you have. MiniSplits typically have a board that controls everything but a typical conventional heat pump in heat pump mode will be controlled by the thermostat. The best way to find out is to look at the schematic or by simply removing the green wire and testing it out.
Hi,
Thanks a lot for this guide, I’m a new homeowner trying to understand how my thermostat are working… I’ve an AC + a gaz heater, the AC is controlled by one specific thermostat and the heat by another… But on my AC thermostat I’ve a W & Y wire both connected and going up to the AC unit… That doesn’t make any sense for me… Have ever see something like this? What could be the W wire plugged on the AC unit?
Thanks! Ivan
One day i had a company agree to hire me cause i knew color codes, this text sure did help me learn alot.
Hi. I have a thermostat question. I replaced the thermostat and discovered that the Green fan wire is Not connected to anything on the air conditioner. Its an older package unit that only had a Red and Yellow wire coming out of the original thermostat to the outside unit. This made the unit run properly. Now ,with the new thermostat the blower will not run. I replaced the blower capacitor because the blower failed in the middle of the night. I installed the new thermostat because the old one was starting to go…What do i need to connect the green wire to on the outside unit. I checked incoming line voltage and its good. Thanks!
you really need to call a professional to look at that as your description is not normal.
I have a new Carrier Infinity System with 2 speed fan. The current thermostat has a battery and is wired with G-green, RC-redish orange, RH-red, Y-yellow and W-white. They are all on the assigned terminals for the thermostat. Also loose in the wall is a blue wire but it is not attached to anything in the thermostat. I want to install a COR thermostat so I can better control the humidity during the cooling season. The COR requires a common line for 24V power because it has no batteries. Do I use the blue wire or is there a combination from the red wire that will allow me to power the COR and take advantage of the humidity feature?
The best way to approach this would be to go to the air handler and see what terminals those wires are connected to at the air handler. From your description it appears the old thermostat used batteries and had no common connected. In that case, you can use the blue wire for the common connection at the air handler and the thermostat. But first, trace it out to make sure. If it is loose in the air handler and not connected to anything then connect it to the common side of the transformer or terminal board in the air handler.
Carrier does have a support site for the COR Thermostat – http://www.carrier.com/residential/en/us/cor/installationsupport/
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!
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!
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?
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!
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
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.
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 ??
beyond that and without looking at it myself I cannot offer you any other suggestions.
Check your out door contractor if it’s shorting out in cooling mode generally u will find its outdoor contractor . If your still having problem
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?
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.
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…
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.
I am trying to put in a new thermostat. I just bought my first home and I would like to use a new smart thermostat. However when I took it off the wall there were wires for A (green wire),B (yellow wire),C (white wire),D (red wire). I returned the nest i bought because it read an error of High voltage. I do have gas heating. Does anyone know of a Smart Thermostat that would be compatible?
Not sure without knowing what type of system you have. Please see the article below and get back to us. You can also upload your photos using our uploader page. See the menu on the right.
https://highperformancehvac.com/thermostat-selection-guide/
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?
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).
with my thermostat in the heat position the furnace and air comes on but the air out the ducts is cold
it is possible you have a problem with your heating system. Best to call for HVAC service.
My old thermostat has a red, yellow, white, green and orange wire. All coordinated with the letter.
My new Honeywell WiFi thermostat does not have an O for the orange wire. Also, what do I do about the C wire. There is a blue wire in the bundle.
you have a heat pump but don’t have the right thermostat. You need a heat pump thermostat. Yes you can use the blue wire for C or common but you have to hook it up to the C or common in the air handler.
Thank you. You helped me for my ex-wife and my daughter’s furnace. Now it works really good. Again thank you
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.