Note: This post was originally published on June 27, 2019. We’re republishing it now as part of a home-focused week at iFixit, and because more people are searching for this kind of fix right now.
If you’re replacing your thermostat for the first time, you might notice that the wires are connected to different terminals, each of which is denoted by a certain letter. Here’s what those letters mean.
Replacing a thermostat is really easy, but sometimes the terminal letters on your old thermostat don’t match up with the ones on your new thermostat. Knowing what each letter stands for and what it’s used for can help you figure out which wire goes where on your new thermostat.
Note: While thermostat wiring only utilizes 24 volts (thus it won’t shock you or is even detectable), it’s still a good idea to shut the power off to your entire HVAC system before replacing the thermostat, so as not to cause any issues with the electrical system as you work on it.
R, Rh, Rc: Your system may have just an R wire, an Rh and an Rc wire, or a lone Rh or Rc wire. If you just have an R wire, it’s responsible for powering your entire HVAC system (through the use of a transformer). If you have both an Rh and an Rc wire, the former powers the heating and the latter powers the cooling (using two separate transformers). If, for example, you have an R wire and an Rc wire, the R wire controls the heating system.
G: This wire controls the blower fan, which is responsible for pushing the warm or cool air through all the vents in your house. It is not a ground wire!
C: This stands for the “common” wire in an HVAC system, and it provides power to the thermostat. Some systems don’t provide this, and it’s not necessarily required by every thermostat, since some can be powered by a battery or just “steal” power from other wires, but if your system has one and there’s a terminal for it on your thermostat, you should absolutely connect it.
Y, Y1, Y2: Whenever your thermostat calls for cooling, the Y wire is used to send a signal to your HVAC system telling it to fire up the air conditioner. Y1 and Y2 wires might be used instead if you have a two-stage system (i.e. a high level for extremely hot or cold days, and a low level for mild days).
W, W1, W2: Just like the Y wire, the W wire(s) control the heating aspect of your system.
O, B, O/B: These wires are responsible for switching the changeover valve in a heat pump system. The O wire reverses the valve from heating to cooling, and the B wire switches the valve from cooling to heating. Sometimes it might be a single O/B wire instead of two separate wires.
X, AUX: Some heat pump systems can provide auxiliary heat. This wire sends a signal to your system calling for auxiliary heat if the outside temperature is too cold for the heat pump to work alone. A secondary heat source located in the air handler will kick on.
E: This works nearly identically to auxiliary heat, but for emergency purposes only. It has to be turned on manually, whereas auxiliary heat can turn on automatically if need be. Emergency heat is only used if there’s a problem with your heat pump.
ACC: Some thermostats have this terminal to connect equipment like a whole-house humidifier or dehumidifier.
L: This terminal is designated for indicator lights on the thermostat, sometimes for when auxiliary or emergency heat is turned on, or if there’s a general problem with your system.
K: Honeywell wire saver modules combine the Y and G wires to form a single K wire that connects to the K terminal on some Honeywell thermostats. It’s used for systems that don’t provide a C wire but have a thermostat that requires one.
S, S1, S2: These wires run directly outside and provide outdoor temperature information to the thermostat.
This is definitely a lot of information to take in all at once, but use it as a simple guide whenever you’re replacing a thermostat. And consult your HVAC and thermostat manuals for wiring diagram information if you’re not quite sure which wire goes where, or what configurations your HVAC system supports.
19 comentários
You definitely need to consult the manual for your HVAC (where the other end of the wires connect) and look at where they attach on that board.
While the old standards (e.g. the 4-wire RGYW system), many HVACs have alternate wirings designed for more advanced thermostats.
For example, Carrier Infinity systems can be wired with the old standards (for third-party thermostats), the colors can also be wired to “ABCD” terminals that provide a data interface to Carrier’s own thermostats (which are effectively terminals that interface with the computer in the HVAC).
shamino - Responder
My old thermostat has two terminals not mentioned in your article. One labeled “HUM” and the other “DHUM”, DHUM has a red wire connected to it. Humidifier?
wdelack - Responder
Thanks delack. Mine also has one marked H. Must be Humidifier. I have an old Lennox and it’s terminals are G, P C, H, V. I can’t get it to work so It may have problems. I assumed the P must be a rubbed off R and V may be a rubbed off Y. But it still won’t start. I wonder if I could use the H terminal to run a wire to W on the furnace. Anyone know if Red and White would at least start the furnace?
Henry Tuesday - Responder
So I have 4 wires and in my old thermostat they were hooked to g y w and rc and then there was a free wire and one end went into rc and the other into the. So on my new honey well thermostat I connected the y g w and rc but there is no rh to connect with the rc but there is an r terminal on the new one. Does this mean this thermostat won’t work? Do I need to get a different kind or can I put that rh wire into the r and connect it to the rc?
Jerilyn Brown - Responder
From what I've read, it goes like this..
R has 24 volts and inside the thermostat it connects to the others depending on what function you select heat or cool or just fan.
So..to tell the
‘G’ to turn on just the fan without heating or cooling.
Or the R can instead tell the
‘W’ to turn on the heater and fan.
Or the Rc which also has 24 volts if connected by your wires..so it can tell the
Y to turn on the cooling.
So in short
R controls W and G
Rc controls Y
The C is used as a common wire to allow the thermostat to work from the R 24 volts all the time without needing penlight batteries as some thermostats have.
This information should help you see how to connect it.
The Rh is the same as the R because R is used for heating.as above
armarra1 -
My old thermostat (honewell TB8575A1000U) has wires connected to R, W/X, Y/A, GL, GM, GH and C. My new thermostat does NOT have these same identifiers, (honeywell RTH2300). It has R, RC B,O G, Y and W. Can I assume that the new thermostat is NOT compatible with my present system?
Joe Brazeau - Responder
I bought a Nest. My old unit had 8 wire system and L and an E connection. What do they equate to in the Nest panel? I connected the other wires to their equivalent letters. G to G etc. I ended up putting L to Y1 and E to Y2 and when I turn on the unit it says AC is on, on the thermostat, but blows heat.
Dawn - Responder
hvac means high voltage alternating current, not heating, ventilation, and air conditioning, which would be hacv for heating, air conditioning, and ventilation, and you could get someone killed making such a thoughtless mistake. the ignorance of inserting the v for ventilation in order to purposefully highjack a term that was already in use in another capacity is astounding.
Dave Laisure - Responder
You are incorrect. HVAC is industry standard.
Dave Durkee -
My current Emerson thermostat with 4 wires connected as 1, 2, C, R.
I can't find any smart thermostat compatible with this configuration.
Anyone have any ideas?
Mike - Responder
Hi @mtrinh00, I can't help you on your quest to find a new smart thermostat but I can direct you to our Answers forum to get a quicker answer. By posting your question here, the it'll get a lot more visibility. When you post it, include a picture of your current configuration, if you can!
Let me know if you need help posting anything by @tagging me (@amber) or emailing community@ifixit.com
Amber Taus -
6 wire from wall to the new hest thermostat dose anyone know what color go to 1st slot L1...2 slot is N1 and 3 N and 4th slot is L???????
mason boggess - Responder
Heat not hest
mason boggess - Responder
I can hear my thermostat kick off but the heat still blows. When looking my Rh W C G and Y/Y2 is hooked up but I have random blue orange and brown wires just loose and my Rc is not connected at all. I'm not a tech or anything but I'm thinking is could be the problem. Any ideas would be greatly appreciated.
Josh Wicker - Responder
Josh, I'm not an expert, but I've been researching and learned a lot. If your system is a heat pump, check out this YouTube video: https://www.youtube.com/watch?v=6qwFra15...
First, check if your Fan is set to ON or AUTO. If it's set to ON it WILL blow all the time. On AUTO some furnaces continue to blow until it cools slightly. If it's a Nest thermostat, check if it's set to blow the fan for additional time every hour to circulate the heat around.
If you have a heat pump and a Nest thermostat, the Nest help line says to put the white wire into W1 when it SHOULD go to W2 (auxiliary). This can cause systems to blow hot when it should blow cold, from comments I've read.
In our upstairs system the brown wire isn't connected to anything (on either end) and isn't needed. On many systems the blue wire is the COMMON or C wire and MAY not be used. Our downstairs sytem has a blue wire connected to the L connector, which apparently isn't needed. Some systems use Rc OR Rh but not both. (See that video.) Good luck!
Jennifer L. - Responder
only have orange and blue wire how do you hook up thermostat for oil heat
laurie scott - Responder
No matter what the actual wire colors, this is how a typical traditional thermostat controls things (basic terminals only):
The letters designate the thermostat terminals, and not necessarily the wire colors.
To activate heat, connect R or Rh to W
To activate cool, connect R or Rc to Y
To activate fan, connect R, Rc or Rh to G
Sometimes Rh and Rc are jumpered together at the thermostat, if there's only one transformer and only one R wire. Some combo heating/cooling thermostats don't need the jumper, and the R wire is connected to either Rc or Rh if separate terminals exist. Read the instructions for the thermostat. It usually doesn't hurt to install the jumper if there's a question.
As noted elsewhere, the C or Common wire is typically used in conjunction with an R wire to power/charge the thermostat as needed. Mechanical thermostats or ones that have non-rechargeable batteries often don't need this wire.
Hope this is helpful. Feel free to correct me if this is wrong.
PJ Blank - Responder
I have two thermostats one shows two terminals that say dry contact. The other has R and W which is for the relay and warm or heat (just an assumption). My question is... Are the dry contact terminals and the RW terminals the same thing?
Confused - Responder
What if your E wire falls out of wall?
Kathy Chaulk - Responder