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.
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 - Contestar
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 - Contestar
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 - Contestar
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 - Contestar
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