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.