The O wire reverses the valve from heating to cooling, and the B wire switches the valve from cooling to heating. O, B, O/B: These wires are responsible for switching the changeover valve in a heat pump system. W, W1, W2: Just like the Y wire, the W wire(s) control the heating aspect of your system. a high level for extremely hot or cold days, and a low level for mild days). Y1 and Y2 wires might be used instead if you have a two-stage system (i.e. 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. 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. It is not a ground wire!Ĭ: This stands for the “common” wire in an HVAC system, and it provides power to the thermostat. G: This wire controls the blower fan, which is responsible for pushing the warm or cool air through all the vents in your house. If, for example, you have an R wire and an Rc wire, the R wire controls the heating system. 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 you just have an R wire, it’s responsible for powering your entire HVAC system (through the use of a transformer). R, Rh, Rc: Your system may have just an R wire, an Rh and an Rc wire, or a lone Rh or Rc wire. 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. 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.

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. 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. 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.

Note: This post was originally published on June 27, 2019.

Honeywell s-88 system