How do I wire my thermostat?

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*There is no standard for which color wire controls each function. When wiring, each wire should be identified by what terminal(s) it connects to — never by color. If you don’t know to which terminal each wire connects, it may be necessary to go to the HVAC system and look at the designations on the control board. Consult your owners/install guide for wiring examples and systems compatible with your thermostat*.

The thermostat uses one wire to control the primary functions of your HVAC system, such as heating, cooling, and fan. See the diagram below for the role of each wire in your system:

S – Indoor and Outdoor Wired Sensors
Y – Compressor Stage 1 (Cooling)
Y2 – Compressor Stage 2 (Cooling)
G – Fan
C – Common
U – Humidifier, Dehumidifier, or Ventilator control
L/A – A – Input for heat pump fault
O/B – Reversing valve for Heat Pump systems
E – Emergency Heat
Aux / W2 – Heat Stage 2 (Heating)
W – Heat Stage 1 (Heating)
R – 24vac (Heating transformer)
Rc – 24vac (Cooling transformer)

*Trade model thermostats must operate “dual-fuel” systems, which use a heat pump for the first 1 or 2 stages and a gas or oil furnace for backup/emergency heating. If you have a dual-fuel system, or are unsure, pause and contact a Professional HVAC Contractor.

Follow the instructions below to guide you through basic wiring:

To protect your equipment, turn off the power at the breaker box or switch that controls your heating and cooling. To confirm your system is off, change the temperature on your existing thermostat, so the system starts heating or cooling. If you don’t hear or feel the system turn on within 5 minutes, the power is off. You can skip this step if you have a digital thermostat with a blank display.

Next, remove your existing thermostat from the wall plate. Most thermostats pull directly off the wall. However, some lift from the bottom and lever off, and others have a locking tab.

Take a picture of your wiring. Make sure the terminal markings are visible.

Review your picture and confirm.

Your new thermostat may not be directly compatible if you see terminals labeled A B C, or 1 2 3, as your system requires a communicating thermostat.

If you see thick, black, or red wires, you have a line voltage system. This type of wiring requires a line voltage thermostat and is not compatible with low voltage thermostats.

If you see wires connected to terminals labeled G1 G2 G3, you need a thermostat capable of controlling multiple fan speeds; none of our retail thermostats are compatible with this system type. G is compatible, but not G1, G2, or G3.

You should typically see an 18-gauge solid core wire. The most common configuration is five wires. However, you could see as few as two and many as ten.

Make a note of any present wire not connected to a terminal. Do not label these wires.

Referring to your photo, remove and label each wire. If a terminal has multiple designations like W and O/B, it will be labeled as W and O/B and not just one or the other.

After removing and labeling all wires, unscrew the old thermostat wall plate and mount the new thermostat’s wall

You can re-connect the wiring after mounting the new thermostat’s wall plate. If we recommend placing a wire in a terminal, do not move it to another terminal if we address it later in the guide. (Example – You have a single wire labeled W-O/B, and we advise placing it in the O/B terminal. If we recommend putting the W wire in the W terminal later in the guide, you will not move this wire, as we’ve already instructed you to place it in O/B.)

Now, let’s cover wiring configurations.

Identify any wires labeled R, RH, or RC. You typically have one or two of those three. If you have one wire — even if it’s labeled RC — it goes into the R terminal, setting the jumper connecting terminals R and RC in place. Thermostats may have a jumper switch, metal staple, or plug. The jumper may also be a wire connecting the two terminals. If you have two wires, R or RH go into the R terminal and RC into the RC terminal. If you have more than one wire (for example, you have a wire labeled R and another wire labeled Rc), remove any jumpers between the R and Rc terminals or push the switch to open the RC terminal to insert a wire.

Next, let’s talk about the C or common wire. If you have a Trane model thermostat and a wire labeled X or B, refer to your thermostat manual. In some cases, one of those wires is your common. If you have a C wire, place it into the C terminal on your wall plate. C wire adapters are available here.

Let’s look at the G wire. This wire goes to the G terminal on your new thermostat.

Of the Y, Y1, and Y2 wires, Y or Y1 go to the Y terminal and Y2 to the Y2 terminal.

The O/B wire can have many configurations. It can be W-O/B, O/B, W-O, W-B, or even separate O and B wires. If you have separate O and B wires, tape off the B wire so it doesn’t make contact and connect the O wire to the O/B terminal.

If your O or B terminal shares a label with another wire — typically W —identify whether you have a heat pump system or not. A heat pump runs your compressor for both heating and cooling. If you don’t know your system type, place this wire in the W terminal. If you have a heat pump system, put it in the O/B terminal.

Locate any unconnected wire labeled W or W1. If you identified an O, B, or O/B wire connecting to the O/B terminal in the previous step and it has a separate W wire, place it into the W2 terminal. Connect the W wire to the W terminal if you don’t have a wire connected to the O/B terminal.

Check your thermostat wiring compatibility.


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