When installing a new thermostat, take a picture of your current thermostat wiring connections. This will prove to be invaluable when hooking the wires up to the new thermostat.
Next, we want to go over what each wire controls.
Red– This wire will be the 24-volt power from the furnace and will go to the terminal labeled, “R”
White– This wire is the heating wire and sends power from the thermostat to the furnace control board, activating the heat. It will be connected to the “W” terminal.
Green– This wire is the fan wire and sends power from the thermostat to the furnace control board, activating the blower. This wire will be connected to the “G” terminal.
Yellow– This wire is the air conditioner wire and sends power from the thermostat to the furnace control board, activating the air conditioner. This wire will be connected to the “Y” terminal.
Blue- This wire is the furnace 24-volt common wire and returns 24 volts to the furnace control board. This wire completes the 24-volt circuit from the furnace to the thermostat and is often used to give power to a digital thermostat so that you don’t need to rely on battery power. This wire is required for all “smart” thermostats as well as “wifi” thermostats. If you do not have a common wire, you may be able to purchase an “add-a-wire kit which will steal power from the fan “g” terminal and use it to power itself.
Turn power to the furnace off either from the furnace switch or the breaker. If you do not do this, you may short the wires and blow the fuse on the furnace control board.
Next, remove the old thermostat and expose the base. As mentioned above, take a photo of the wiring on your existing thermostat. This is important because some wires may not match the colors of your wiring. I’ve seen old installations where wires were added and replaced and end up with several of the same color wirings. If this is the case, be sure to mark which location each wire is at before you take it off. The best is different colored electrical tape and use the color for what each wire should be. Another option would be to label with some masking tape. Some new thermostats come with wire labels, so check to see if yours does. Once this is done, remove the wires and the thermostat base.
Install your new thermostat base and make your wiring connections. If Your new thermostat has a jumper between “RC” and “RH”, (photo above) leave it in place and just install your red wire to either of the two terminals. This is to allow for power to be sent for heating and cooling. Without it, your air conditioning may not work.
Last, install the batteries (if your thermostat uses them), connect your new thermostat to the base, and restore power to your furnace. Then turn up the heat or turn down the ac (depending on the climate) and test your new thermostat functions to be sure that it is working properly.