How to Wire a Thermostat

To install your unit, you’ll need to connect the correct wires to the corresponding terminals on the back of your new thermostat. Here is the industry standard color code for thermostat wires used for most systems:

• The W wire is connected to your heating system. If your thermostat controls your heat, you will have a white wire.
• The Y wire is yellow and connects to your air conditioning compressor.
• The G wire is green and connects to the fan.
• Rc and Rh: The red wire(s) are the power source for your thermostat. If your thermostat is dedicated to air conditioning only, it will have a red Rc wire. For heating and cooling systems, it will have a red Rc and a red Rh wire. Though both wires are red, they are not interchangeable. For the thermostat to work properly, the red wires must be connected to the correct corresponding terminal.
• The C, or common wire, provides smart thermostats with continual power. It’s usually blue, but it may also be black, brown or purple.
• The O or orange wire connects to your heat pump (if applicable).

While it’s important to know these industry standards, your wiring may be different. Use this guide as a reference only. If you are unsure about any wiring to your thermostat, contact a professional electrician.

Tip: The terminal letters can be very small, so use a magnifying glass to make them easier to see.

Once you understand wiring, it's time to prepare to install your new thermostat. First, turn your existing thermostat to the OFF position. Wait for any cooling or heating to stop, then turn off the power to your HVAC system at the main breaker box. Remove the faceplate or control panel of your thermostat according to the user’s manual to reveal the wires. Do not remove the rest of the thermostat from the wall or remove the wiring at this point.

Look for a wire connected to a terminal labeled with a “C” on the thermostat. If you have one, that’s your c-wire, and you’re ready for general installation.

The common wire, sometimes referred to as the c-wire, is what’s responsible for delivering constant power to your thermostat. For programmable and smart thermostats, c-wires are required for setting a heating or cooling schedule. While some units can run without a c-wire, most brands suggest having one for optimal function.

If you don’t have a “C” wire:

• Check the area behind the thermostat to see if a c-wire was tucked into the wall by your electrician or the installer. Many older thermostats did not need c-wires, but they were available for use.
• Check your furnace to see if you have one. You may find a c-wire connection and lead. This means that you do have a c-wire, but you need to find the end of the lead near your thermostat.

If you can’t find the c-wire or need help with the difference between a 4-wire thermostat and a 5-wire thermostat, call a professional electrician. Or look for thermostats that specifically say they do not require a c-wire to operate. Keep in mind that choosing a model that doesn’t require a c-wire will limit your choices and overall functionality.

Identify and label the wiring in your existing thermostat before you disconnect the wires:

• Take a picture of your current wiring scheme. Check to see if they match the industry standard listed in Step 1. If they do, you can use that as your guide to connect your new thermostat. If not, make your own list with a pencil and paper, and label each wire with a strip of painter’s tape and a permanent marker.
• Disconnect the wires and the corresponding mounting screws. The back plate from your thermostat should come free. Tape the wires to the wall with painter’s tape so they don’t fall in the hole while you prepare your new thermostat.

Tip: In some cases, you may need to remove the old anchors. Repair the drywall with a patch to ensure a secure fit when installing your new thermostat.

Place the plate of your new thermostat against the wall where it will be located. Check the plate with a level, and mark the screw holes with your pencil.

• If applicable, drill guide holes before adding anchors to the wall.
• Pull the wires through the opening of the thermostat’s backplate.
• Attach the back plate to the wall with the provided screws. Check to make sure the thermostat is secure.
• Reconnect the wires according to the industry standard color codes or your unit-specific notes from Step 4. Double check that they’re connected in the right place. If necessary, refer to the photo you took of the old wiring scheme or the instructions that came with the thermostat.
• Replace the thermostat cover.
• Turn the HVAC power back on at the main breaker. Follow the manufacturer’s instructions to connect to Wi-Fi and set up user programs.

Tip: Before you connect the wires to the new thermostat, check them for wear. Wrap them with electrical tape if they look worn out.