The common wire, sometimes referred to as the c-wire, is

what’s responsible for delivering constant power to your thermostat. For programmable thermostats and smart thermostats, c-wires are essential for setting a schedule or having your heat or air conditioner turn on at a desired temperature. While some units can run without a c-wire, even brands like Nest recommend having one for optimal function. Here’s how to determine if you have a c-wire thermostat:   Turn your thermostat to the off position. Wait for any cooling or heating to stop, then turn off your furnace/AC breaker.  Detach your current thermostat from the wall. 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.

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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 that a c-wire connection and lead is present. This means that you do have a c-wire, you just 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 2-wire thermostat and a 4-wire thermostat, call a professional electrician. Alternately, look for thermostats that specifically say they do not require a c-wire for operation. However, choosing a model that doesn’t need a c-wire will limit your choices and overall functionality.

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Once your c-wire is ready, follow these steps to install your new thermostat.

• Step 1: Turn off your existing thermostat. Flip the furnace/AC breaker to the off position.

• Step 2: Label where the wires are connected on the thermostat. Check to see if they match the industry standard listed above. 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. Take a picture of your current wiring scheme before you move to step 3.

• Step 3: Disconnect the wires and the corresponding mounting screws. The back plate from your thermostat should then 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.

• Step 4: Put the plate of your new thermostat to the wall. In some cases, you may need to remove the old anchors and repair the drywall for a secure fit when installing your new model. 

• Step 5: Check the plate with a level and mark the holes with your pencil. Drill guide holes before adding anchors to the wall.

• Step 6: Pull the wires through the thermostat opening and attach the back plate with the provided screws. Check to make sure the thermostat is secured.

• Step 7: Reconnect the wires according to the industry standard color codes or your unit-specific notes. Double check that they’re connected in the right place.

• Step 8: Put the thermostat cover in place and turn the power on. Follow the manufacturer instructions included with your thermostat for setting up user programs or Wi-Fi, if applicable.