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How to Connect your New Thermostat to your AC

A thermostat allows you to control your home’s heating and cooling system. Installing a programmable or smart thermostat is easier than ever, but there are several steps to it. Here’s how to set up your new thermostat.

Locate the Common Wire

To get the most out of your new thermostat, you need to have a common wire, aka c-wire. This wire supplies constant power to your thermostat. Without one, you won’t have access to all the great features of your new thermostat.

To find this wire, turn off your thermostat. You should then turn off the furnace and air conditioning breakers in your electric panel. Take your current thermostat off the wall and look for a wire connected to a terminal labeled “C,” which is the c-wire.

If You Can’t Find a Common Wire

If you don’t see a wire attached to a “C” terminal, look behind the thermostat. An electrician or technician may have tucked it into the wall since it wasn’t needed for the existing thermostat. Many older thermostats didn’t need this wire.

If your home doesn’t have a c-wire, you’ll need to have an electrician install one. You can choose a thermostat that doesn’t need a c-wire, but you’ll be missing out on some great modern features and will be severely limiting the number of thermostats you can choose from. A thermostat that doesn’t need a c-wire will say so on the packaging.

Label the Wires

There will be several wires attached to your old thermostat. You should first take a picture of the wiring so that you can reference it. The following is the industry standard for thermostat wiring:

  • White wire – connects to heat
  • Redh wire – connects to heating system
  • Yellow wire – connects to compressor
  • Redc wire – connects to cooling system
  • Green wire – connects to fan
  • Blue wire – common or c-wire
  • Orange wire – connects to heat pump (if you have one)

Note that not all thermostats use the industry standard, so this is just a guide.

Step 1

Turn off the old thermostat and then flip the furnace and AC breakers to their off positions.

Step 2

Label each wire using painter’s tape and a marker. Mark each label indicating where the wire attaches.

Step 3

Disconnect the wires and remove the mounting screws. The backplate will come off the wall. You should tape the wires to the wall so that they don’t fall into the hole behind the thermostat.

Step 4

Attach your new thermostat’s backplate to the wall. You’ll probably need to remove the old one’s anchors and patch up the drywall so that your new one fits snugly against the wall.

Step 5

Some thermostat back plates have a small level in them so that you can make sure it’s straight. If not, use a level and mark where the screws go in the wall with a pencil—drill guide holes before you put anchors into the wall.

Step 6

Pull all the wires through the opening in the backplate. You can then attach the backplate to the wall using the screws that came with your new thermostat. Double-check that each screw is secure before proceeding.

Step 7

Reconnect all the wires. You can use the wire code above as a guide. You can refer to the picture you took and your labels if your wire colors are different.

Step 8

Put your thermostat cover back into place. Flip the furnace and AC breakers back into their on positions. You can now turn the power on for your new thermostat.

Finishing Up

You’ll need to follow the manufacturer’s instructions for your new thermostat to complete setting it up. A Wi-Fi connection will need to be set up, if applicable, to set all of the user settings. You’ll also likely have access to a mobile app you can install on your smartphone and/or tablet.

All Temp Co., Inc Air Conditioning, Heating, Plumbing, & Electrical is a heating and cooling firm based in Harrisburg, NC. We can install programmable and smart thermostats for you and explain all of their great features. We’ll show you how to use it most effectively so that you save money on your energy bills while your home stays nice and comfortable.

We install, repair and tune-up heating and cooling systems. We also install insulation, perform energy audits, and can improve your home’s indoor air quality. Please call All Temp Co., Inc Air Conditioning, Heating, Plumbing, & Electrical today to schedule an appointment.


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