Thermostat Wire

Air conditioners, furnaces, and your entire HVAC system are controlled by a thermostat. As such, your thermostat must be wired correctly. Each thermostat wire has its own purpose that is important to know. You’ll find colored wires once you remove the front panel of the thermostat. To get started, it’s first imperative you understand what each color means.

There are several configurations, including 2, 3, 4, and 5 wire thermostats. Wire colors are selected based on a code set by industry standards, and include:

  • Yellow: Connects to the AC compressor.
  • White: Connects to the furnace or other heating component.
  • Orange: Attaches to the heat pump, if you have one.
  • Green: Powers the HVAC fan.
  • Red: An RC wire connects to the cooling system, while an Rh wire powers the heating system.
  • Blue: A “C” wire used to power newer thermostats.

Steps to Thermostat Wiring

Before you start unscrewing terminals and detaching wires, take a picture so you have a reference when installing the new unit. If you remove the thermostat too quickly, you’ll end up having to reach into the wall or break it apart to fish for the wires.

  1. Turn Off the Thermostat: In addition to turning it off manually, flip the circuit breaker serving your HVAC system.
  2. Label the Wires: Use a marker and painter’s tape to create labels for each wire. Use each wire’s color and corresponding terminal code (labeled on the thermostat) as a reference.
  3. Disconnect the Wires: Bend the wires outward so they don’t fall back into the wall. Disconnect them and remove the mounting screws to free the back plate of the thermostat.
  4. Prepare the New Thermostat: Remove any old anchors and repair the drywall if necessary. Then put up the new thermostat plate, checking it with a level and using a pencil to mark holes to drill. Before you add anchors, drill guide holes first.
  5. Attach the Back Plate: Once the wires are pulled through the opening, screw on the plate and verify that it fits tightly.

 Whether installing a new thermostat or replacing the wiring (for example, when upgrading your HVAC system), it’s important to follow the right steps. The right type of wire depends on the type of thermostat you have. Wires are sold in bundles; for heating and AC equipment, it will have at least five wires, but you can use eight-wire bundles to plan for future upgrades. Thermostat wires are typically 18 gauge, so are labeled 18/5, 18/8, and so on based on wire count.

Wiring the thermostat requires ensuring reconnecting each wire to the appropriate terminal. For example, make sure the red wire connects to the R terminal, green wire to the G terminal, black wire to the C terminal, white wire to the W1 terminal, and blue or yellow to the Y terminal. To proceed with installation:

  1. Strip about ½ inch of insulation of each wire, using a wire stripper, and add wire connectors if necessary.
  2. Referencing the color code and diagram included with the thermostat, attach the wires to the terminals and screw the ends of the conductors in place.
  3. Attach the corresponding wires to the terminals on your furnace, AC unit, and other components the thermostat will control.
  4. Clamp the new wire bundle approximately 8 inches from the wall, wrap any unused wires around it, and gently insert the bundle into the wall.
  5. Secure the thermostat on its base plate.

Contact Express Electrical Services

We’re familiar with all aspects of thermostat and home wiring. Customers throughout Southern California rely on us for any type of professional electrical work. For help installing and wiring your thermostat, give us a call at 323-727-7799 today!