How to Install a Smart Thermostat

By
Lee Wallender
Lee Wallender
Lee Wallender

Lee has over two decades of hands-on experience remodeling, fixing, and improving homes, and has been providing home improvement advice for over 13 years.

Learn more about The Spruce's Editorial Process
Updated on 03/29/23
Smart Thermostat

Melissa Ross / Getty Images

Project Overview
  • Total Time: 1 - 3 hrs
  • Yield: One smart thermostat, installed
  • Skill Level: Intermediate
  • Estimated Cost: $150 to $350

Smart thermostats have revolutionized home heating and cooling by giving users far more control than analog or even programmable or Wi-Fi thermostats.

Packed with features like geofencing, proximity sensors, energy tracking, and smartphone interfacing, smart thermostats save energy and money while maintaining a comfortable temperature throughout the house. Homeowners who install their own smart thermostat on a pre-installed five-wire thermostat cable can usually finish the project in about one hour.

The Best Smart Thermostats for a More Energy-Efficient Home

What You Need: Five-Wire Thermostat Cable

A five-wire thermostat cable is preferable when installing most smart thermostats. This bundled cable is split into five separate wires: red, white, yellow, green, and blue or black.

The blue or black wire is the C-wire or common wire. If your current thermostat has a C-wire, you should be able to install nearly any smart thermostat brand or model. If your current thermostat is a Wi-Fi thermostat, it will have a five-wire cable that includes the C-wire.

Wire Terminal on Thermostat
White W
Yellow  Y 
Green G
Red R, Rc, or Rh
Blue or black C

3 Workarounds When There Is No C-Wire

The C-wire is a vital, preferred feature of most smart thermostats as it delivers a consistent flow of 24 volts AC to the device. Your choice of smart thermostats will be limited to a smaller number if you decide to go with the other two workarounds: thermostats without C-wires or C-wire adapters.

Install a New Wire

If the home's existing thermostat does not have a C-wire, a new wire can be installed from the HVAC central system (the furnace or air handler) to the thermostat.

The existing two-, three-, or four-wire thermostat cable will be abandoned. A new five-wire cable will be fished through walls, ceilings, or floors.

Buy a Thermostat That Does Not Require a C-Wire

Some smart thermostats do not require a C-wire because they have onboard lithium-ion batteries. Smart thermostats that operate without C-wires may not be as accurate since the thermostat occasionally powers up to check on the temperature, before powering down again. Room temperature swings may result.

Tip

Even smart thermostats that do not require a C-wire will have C-wire capability. Attach these thermostats to a C-wire, if possible, to avoid battery drains and other power issues.

Install a C-Wire Adapter

A C-wire adapter wirelessly signals between the thermostat and the HVAC central system, eliminating the need for a C-wire. The C-wire adapter is installed on the HVAC unit's terminal board. Most C-wire adapters cost $20 to $40. Some smart thermostats include a C-wire adapter.

When to Install a Smart Thermostat

Installing a smart thermostat can be an involved process, especially if you also need to install a C-wire or a C-wire adapter. Install the thermostat on temperate days when you know that you will not need to use the HVAC system for a day or two.

Safety Considerations

Line voltage thermostats contain 120V or 240V electricity. Do not replace this type of thermostat with a smart thermostat. Not only are the two systems incompatible, but this type of electricity can cause injuries or can be fatal. Zoned heating devices like electric wall heaters and baseboard heaters often use line voltage thermostats.

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What You'll Need

Equipment / Tools

  • 1 electric drill
  • 1 flathead screwdriver
  • 1 Phillips head screwdriver
  • 1 hammer
  • 1 wire stripper
  • 1 pencil
  • 1 roll painter's tape or labels

Materials

  • 1 smart thermostat
  • 1 C-wire adapter (if needed)

Instructions

  1. Turn Off the HVAC System

    Turn off the HVAC system at the electric service panel or circuit breaker board. Often, HVAC systems are controlled by two circuit breakers with connected switches, or the breaker may be a large breaker with a wide switch.

    Tip

    Turn on the HVAC system at the thermostat to confirm that no power is present.

  2. Remove the Thermostat and Cover

    Remove the existing thermostat cover. The cover may unsnap or you may need to use a flat-head or Phillips head screwdriver to remove it. Gently pull the thermostat away from the wall. Thermostat wires are thin and loosely attached, so be careful.

    Warning

    Do not touch cables printed with "14" or "12." These numbers indicate thicker gauge wire for line-voltage thermostats. Individual 14-gauge wires are as thick as a U.S. dime and 12-gauge wires are as thick as a U.S. nickel.

  3. Inspect the Existing Thermostat Wires

    The proper wiring for most smart thermostats is 18/5: five separate plastic-coated thin wires, each 18-gauge. The wires will be encased in a larger bundle. As the wires emerge from the wall, the wires will splay out and attach to the back of the thermostat. Taking a picture of the wires helps you reference them later on. The C-wire is usually blue or black. If the C-wire is in-use, it will be attached to the terminal marked "C."

    Tip

    Sometimes, a C-wire will be present but it is not being used. The wire may be wrapped back on the wire bundle and can be used. Keep in mind that the other end will need to be attached to the HVAC unit's control board.

  4. Disconnect Wires From the Existing Thermostat

    Remove the wires from the back of the existing thermostat. Before you remove each wire, attach a label to the wire with its designation (C, W, Y, and so on). Sometimes, a short wire will connect Rc, Rh, or R terminals to each other. Do not disconnect these wires.

    Color  Designation Purpose
    Blue or black C Common
    White W Heating
    Yellow Y A/C compressor
    Green G Fan
    Orange O Heat pump (optional)
    Red Rc A/C-dedicated thermostat (optional)
    Red Rh Heating and cooling thermostat (optional)
    Red R 24V hot

  5. Remove the Old Trim Plate or Backing Plate

    With the drill or Phillips head screwdriver, unscrew the old thermostat's trim plate or backing plate from the wall.

  6. Install the New Backing Plate

    Screw the new backing plate onto the wall. Use drywall anchors if necessary. Make sure that the hole on the trim plate is aligned with the hole in the drywall with the thermostat wire. Pull the wire through and temporarily tape it to the wall to avoid it dropping into the wall.

    Tip

    Many smart thermostats come with an optional trim plate that can be installed to cover up screw holes and marks left by the old thermostat. The trim plate often will snap onto the backing plate.

  7. Attach the Red Wire

    More than one red wire may extend from the wall. These wires may have several acceptable hookup combinations on the thermostat terminals.

    Wire Acceptable Terminal on Thermostat
    R Rc
    R Rh
    Rc Rc
    Rh Rh

  8. Connect the Remaining Thermostat Wires

    The rest of the thermostat wires are a color-to-color match with the thermostat.

    Thermostat Wire Terminal on Thermostat
    G G
    Y Y or Y1
    Y1 Y or Y1
    W W or W1
    W1 W or W1
    C C

  9. Attach the Smart Thermostat to the Trim

    After double-checking that the wires are secure, push the wires into the wall. Snap the thermostat onto the trim.

  10. Turn on the Power

    At the electric service panel, turn on the power to the HVAC system.

  11. Set Up the Connection

    Active home internet with a Wi-Fi connection is required. Smart thermostats have apps that must be downloaded to your smartphone. Most smart thermostats will continue to work if your home Wi-Fi goes down, but you will not be able to access the thermostat through your phone.

When to Call a Professional

Call a heating or cooling company to install a C-wire if the thermostat doesn't currently have one. The HVAC unit must be opened up and one end of the C-wire must be attached to the terminal block on the electronic control board.

If you're unsure about the type of existing thermostat and whether it contains 120V or 240V electricity, call an electrician for an assessment.

FAQ
  • Can I install a smart thermostat myself?

    A homeowner with limited electrical skills can install a smart thermostat in about an hour as long as a five-wire thermostat wire is present.

  • How much does it cost to get a smart thermostat installed?

    The average cost to get a smart thermostat installed is about $250, ranging up to a high cost of around $500. The smart thermostat is an additional cost. If a new wire needs to be run, that will raise the cost to install the smart thermostat by $70 to $110 per hour of labor.

  • Can I replace my thermostat with a smart thermostat?

    You can replace any type of thermostat with a smart thermostat, as long as the current thermostat is hooked up to a five-wire thermostat cable. If a five-wire cable is not in place, a smart thermostat can still be installed, as long as a new cable is added or an adapter is installed. Some smart thermostat models are capable of functioning without a five-wire cable.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. How to Choose a Smart Thermostat. EnergyStar / U.S. Department of Energy

  2. Learn about the common or C-wire. Google Nest Support