How to Replace an Analog Thermostat

Step 1: Carefully Select New Thermostat.

Before installing a new thermostat, you should check your home's compatibility. Homes are wired in two ways, line voltage and low voltage, and you will need to purchase a thermostat that will work with your specific heating and cooling system.

Check out our blog Line Voltage vs Low Voltage for a more detailed comparison.

If you are unsure about the kind of thermostat connection your home has, contact HomElectrical at 1-888-616-3532 and we will be glad to answer any questions.

Step 2: Shut Off Power to The Furnace

Important: Turn off all power to the heating and cooling system prior to doing any electrical work to avoid any chance of electrocution or injury.
Circuit breaker with flipped switch

HVAC systems can be powered down at the breaker box. The power for some furnaces can be located directly on the boiler, usually with a toggle switch.

It is also a good practice to turn your current analog thermostat all the way down before disconnecting.

Step 3: Remove the Faceplate From the Current Thermostat

With the wires exposed, you will be able to test the wires with a circuit tester to confirm the power is disconnected. Look at the type and number of wires in order to determine your thermostat's compatibility. Line thermostats will have two thick wires, while a low voltage thermostat will have anywhere from two to nine small-gauge wires.

Caution: Many older analog thermostats contain a glass tube of mercury. Mercury is considered a hazardous material and must be disposed of properly.

Step 4: Label the Wires

Thermostat wiring colors and terminal labels

You will want to label, take a picture of, or make note of where the wires are connected before disconnecting them.

Note: The wires are not always color coded or the same for each thermostat.

For example, if the green wire goes to the G terminal, the same wire will need to be inserted into the G terminal of the new thermostat.

Step 5: Disconnect the Wires and Remove the Wall Plate

You will need to unscrew each wire from the terminal and unscrew the wall mount as well. The wall mount will typically have one screw on each side.

Check to see if the current drywall anchors align with the new thermostat wall mount. If they don’t, you will need to install the drywall anchors in the appropriate location.

Note: We recommend that you secure the loose wires, such as taping them to the wall, so they don’t retract into the hole.

Step 6: Mounting the New Thermostat

If the new thermostat does not line up with the existing holes, you will need to drill new ones.

Place the new thermostat flush with the wall and mark the holes on the wall with a pencil or marker. Be sure to use a level to ensure the mounting isn’t crooked.

Drill into the marked locations on the wall.

Note: The drill bit will need to be smaller than the screws provided with the new thermostat. If needed, insert the drywall anchors.

After the holes have been drilled, screw the new thermostat's back plate onto the wall. Feed the wires through the hole in the back of the thermostat, and you will be ready to re-wire the new thermostat.

Step 7: Rewiring

Thermostat wiring

Begin attaching each wire to the proper terminal.

Refer to the picture or notes to ensure the correct wire is connected. In some cases, you will have an extra wire.

The extra wires are the wires that were not connected to the old thermostat. This is ok and you do not have to worry about the exposed wires.

Step 8: Attach the Faceplate

Attaching face plate to thermostat

Most modern thermostats will need batteries to operate properly.

Insert the compatible batteries, then snap the faceplate onto the front of the new thermostat. Consult the user manual if you run into any issues attaching the faceplate.

Step 9: Test Out the New Thermostat

With the new thermostat attached, you will want to confirm the installation was done correctly. Restore the power to the HVAC system. Turn the heating and air system back on and test the thermostat. To do this, just turn the thermostat to a temperature higher or lower than the current temperature. For example, if the current temperature is 76°F, you might want to turn it down to 72°F. Within a few moments, you should hear the HVAC system's fan fire up.

If nothing happens, cut off power to the system and check the wiring. Feel free to call HomElectrical at 1-888-616-3532 and we will be glad to answer any questions you may have. Find a selection of analog thermostats at HomElectrical!

Tyler Karstensen
Tyler Karstensen

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