How to Replace a Thermostat in Your Home

Purchase a Compatible Thermostat

The most important first step in replacing a thermostat is buying a compatible model. Most thermostats list their voltage and wiring details on the back of their faceplates. Local compatibility information in the instruction manual of your new thermostat and speak with a local HVAC company when in doubt.

Depending on the age of your system, home, and even current thermostat hookup, you'll likely have a 24-volt or 240-volt hookup—quite the big difference. Attempting to attach incorrect voltage to your HVAC system is dangerous, and can damage your system.

Additionally, upgrading to a smart thermostat from a manual or simple programmable one could require the addition of a C-wire. If you already have a C-wire in your old thermostat—more on this below—proceed as usual. If you need to add a C-wire or find it in the wall, call a professional.

Read Your Instructional Manual

Thermostats share many similarities, but with so many models on the market, it's very important to read the manual. Before breaking out the tools and getting started, sit down and read the manual from front to back. Much like baking a cake, you don't want to get halfway through the project and realize you need a specific tool or more time than expected.

Turn Off the Power to Your HVAC

Always begin by cutting the power to your HVAC system and thermostat. Many thermostats have backup battery power, so it may not lose its settings if you're simply checking out the current system before heading to the hardware store.

Look for the power at the main service panel (the HVAC circuit breaker should be clearly labeled). If you are at all in doubt about how to fully cut power to your HVAC system and thermostat, speak with an electrician or local thermostat installer for guidance.

Remove the Thermostat Faceplate

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With the power shut down, remove the faceplate from the front of your thermostat control panel. In most models, simply pinch the sides of the faceplate and pull it towards you. Use this moment to double-check the voltage information again on the back of the faceplate to compare it to your new model.

If Necessary, Install a C-Wire

At this point, you should be able to see the wires connected to the current thermostat. Do not remove them quite yet.

If you’re upgrading an older model thermostat, it’s possible you only have two wires currently. But you’ll need five wires—including the C-wire or Common wire—for a smart thermostat. At this point in the project, it might be wise to stop and call in the pros to ensure you have the equipment to meet capacity. They may choose to install a C-wire, a power extender kit, or a 24-volt adapter.

If you see five wires behind the faceplate, this indicates you already have the C-wire, but always check the existing labels to make sure.

In any case, take a picture of the wires and how they relate to the current system. Some wires follow a color-coordination system, but others do not, so you’ll need to take extra precautions to label your wires. Use small pieces of masking tape to mark your wires with a letter to note how it corresponds to the current hookup.

Disconnect Wires

Disconnect the wires from the current thermostat by either unscrewing them with a small screw bit or pressing the corresponding button to release the wire. The setup will depend on the age and type of system.

Safety note: Make sure that the wires do not fall back into the wall when you disconnect them from the thermostat. You can temporarily attach them to a pencil with masking tape or tape them to the wall as you move on to the next step.

Remove the Baseplate

Unscrew the base plate and delicately pry it away from your wall. String the wires through the opening of the old baseplate and, yet again, make sure they don't slip into the wall. Retape them if necessary.

Line Up the New Baseplate

Photo: yunava1 / Adobe Stock

Take this moment to determine if you need to fix any areas behind your old thermostat. If your new plate doesn't cover the old area, you may need to add a bit of paint or spackle to make it look smooth.

It's also important to make sure your thermostat is level. Even though new thermostats don’t depend on mercury, it will look best to have a level display. Some models include a built-in level for easy installation, but you can also use a small separate one.

Mark the screw holes with a pencil to stay on track.

Screw New Base to the Wall

String your bundle of wires through the new baseplate and drill the screws into the holes at your pencil marks. If you're working with drywall, attach the base of the drywall screws first and then secure it with a screwdriver.

Attach the New Wires

Attaching new wires to your thermostat will significantly vary by the model. Many new systems make it quite easy for DIY homeowners by including small buttons. Simply press the button to open the connection and place the coordinating wire underneath it. In other cases, you may need to attach the wires to small screw terminals.

We'll say it one more time for the road: if you are not sure how your old wires should correspond to your new system based on the manual, pause and call a professional.

Depending on the instructions in your manual, at this point, you should be able to add new batteries to your thermostat if required. Not all systems include batteries, but some include them as an extra power source.

Mount the New Thermostat Faceplate

Snap or screw on the new faceplate according to your new system's instructions. Double-check that everything is attached to the wall properly and in the right alignment.

Restore HVAC Power

Flip back on the power and go through the steps in your manual for programming and starting up your system.