How to Install a Thermostat

1. Turn Off Power to the HVAC System

The first step in how to install a thermostat is making sure the power is turned off to your existing thermostat.

• Turn off the power for your heating and air conditioning system at the main service panel. Its circuit breaker should be clearly labeled.

Safety Tip: Make sure you don’t install a new thermostat on circuits exceeding manufacturer-specified voltage. Higher voltage thermostat wiring will damage the controls and may cause shock or become a fire hazard. Additionally, short or incorrect thermostat wiring will damage the unit and may cause personal injury and/or property damage.

2. Remove Existing Thermostat

• Remove the old thermostat cover plate with a screwdriver.
• Unscrew the thermostat mounting screws and remove the thermostat body.

3. Separate & Label Wires

• Label the wires using masking tape so you can remember their screw-terminal locations. There is no standard color code, so use letter codes to make the wires easy to identify.
• Disconnect the wires.
• Tape the wires to the wall or wrap them around a pencil to prevent them from slipping into the wall cavity.

4. Install a C-Wire if Needed

Smart thermostats can have up to five thermostat wires that always include a “common wire” or C-wire. The C-wire provides a continuous flow of power to the thermostat without necessarily turning your other appliances (such as the fan or cooling) on or off.

If you’re replacing a smart thermostat with another smart thermostat, you’ll already have a C-wire connected to the C terminal. If you’re upgrading to a smart thermostat, you’ll need to find and identify the C-wire or install a new C-wire.

• Even if your old thermostat was not using a C-wire, an unused C-wire may be stuffed in the wall behind the thermostat’s backplate. Check your HVAC control board to see if it has a wire connected to its C terminal. The HVAC control board is usually located near the heating and air conditioning system or inside of a furnace access panel. If it has a C-wire, the other end of the wire should be in the wall, and you can use it for your new smart thermostat.
• If there is no C-wire, one option is to use an “add-a-wire” kit or a power extender kit. Some smart thermostat models may come with this kit and other thermostat parts included. Follow the manufacturer’s instructions for installing this kit at the HVAC control board.
• Another option is to use a 24-volt adapter that provides a C-wire and plugs into a nearby outlet to power your thermostat.
• If your HVAC control board has an empty C terminal, you can also install a C-wire yourself and run it through the walls to your thermostat.
• If you’re ever in doubt, call an electrician to add a C-wire for you.

5. Install the New Thermostat Base and Wire

Whether you’re learning how to install a basic non-programmable thermostat or learning how to install a smart thermostat, the next step is generally the same. Of course, it’s always important to follow manufacturer instructions for any small differences that exist from one unit to another.

• Remove the existing thermostat base by loosening the mounting screws.
• Separate the front of the new thermostat from the base. Begin by installing the base first.
• Thread the low-voltage wires through the specified opening of the new thermostat base.
• If your new thermostat is not self-leveling, use a small level to make sure it’s properly aligned.
• Mount the thermostat base on the wall using the screws included with the thermostat.
• Connect the low-voltage wires to the appropriate screw terminals on the new thermostat base.
• If your connections do not match, consult the installation diagram in the new thermostat’s owner’s manual.
• Tighten any loose wire connections and make sure the wires and sheathing are in good condition.
• Install the battery or batteries in the thermostat body (if applicable). Make sure the battery size and polarity are both correct.

Tip: When shopping for your home thermostat replacement, check the package to ensure it is compatible with your home’s system.

6. Mount Thermostat and Restore Power

• Mount the thermostat base by gently pushing the excess wire back into the wall opening and plugging the hole with a fire-resistant material. Fiberglass insulation is great for preventing drafts from affecting thermostat operation.
• Mount the control unit to the base using the screws included with your thermostat. Tighten the screws securely.
• Restore power to your heating/cooling system, then press the reset button to clear the program memory.
• Program the thermostat according to the manufacturer’s instructions.
• Test the unit to make sure both the furnace and air conditioning unit are cycling properly.

7. Thermostat Wiring Tips

• Mount the thermostat base by gently pushing the excess wire back into the wall opening and plugging the hole with a fire-resistant material. Fiberglass insulation is great for preventing drafts from affecting thermostat operation.
• Mount the control unit to the base using the screws included with your thermostat. Tighten the screws securely.
• Restore power to your heating/cooling system, then press the reset button to clear the program memory.
• Program the thermostat according to the manufacturer’s instructions.
• Test the unit to make sure both the furnace and air conditioning unit are cycling properly.

8. Thermostat Wiring Tips

• Installation varies for two-wire systems (heating only or cooling only).
• If you have a two-wire system, refer to the instruction manual provided by the manufacturer for a more detailed installation guide.
• Tighten any unused terminals securely.
• If your system has more than two wires, follow the manufacturer installation diagram to determine correct wire connections.
• Heating and cooling needs vary from season to season, so adjust the programming as needed to accommodate your lifestyle.
• If you have more than one heating or cooling system, placing an adjustable thermostat on each one will ensure greater energy efficiency.

9. Tips for Installing Programmable Thermostats