Project Guide

How to Install a Thermostat

Turn Off Power to the HVAC System
Someone turning off main power at fuse box.

Before you begin to change the thermostat in your home, turn off the power for your heating and air-conditioning system at the main service panel. 

Safety note: 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 the damage unit and may cause personal injury and/or property damage. 

Remove Existing Thermostat
Someone unscrewing existing thermostat from wall.
  • Remove the old thermostat cover plate with a screwdriver. 
  • Unscrew the thermostat mounting screws and remove the thermostat body.
Separate & Label Wires
Someone separating thermostat wires.

Label the low-voltage wires to identify their screw-terminal locations using masking tape and then disconnect the wires. There is no standard color code, so use letter codes to make the wires easy to identify.

Tip: Tape the wires to the wall or wrap them around a pencil to prevent them from slipping into the wall cavity.

Install the New Thermostat Base & Wire
Someone screwing the base plate of thermostat into the wall.
  • 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. Or you can locate the low-voltage transformer that powers the thermostat. The transformer is usually located near the heating and air-conditioning system or inside of a furnace access panel.
  • 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.

Mount Thermostat & Restore Power
Woman adding face plate to programmable thermostat.
  • 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.
Thermostat Wiring Tips
Programmable thermostat installed on gray wall.
  • Installation varies for two-wire systems (heating only or cooling only). 
  • If you have a two-wire system, refer to the instructions 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. 

Although installing a programmable thermostat or smart thermostat makes the most economical sense, you can also use the steps above to install manual varieties. Our thermostats buying guide offers additional tips for understanding the different thermostat types and their features. When you’re ready for a new thermostat, remember we offer free delivery on over a million online items.