How to Replace a Furnace Thermostat


When your furnace thermostat malfunctions, it’s time to replace it. Check the type of thermostat that you have and purchase a new one just like it. Determine the number of wires and types of terminals in the unit so you purchase the proper replacement unit. Write down the number and types of terminals and the color of wire attached to them. Also consider replacing your old thermostat with a programmable one; and a unit that has a battery backup that will retain the information if the power goes out. Using programmable thermostat lets you set the temperature according to the time of day, saving you money in heating costs. And by replacing your thermostat yourself, you’ll save on a costly service call. As always, follow the thermostat manufacturer’s directions for installation for the brand of thermostat you purchase.

Things You'll Need

  • New thermostat with mounting screws
  • Stick-on labels
  • Black magic marker (thin tipped)
  • Screwdrivers (flathead and Phillips)
  • Level
  • Shut off the power to your furnace at the circuit breaker box. Lift off the cover to the broken thermostat. Disconnect the wiring from the wall to the old thermostat by unscrewing the screws holding them in place with a Phillips screwdriver.

  • Make a notation of where the wires from the wall were connected to the old thermostat wiring with a black marker and labels. Write a letter on each of the old thermostat wires and one on the wire from the wall that corresponds to it. This way you’ll know what corresponding wires on the new unit to use when attaching them to the wall wires.

  • Unscrew the mounting screws holding the old thermostat mounting plate to the wall with a flathead or Phillips screwdriver, depending on the type of screw. Lift off the mounting plate. Attach the new mounting plate for the new thermostat in its place with the screws and screwdriver. Set a level on top of the mounting plate to position it evenly; tighten the screws fully.

  • Connect the wiring for the new thermostat to the corresponding screws you labeled. Wrap the ends of the wires around the screw posts and tighten the screws with a Phillips screwdriver so they are secured under the screws.

  • Place the cover for the new thermostat in place according to the manufacturer’s directions. Turn the power back on at the circuit breaker box.

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