How to Replace a Heating and Cooling Thermostat

Programmable thermostats allow you to set different temperatures for different times of the day.
Programmable thermostats allow you to set different temperatures for different times of the day. (Image: Comstock/Comstock/Getty Images)

Just by making some adjustments in your heating and cooling routine, you can save about 10 percent a year on your energy bills, according to the U.S. Department of Energy. The best way is to install a programmable thermostat so you can easily make those adjustments. So if you want to save some money, or perhaps your old thermostat is petering out, you can replace it with just a couple of tools.

Things You'll Need

  • Masking tape
  • Small flathead screwdriver
  • Small Phillips screwdriver
  • Pencil
  • Small level
  • Drill and bit
  • Plastic wall anchors
  • Small wire strippers

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Turn off the central air conditioner and all power running to it. On most thermostats, the "Off" selection will be located with the heat and cool selections. Also locate the the breaker for the air conditioning and heating unit or units and turn it off. This will cut off all power to the system and the thermostat. Be sure to double check that you have selected the correct breaker by attempting to turn the air conditioning unit back on.

Remove the cover on your old thermostat. This step will allow you to note the settings that your current thermostat utilizes and expose the screws that hold your thermostat base on the wall.

Mark your thermostat wires by writing the correct connection location on masking tape and wrapping the masking tape around the corresponding wire. Thermostat wire comes in a variety of colors and wire counts, but the most important wires typically follow this pattern: Red is the hot source from the transformer. It can be marked RH, RC or R. Green is the wire that controls the fan or blower relay. It goes to the G terminal. Yellow is the air conditioning control. It goes to the Y terminal. White is the heating control, and it goes to the W terminal. These are the four wires that control your thermostat operation, but don't worry if you notice additional wires that are not connected to anything. The installer may have used a different wire and those wires do not have to be connected.

Remove the wires from the old thermostat. As you remove the wires from the old thermostat base, wrap them around a pencil so that they do not fall back into the wall opening and they stay in a usable order.

Remove the old thermostat base by carefully removing the screws that hold it in place.

Draw the thermostat wires through the new base, level it and secure it to the wall. Remove the wires from the pencil, pull the wires through the base unit, level the thermostat, mark where the screw holes need to be drilled, drill the holes, place the wall anchors in the holes and connect the base unit to the wall.

Attach the wires carefully to the correct wire terminals. You want to make sure that the wires are connected to the correct terminals that you marked and that all of the connections are tight. You may have to use wire strippers to expose additional bare wire, but be sure not to have any bare wire touching anything besides its correct terminal.

Attach the thermostat to the base with the built-in screws. Make sure that you have set the thermostat settings to the same as the old thermostat before continuing to the final step.

Attach the cover, restore power and check the unit operation. The cover should snap into place easily now. Simply turn the thermostat to cool, restore the power and your new thermostat should be fully operational.

Tips & Warnings

  • Be sure to level, use the correct drill bit and verify placement of your new thermostat.
  • Sometimes it is easier and safer to pay for a repair call. Your thermostat may not be your problem.


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