Step 1: Remove the Old Thermostat

person unscrewing an old thermostat in order to remove it
Photo: Ryan Benyi
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Cut off power to the furnace using the emergency shutoff or breaker switch, then unscrew the thermostat's mounting bracket. As you disconnect each wire from its terminal, label the wire with the letter of the terminal it came from. Tape the wires to the wall so that you don't lose them. Some older thermostats contain mercury; for information on proper disposal, go to the Thermostat Recycling Corporation.
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    Tools List

    • flathead screwdriver
      Screwdriver
    • needlenose pliers
      Needlenose pliers
    • drill
      Drill/driver, for mounting the outdoor sensor

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    Programmable thermostat