How to Replace Thermostat


If you have an old inefficient thermostat, consider purchasing a new programmable thermostat. You could end up saving hundreds of dollars a year. But if the wiring in your current thermostat looks like a jumbled mess, you might think you don't know enough to install a new thermostat. New thermostats, though, come with thorough manuals that guide you through the wiring process. Replacing a thermostat is a common do-it-yourself task.

Things You'll Need

  • Screwdriver
  • Masking tape
  • Level
  • Drill
  • Hammer
  • Turn off the circuit breaker to your air conditioning unit and furnace before working on the thermostat. Some AC units have a separate breaker right next to the unit; if not, you will need to turn it off at your main breaker box.

  • Remove the face of your current thermostat. There should be several wires coming out of the wall and inserted into connectors on the thermostat. The connectors should have letters stamped on the plate. Wrap a small piece of masking tape on each wire and write the connector code letter on the tape so you can match the correct wire when installing the new thermostat.

  • Remove the wires from the old thermostat. You may need a small screwdriver to loosen the connection and pull the wire out. Do not let any wires fall back into the wall.

  • Remove the old thermostat from the wall. If it contains a mercury switch, do not throw it in the trash. You must call your waste management authority for instructions on how to dispose of the mercury. Unscrew and remove the old backing plate.

  • Hold your new thermostat backing plate up to the wall and pull the wires through it. Use a level to straighten the wall plate. Use a pencil to mark where the new screws will be inserted.

  • Drill pilot holes into the drywall. Your new thermostat should come with screws and anchors, and the manual will tell you the size of the pilot hole to drill.

  • Insert the anchors into the drywall. You may need to tap them with a hammer to push them all the way in, but be careful not to damage the drywall.

  • Hold the new backing plate up to the wall and use a drill or screwdriver to insert the screws through the backing plate. The backing plate should now be mounted firmly on the wall.

  • Connect the wires to the same letters on the new thermostat. For example, if the yellow wire previously went to "Y," hook up the yellow wire to "Y" on the new thermostat. There will be some kind of clamp or small screw to hold the wire in place.

  • Mount the front of the thermostat to the wall plate.

  • Turn on power at the circuit breaker.

  • Set the temperature.

Tips & Warnings

  • Refer to your new thermostat's manual if the letters from your old thermostat do not match your new thermostat. It will have instructions for where to connect the wires. For example, the manual will tell you "Connect C, X, or B to the C terminal."
  • Read and use the new thermostat's manual as a guide while wiring.
  • Be careful with electricity. Be sure to turn off the circuit breaker before beginning work.
  • If you are uncomfortable dealing with wiring, you can always call a qualified contractor for help.

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