How to wire a 2-wire thermostat to a 4-wire system

Written by marshal m. rosenthal
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A thermostat is designed to control the temperature of a heating or cooling unit in the home. Replacing an outdated four-wire thermostat with a newer model that uses two wires is not complicated, once you purchase the thermostat from a home-and-garden shop or hardware store. The procedure to install the new thermostat is straightforward and does not require the assistance of a certified electrician. A few tools found around the house are all that's needed.

Skill level:
Moderate

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Things you need

  • Phillips jeweller's screwdriver
  • Replacement two-wire thermostat

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Instructions

  1. 1

    Remove the fuse or trip the circuit breaker in the fuse box that supplies power to the thermostat and the unit that it controls. Details as to which fuse/circuit breaker affects which electric line is typically noted on the inside of the fuse box or beneath the fuse/circuit breaker.

  2. 2

    Remove the cover from the thermostat. Typically, this involves pulling the cover off, although in some cases you might first need to wiggle one side loose and then the other.

  3. 3

    Remove the screws from around the thermostat with the Phillips jeweller's screwdriver. Pull the thermostat out from the wall.

  4. 4

    Aim a flashlight at the terminal block inside the wall to determine which screws are connected to the red and white wires attached to the thermostat. Loosen the screws on the terminal block with the Phillips jeweller's screwdriver. Unwind the four wires attached to the thermostat from around the screws. Remove the thermostat and dispose of it properly.

  5. 5

    Wind the exposed end of the red wire attached to the replacement thermostat around the screw that formerly held the red wire from the old thermostat. Wind the exposed end of the white wire attached to the replacement thermostat around the screw that formerly held the white wire from the old thermostat. Tighten these two screws as well as the other two screws on the terminal block.

  6. 6

    Place the replacement thermostat against the wall and screw it on using the Phillips jeweller's screwdriver. Place the cover over the replacement thermostat. Restore electric power to the thermostat and the unit it controls.

Tips and warnings

  • Verify that the electricity is turned off by plugging a table lamp into a wall outlet and turning it on to see if it works.
  • Tape the fuse box closed with strips of duct tape to prevent usage while wiring the thermostat.
  • If you have any concerns about wiring, wear electrician's rubber-insulated gloves to protect your hands against electric shock.

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