How to Wire a A/C Thermostat With 2 Wires


Air conditioning condensing units communicate with the furnace via two thermostat wires. The two wires are encased in insulation to make one two-wire thermostat wire. One of these wires runs from the air conditioning condensing unit to the furnace. The main thermostat wire then runs from the furnace to the thermostat. Wiring your A/C thermostat with two wires is actually the among easiest steps in installing an air conditioning condensing unit.

Things You'll Need

  • Tape measure
  • Thermostat wire
  • Wire stripper
  • Wire nuts
  • Zip ties
  • Screwdriver
  • Turn off the power to the furnace and air conditioning condensing unit by turning off the correct circuit breaker in the circuit breaker panel.

  • Install a two-wire thermostat wire from the air conditioning condensing unit to the furnace. Strap it to the copper line sets that run from the furnace to the condensing unit with zip ties.

  • Strip about 2 inches of the outer layer of insulation back on the thermostat wire at both the condensing unit and the furnace to expose the two wires inside. Strip each of these wires back 1/4 inch to expose the bare copper.

  • Connect the red and white wires of the thermostat wire to the red and white wires coming from the condensing unit with wire nuts.

  • Connect the red thermostat wire to the "C" terminal on the furnace control board. Connect the white thermostat wire to the "Y" terminal on the furnace control board.

  • Check to see that the yellow wire coming from the thermostat is connected to the "Y" terminal on the furnace control board also. If it isn't, connect it.

  • Go to the thermostat and remove it from the wall by applying gentle pulling pressure. Check to see that the yellow wire is connected to the "Y" terminal. If it isn't, connect it.

  • Push the thermostat back on to the wall plate by pushing with gentle pressure until it snaps into place. Turn the power back on to the furnace and air conditioning unit. Set the thermostat to your desired setting and your air conditioner should function properly.

Tips & Warnings

  • Make sure the power is off to the furnace and air conditioning unit. Failure to turn the power off could result in a severe electrical shock.

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