How to Run Common Wire From a Thermostat to a Carrier Furnace


The common wire is a low-voltage wire that runs from the furnace to the thermostat. Thermostats receive their power two different ways. One way the thermostat receives its power is from batteries. The other way the thermostat receives its power is through the common wire. Thermostats can use batteries only, receive power from the furnace with battery backup, or use power from the furnace entirely. Some Carrier furnaces require a common wire to the thermostat; installing one is a simple job.

Things You'll Need

  • Screwdriver
  • Wire stripper
  • Turn off the power to the Carrier furnace by turning off the breaker in the circuit breaker panel.

  • Remove the Carrier furnace access panel. Locate the control board. On the control board there will be a terminal strip the thermostat wires tie into. The common wire is usually the black wire and it connects to the "C" terminal. Make this connection if it is not already made.

  • Replace the furnace access panel and go to the location of the thermostat.

  • Remove the thermostat from the wall by applying gentle pressure until it snaps off the wall plate.

  • Locate the black wire and strip it back with your wire stripper to expose 1/4 inch of bare copper wire.

  • Connect the black wire to the "C" terminal on the thermostat. Push the thermostat back onto the wall plate.

  • Turn the power back on to the furnace. Your thermostat should now be powered by the furnace entirely, unless the thermostat has batteries. Then it will be powered by the furnace when the batteries fail.

Tips & Warnings

  • If the thermostat uses batteries, replace them at least yearly. If for some reason the common wire doesn't transmit power to your thermostat, batteries are vital to keep the thermostat operating.
  • Make sure the power is off to the Carrier furnace when installing the common wire. Failure to do this may result in the control board shorting out. You may also incur electrical shock from the live components in the furnace access panel.

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  • Timothy Gonyo; Sheet Metal Workers Local #18; Milwaukee, WI
  • Photo Credit Hemera Technologies/ Images
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