How do i hook up a new 5 wire cable to an existing 4 wire furnace

furnacethermostat-c-wire

I would like to replace the existing thermostat wires on my 2000's era janitorial furnace with the upgraded 5 wire setup that will allow fan control of my Honeywell wifi 9000 thermostat. I was going to just try and figure out how to jump the green G wire, but i would really prefer to just run a new wire.

What I would like to know is how do I wire the new wires to the old unit? As in who do each of the new wire's correspond with the existing 4 wires that connect to the circuit board on the existing furnace.

I do believe that the other "pink wire" that connects to the Y wire connects to the AC condenser… not 100% sure but I believe so.

Photo 1: Back of thermostat face plate and wires
Photo 2: Wires right before going into HVAC unit
Photo 3: Wires being connected to the HVAC unit

Any help would be greatly appreciated!

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Best Answer

The thin blue wire at the thermostat is connected to the Y terminal, which means it is used to call for cool (air conditioning). You'll notice at the furnace, that the thin blue wire connects to the thin red wire. The thin red wire goes to the contactor coil in the A/C unit outside.

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Once you have the new cable run, you'll connect the wires as described below.

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At the furnace

  1. Remove the old thermostat wires (leaving the wires for the A/C unit alone).
  2. Connect the green wire from the new thermostat cable, to the thick green wire from the furnace.
  3. Connect the red wire from the new thermostat cable, to the thick red wire from the furnace.
  4. Connect the white wire from the new thermostat cable, to the thick white wire from the furnace.
  5. Connect the yellow wire from the new thermostat cable, to the thin red wire from the A/C unit.
  6. Connect the blue wire from the new thermostat cable, to the thick blue wire from the furnace and the thin white wire from the A/C unit.

At the thermostat

  1. Connect the red wire to R
  2. Connect the green wire to G
  3. Connect the white wire to W
  4. Connect the yellow wire to Y
  5. Connect the blue wire to C
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