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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  • The thin blue wire at your thermostat is almost certainly not ground. It is not connected to the fat blue wire at your furnace.
    – brhans
    Commented Nov 27, 2017 at 21:36
  • It looks like the blue wire at your thermostat is actually your Y wire to your air conditioner compressor. Do you have central air, and if so, does that 2-wire cable from your furnace go to your compressor (outdoor unit)? Commented Nov 28, 2017 at 0:05

3 Answers 3


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
  • Thank you for the diagram Tester! I hooked it up exactly as you described and it works perfectly!! Thanks gain!
    – schwein
    Commented Nov 29, 2017 at 12:55

Your wifi thermo is going to need both power leads R and C.

Your furnace has (Red White Green Blue)
Your A/C has (Pink White)
18/5 thermostat wire typically has (Red White Yellow Green Blue)

Most of your new setup will be similar as before, except for blue and yellow:

  • R to Red to Red/furnace (Jumper remains for Rc)
  • C to Blue to Blue/furnace & White/AC


  • W to White to White/furnace
  • Y to Yellow to Pink/AC
  • G to Green to Green/furnace

The thermostat also needs to be configured for Conventional 1H/1C, single transformer

  • Thank you Starvr! It seems like your response is in line with Testers comment above, Thank you for your response!
    – schwein
    Commented Nov 28, 2017 at 18:42
  • what do you mean by having to configure the thermostat for conventional 1H/1C, single transformer? Would I not be able to simply plug the new 18/5 cable into the Honeywell Wifi 9000 thermostat and have it work?\
    – schwein
    Commented Nov 28, 2017 at 20:29

I had the same problem with my new wifi thermostat. Thought that it may help to someone who has similar issue with my old unit.

So this was my problem. Here most of the people explained new furnace units with control boards. My unit is one of the oldest units which doesn't even has a control board. It only has power supply (transformer) and wire connections. While i was trying to figure out whole system, found attached furnace wiring diagram.

furnace wiring diagram

As you may see from diagram there suppose to be a c wire at my furnace transformer. Checked transformer and traced each cable which goes to thermostat. Even though there were spare wires coming from thermostat cables, nothing was connected as c wire. There was only green, yellow, red and white color cables were connected. Found c wire connection on transformer and connected one of the spare cable which goes to my thermostat. transformer and connected wires

Finally i was ready to hook up my c wire to my new thermostat to work it. But nothing happened. Checked transformer red and c wire connections with my multimeter and there was 24v ac. Really confused and started to think about having faulty unit option. Right before dumping my thermostat, remembered an important detail. What if i miss something at my thermostat cabling. Watched tons of videos about how to work it with an external power supply. All videos has a simple common point which is; there suppose to be a "-" and "+" polar to work any electronics :)

thermostat connection pins

I was assuming that red and c wire are enough to work unit but it wasn't working. If you have an old unit like mine and if you don't have seperate "Rc" cable, u need to add jumper cable between "Rc" and red cable as it is stated at unit manual. After i add jumper to thermostat, finally could work it.

By the way; these units come with factory default settings. If your fan, ac, heater doesn't work don't panic. Just rearrange your thermostat setting from the menu according to your furnace system by fallowing mannual.

Be Carefull!!! If you do not have enough knowledge about electricity or electronics, please don't risk your life and don't touch your system. If you still want to do it, don't trust anything like furnace cover security switch (which shuts off the whole unit when you open front cover), just work when you turn off home furnace main switch.

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