I am having some difficulty figuring out the correct wiring for my new wifi thermostat. Before I bring out the A/C technician, I was wondering if anyone here might be able to help? Searched other threads and did not see one that applied to my situation...

It is a Honeywell RTH65800WF, watched all of their videos and spoke with their support department and still no luck, so here it goes:

Existing Thermostat Terminals:

  • G - Green Wire,
  • Y - Blue Wire (previous owner wrote a B next to Y label),
  • W - White Wire,
  • RH - Red Wire (with jumper to RC terminal) (previous owner appears to have highlighted the R),
  • B - Empty,
  • O - Empty,
  • RC - Jumper to RH terminal

Existing AHU Terminals:

  • C - Blue Wire,
  • R - Red Wire,
  • G - Green Wire,
  • Y/Y2 - Yellow and Gray Wires (not sure where these lead to , but I think to outside unit),
  • O - Empty,
  • W1 - White Wire,
  • W2 - Empty,
  • Y1 - Empty

New Thermostat Terminals (need to connect to): C, G, Y, W, R

So it appears the C terminal at the Air handling unit (AHU) block is currently connected to the Y terminal at the existing thermostat? Is that possible?

I will attempt to include photos of the AHU and Existing Thermostat terminal blocks:


enter image description here

Ex. Thermostat

enter image description here

  • I don't have an answer, just a note: I installed a NEST in my house and blew a fuse on the furnace by not shutting it off before doing this. Also, my furnace didn't have a ground and, despite NEST saying it wasn't needed, it was needed for NEST to work so I ran a ground.
    – user19347
    Oct 21, 2014 at 19:17
  • Thanks for the tip. Nothing mentioned on grounding in the directions, but I assume the AHU unit itself is grounded, and the thermostat utilizes that ground. I did find the main breaker for the whole AHU system and shut it off before opening the panel to expose the main block. The Honeywell directions were clear about that... The problem is what I found inside does not match anything else I've researched! Thanks
    – Hugh
    Oct 21, 2014 at 19:42

1 Answer 1


I've labeled your photo, which should help you understand what's going on.

enter image description here

What you describe as a "blue" wire at the thermostat, you describe as "gray" at the air handler. I can't be sure, since I'm not there, but I think they are one in the same.

If you want to install a fancy new WiFi thermostat, you're going to have to pull an additional wire from the air handler to the thermostat. Once you have that, you'll connect one end to the C terminal of the air handler, and the other end to the C terminal of the thermostat.

  • This answer, and this answer might also be helpful.
    – Tester101
    Oct 21, 2014 at 20:11
  • Thanks! I'll double check when I get home, I was pretty sure the wire at the Y terminal of the thermostat was blue as well, but the picture is kind of dark, maybe it is the gray wire from the Y terminal at the AHU. That would make more sense, not sure how it would be working if the C was connected to Y... So I just need to run a new 18 ga wire for the C terminal connection between AHU and Thermostat? Is it ok to double up the connection at the C terminal on the AHU? Thanks again for the help!
    – Hugh
    Oct 21, 2014 at 20:56
  • One more question, if I am going to through the trouble of running a new wire, would it be worth replacing the old wire with a 5 way wire bunch cable? It is a pretty straight shot down from the attic, I figure I could tie of one end of the new cable to the old one and pull it through fairly easily. May be easier than just trying to run a single strand to since I'll have somewhat of a pull string (the old 4 bunch cable). Thanks!
    – Hugh
    Oct 21, 2014 at 21:04
  • Yes, if it's not too hard pulling a new cable that's the way to go.
    – Tester101
    Oct 21, 2014 at 21:30
  • And connecting 2 wires to the C terminal at the AHU block isn't a problem?
    – Hugh
    Oct 22, 2014 at 1:23

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