C Wire for Dual Transformer System?


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Old 07-06-22, 02:17 AM
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C Wire for Dual Transformer System?

Hello, I'm trying to figure out if my system is compatible with a smart thermostat like Nest or Ecobee would love some help! A quick summary up front - I figured I had a dual transformer system since I have two bundles of wires separate, one for cooling and heating. The cooling bundle behind the thermostat has many unused wires, but I can't seem to figure out if any of those can be used as the C wire and where to connect them at the air handler if I can. It should also be noted that I have two thermostats on different floors but they are not zoned differently for cooling.

Now for the detail!
I have an old mercury round thermostat and these are how the wires are attached to that. There are Rh (red) and Rc (yellow) wires connected.

Here is what the wires look in the wall behind my thermostat. At first I was excited there are extra wires unused in the cooling bundle (seem to be all kinds of colors, blue, brown, orange, black, white). Can any of these be my C wire?


So I went to my air handler thinking I could find these unused wires hanging around, but I don't see those colors anywhere.
Overview of the wires in the air handler:
Don't see orange, blue, light brown, etc. that was unused in my thermostat wires.

Here's what the control board(?) looks like:

Closeup of the wires in the cage if helpful:


Can any of my thermostat wires be made a C wire?
Also as I noted in the beginning I have two thermostats, one of each floor. Will this affect how the C wire needs to be hooked up like more than one C wire is needed or something?

Sorry for all the questions, this ended up being more complicated than I imagined, any help is greatly appreciated! Let me know if more pictures are needed. I haven't opened the second thermostat up yet, wanted to see if I could figure it out with this one first to see if it's even worth the effort. Thank you in advance!
 
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Old 07-06-22, 09:40 AM
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Welcome to the forums.

Since you have separate heating and cooling you have two cables.
What is the heating ?

Four wire heating cable
Rh -------> yellow
W --------> green (blue?)
red and black are spares

Seven wire cooling cable
Rc -------> red ---------> Rc
Y --------> yellow ------> Y
G --------> green ------> G
C --------> blue --------> C
white
brown
orange

Please turn power off to the air handler and carefully pull the splices out of the junction box so we can see the wiring. Post another pic or two.
 

Last edited by PJmax; 07-07-22 at 09:18 AM.
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Old 07-06-22, 03:19 PM
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I have a gas boiler for heating.

Here are some more pictures of the wiring in the air handler.


as noted in the photo one of the brown wires just connects here for some reason instead of going through the hole in the ground:
here's a pic in case important


Hope this helps, thank you!
 
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Old 07-07-22, 09:21 AM
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I relabeled your diagram.
You had the a/c condenser and the float switch reversed.
The float switch is that device down by the floor.
It interrupts the Rc line to the thermostat is water is detected.

There is a spare blue wire in the stat cable.
Connect the stat end to C and the air handler line to C (white and brown splice).
 
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Old 07-07-22, 01:00 PM
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Thank you so much for your help! To confirm, I should connect the light blue wire shown at the Stat label to the blue nut labeled C?

However at the thermostat there is no light blue wire end. Which wire would I connect to my thermostat? The dark blue one? There were only these wires:
 
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Old 07-07-22, 03:19 PM
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It should be the same cable.
Unless you have another splice on top of all you have now.
 
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Old 07-07-22, 06:57 PM
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I think you're right... the wire must have been spliced somewhere but I'm not sure I can find it in the wall now. Tried pulling the thermostat wire further out but it didn't come out much more before I felt resistance and didn't want to keep tugging.. I guess this means I'm out of luck for connecting the C wire?
 
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Old 07-07-22, 07:02 PM
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Try it and see.
If not try another color like white.

If you had a meter you could check for C (24vAC from it to Rc).
 
 

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