adding a C wire


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Old 04-13-21, 10:05 PM
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adding a C wire

Greetings!

I see this is a common question, here, but haven't been able to find a definitive answer, so if there is one, please kindly point me to the thread.

Anyway, I recently bought a Honeywell WiFi thermostat and determined there is no C wire—only four run to the thermostat. I checked the furnace and did not see an obvious terminal where I could add a C wire (and run new 18/5 to the thermostat). Then, I noticed that Honeywell was kind enough to include a voucher for a free kit that can be used instead of a C wire. Perfect!

That arrived today and it requires a C wire terminal on the furnace's control board, so back to square one. For the better part of the evening, I've been trying to research if/how I can add a C wire to my set up (again, running new 18/5 to the thermostat—it's a short run and should be pretty easy). I read that you can attach it to the wire that goes to the AC compressor, but my wiring is a bit different as the city has some sort of governor/regulator to which this attaches.

Anyway, here are some pictures with what I think is going on:


bundle of wires connecting to 4 wire from thermostat and 2 wire from AC

furnace control board

thick blue wire from ?ground? to white wire going to the "governor"

furnace wiring diagram

As you can tell, I do not work much with HVAC, but am generally handy. That said, any guidance you can provide would be greatly appreciated!

Thanks!
 
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Old 04-14-21, 08:52 AM
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Assume "governor" is float overflow switch or similar safety.
So C would be the thick blue wire that attaches to the "governor" and ground. If you look at the top of the diagram it shows "chassis grnd. to condenser" with a B for blue wire.
 
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Old 04-14-21, 09:22 AM
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Thanks for the reply! The "governor" is actually a box that is also wired to our water heater that the city uses to regulate power during peak times—I assume—to prevent brownouts. I believe this allows the fan to run, but prevents the compressor from turning on at the city's whim.

Regardless, if I understand correctly, would I be able to run 18/5 to the thermostat and connect the new C wire to this thick blue wire you reference?

Currently, at my thermostat, the green wire is connected to G, white is W, blue is Y (which makes sense, since it is connected to the white wire going to the AC), and red is R. When I replace this with 18/5, would it be better to redo this a bit so that the yellow wire in the new 18/5 connects to the AC (instead of the current blue wire; and I'd put the yellow wire on the Y terminal on the thermostat), and then wire the new blue wire—which will be the C wire—from the 18/5 to the thick blue wire—the one in the picture that is connected to the white wire going to the "governor?"
 
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Old 04-14-21, 01:30 PM
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Hi, what is this Governor? What is the model of your old Stat ? is there 2 transformers in this system, that diagram doesn’t show how the control board interfaces with that furnace.
18/5 cable Red to R, White to W, Yellow to Y , Green to G, Blue to C and at the furnace it gets a bit confusing, Red to R, White to W , Green to G , Yellow to the Red conductor that goes to the AC , Blue to the White going to the AC.
Geo 🇺🇸
 
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Old 04-14-21, 02:36 PM
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@Geochurchi

As I noted in my reply to Astuff, as far as I know, it's something the city uses to regulate usage during peak hours. It's actually in a different part of the basement, by my water heater, and the white sheathed wire you see in the first picture of my original post—with the inner white wire connecting to the blue wire (Y) to the thermostat), and the inner red wire connecting to the red wire to the AC—runs to it. I've posted a picture of it below:



governor

My existing thermostat is a Honeywell RTH6350 and it is wired as I described in my reply. I've attached an image of how it's wired below as well.


Here's the rest of the wiring guide from the inside of the furnace—save for a bit cut off of the top. That's all that is posted inside.

thermostat wiring

top of wiring diagram

Thanks!
 
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Old 04-14-21, 06:07 PM
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I assume the governor is a simple relay that is to disable the a/c compressor only. For some reason they installed it on the C connection to the outside unit instead of the Y.

So the C (blue wire) from the thermostat should still go to the thick blue, not the white going to the a/c or you will have power issues when the governor kicks in.
 
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Old 04-14-21, 08:10 PM
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So the C (blue wire) from the thermostat should still go to the thick blue, not the white going to the a/c or you will have power issues when the governor kicks in.
I assume you are referring to when I run the new 18/5, correct? Currently, everything is working as expected. I've taken a new picture that better illustrates how everything is currently wired:


existing 18/4 wiring: bottom four are from the thermostat and the top, right two are from the AC (with blue and orange caps)

STAT green —> thick green wire
STAT white —> thick white wire
STAT red —> thick red wire
STAT blue —> AC white
AC red —> "governor" red
governor white —> thick blue wire

So, if I'm following what you said correctly, after I run new 18/5, I'd wire thusly:

STAT green —> thick green wire (per current wiring above)
STAT white —> thick white wire (per current wiring above)
STAT red —> thick red wire (per current wiring above)
STAT blue —> thick blue wire (as opposed to white from AC)

Where I'm a little lost is the following:

STAT yellow —> ??? to AC white ???
AC red —> ??? still to governor red ???
AC white —> ??? STAT yellow ???
governor white —> ??? where does this go ???
governor red —> ??? still to AC red ???

Thanks!


 
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Old 04-14-21, 09:23 PM
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STAT G green —> thick green wire (per current wiring above)
STAT W white —> thick white wire (per current wiring above)
STAT R red —> thick red wire (per current wiring above)
STAT C blue —> thick blue wire (as opposed to white from AC)
note: tape off blue at old stat so it doesn't accidentally short out

STAT Y yellow —> AC white

AC red —> still to governor red
governor white —> thick blue wire (along with Stat blue)
 
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Old 04-18-21, 01:28 PM
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That worked great, thanks!
 
 

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