Adding C wire

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Old 07-15-19, 12:36 PM
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Adding C wire

I got a Nest thermostat and had it installed for about 2 weeks before it stopped holding a charge. I put the old regular thermostat back and its working fine with the old thermostat.
I know that I need to install a C wire. My thermostat has 3 wires for the AC. It does not have a C wire. So I went and purchased 18/2 thermostat wire and ran it from the thermostat to the AC handler. The problem is that I don't know what to do next. I am attaching pictures here. I don't believe I have a circuit board that I can just attach the end of the thermostat to in the C terminal. Instead, it looks like the wiring is "pig tailed" between the thermostat and AC unit. Can anyone provide any insight?
 
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Last edited by PJmax; 07-15-19 at 05:26 PM. Reason: resized/labeled picture
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Old 07-15-19, 05:27 PM
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Welcome to the forums.

In the last picture I labeled the C wire. It's brown. Follow it for a place to connect to it.
You can connect to that brown wire at any point.
 
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Old 07-15-19, 06:44 PM
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Great. I dont see an actual circuit board that has the C on it so does that mean I need to pigtail it?

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Last edited by PJmax; 07-15-19 at 09:34 PM. Reason: added labeled diagram
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Old 07-15-19, 09:33 PM
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They show four connection points inside the wiring junction area...... white, red, gray and brown.
So they brown should be with the other wires. You may find it already connected with a wire.
That's fine.
 
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Old 08-13-19, 06:57 PM
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Thanks for the drawings. It helped, but I must have done something wrong. I found the brown wire coming from the PCB. It was pigtailed with a black wire and a red wire in it. I add the new thermostat wire into that connection and put the cap back on. When I went to turn my AC on, it wouldn't come on. The thermostat kept blinking cool. I then went back in and removed the thermostat wire I just installed and went and turned the AC back on and the AC came on. Not sure if I didn't pigtail it correctly. Is it odd that there is another 2 wires which look like thermostat wires in the nut with that brown wire?
I'm wondering if there is a way I can somehow connect the new thermostat wire to somewhere where the existing thermostat wiring connects to? That is, the three thermostat wires go from my thermostat and terminate with three other wires that are nutted which those wires then go into the AC unit behind the panel.
 

Last edited by obeythelaw; 08-13-19 at 07:18 PM.
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Old 08-14-19, 12:47 PM
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Is it odd that there is another 2 wires which look like thermostat wires in the nut with that brown wire?
Not at all. That two wire cable is the wiring to the outdoor condenser. It gets connected to common and the Y connection.

There should be four wires for your thermostat to connect to. Very possible that someone pulled the brown wire out of the wiring area because it is shown as being there.
 
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Old 08-14-19, 07:06 PM
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Would you have an idea what those numbers "4,3,2,6,1" mean in the diagram? Also, upon closer inspection, the three current thermostat wires which are green, red and white are each individually pigtailed to a gray wire that has 5 thermostat wires of its own of which the green and orange in that 5 wire bundle is pigtailed to the brown wire that has 3 thermostat wires coming out of it and to which the orange wire and green wire in that 5 wire bundle are pigtailed to a red wire coming from the 3 wire bundle and the green wire in the 5 wire bundle to a black wire (which I know is connected to the emergency float switch). I just don't get how messed up the wiring can be. And then the 3 thermostat wires from my actual thermostat are each connected to the green/green, white/white and red/gray of the brown wire bundle coming from the AC.
 
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Old 08-14-19, 07:59 PM
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In an effort to try and provide more info, I am linking to some pictures I took that seem to show the blue, white, violet and brown wires going into the "box" of wires inside the AC handler. The brown wire, which I am guessing is the common wire has a red type and black type thermostat wire connected with a wire nut. This is the one that I said I attached my new thermostat wire to yesterday but after I did that, my AC didn't turn on. Perhaps it was because I didn't use a large enough wire nut to get all of those wires inside the nut? Am I guessing wrong that the light brown wire is the common wire?
https://www.dslreports.com/speak/sli...y10by1jb21tb24
 
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Old 08-14-19, 08:17 PM
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I can't see the splices inside that box.
Try to pull the wires and splices out of the box and post a pic.
 
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Old 08-14-19, 08:44 PM
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"Would you have an idea what those numbers "4,3,2,6,1" mean in the diagram?"

Connection plug for electric heating elements. I assume this a cooling only application. You lost me at the gray wire, sounds like a rats nest of wire junctions. Very basic diagram below, wire colors are not as important as where they go to. Follow your wires and compare with the diagram. Your pump safety switch may not break the Y (cool) as shown but may break the R (going to stat) or the C (common) to the outdoor unit or transformer. If your unit does have heat, connect the W stat to W air handler. If this was my unit, I'd pull all the wires apart, and route the thermostat, condensing unit and pump safety switch into your junction box with no external wire junctions if possible.

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Old 08-15-19, 07:09 AM
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Thanks. When I get home later I'll try pulling some of those wires out of the little box. There are so many wires/splices in there. Yes, I only have AC cooling. No heat (my heat is a separate furnace/baseboard hot water/gas heat).
 
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Old 11-08-19, 05:47 AM
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Ok, so I'm circling back here for some more input. I think I am almost there in figuring this connection out. As my previous post noted, I had connected my new thermostat wire I ran to the brown common wire. I had then connected that new wire into the "C" terminal in the Nest but the nest didnt' turn on. Do I have to take all of the thermostat wires and plug them into the Nest for the "C" to work? I think this may be the reason why my Nest didn't turn on because I didn't go and take the other thermostat wires from my old thermostat and plug them into the Nest because I assumed that the "C" wire I just hooked up in the air handler had current running through it that it would power it up all by itself. However, I've been reading more that the Common wire is basically just a wire that creates the connection between it and the R wire (in my case the Rc) wire. So I'm guessing since I didn't plug the Rc wire (or the Y,G,W, Rh) into the Nest, the common wire wouldn't work. Am I onto something?
 
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Old 11-08-19, 11:17 PM
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Yes.... you are on to something. This system is AC. No polarity but if it were a DC system..... the R wire would be hot or plus and the C wire would be ground or minus. So both wires are needed to power the nest.
 
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Old 11-10-19, 08:36 AM
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I got it working! Thanks to everyone her for their input. I wouldn't have been able to do it without the assistance.
 
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Old 11-10-19, 12:17 PM
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Good job. Name:  thumb.jpg
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Size:  1.6 KB Thanks for letting us know how you made out.
 
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