Connecting Common Wire on Furnace


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Old 01-11-16, 09:33 PM
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Connecting Common Wire on Furnace

Hey folks. I'm installing a Nest thermostat. The old thermostat had the 5th Common (blue) wire, so I connected all five (Rh, C, Y, G, and W) wires just as they were.

However, I got an E74 Nest error, which indicates that the Rh wasn't getting power. Nest's troubleshooting eventually suggested disconnecting the C wire altogether.

That worked, and I am now trying to figure out if something is wrong with the Common wire. I'd like to see if I can connect it properly, so it charges the Nest.

I traced it back to the furnace (there is a junction along the way where the thermostat, boiler, and air handler lines connect) -- and the Common from the thermostat is connected at the air handler's R terminal -- along with Rh from thermostat and R from the boiler. C and Rh are at the same voltage, which I guess explains the E74 error.

Outline of the Thermostat Wiring

G (therm) connects to G (air handler)
Rh (therm) connects to R (air handler) and R (boiler)
C (therm) was connected to the R (air handler); I disconnected the side at the thermostat; other side is still at R on furnace
W (therm) <--> W (boiler)
Y (therm) <--> Condensation pump's Red terminal
Condensation pump's White goes offscreen (unsure where) and returns to the C terminal on the furnace. Perhaps the condensation pump and the outdoor unit in series? Here's a picture. Let me know if you can't zoom in to read the notes, which explain things pretty well.

Name:  furnace_notes.jpg
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The main question is how to determine whether I can disconnect the Blue 5th wire that runs to the thermostat from the R terminal, and move it to the C terminal. Then attach the other side to the C terminal on the Nest. I'm also trying to avoid a short/overload, which happened in a very similar situation in this thread.

Thanks so much for the help.
 
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Old 01-11-16, 10:33 PM
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Welcome to the forums.

You're saying furnace.... you don't have a furnace. You have a boiler and an air handler.... is that correct ?

It's hard to read your text and see the picture. This should be hosted on an outside host and linked here, in order to see wiring.... I need a BIG picture and the board doesn't allow it here.

You need to be careful because you are working with two different transformers.... one from the boiler and one from the A/C. They cannot meet.

http://www.doityourself.com/forum/el...-pictures.html
 
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Old 01-12-16, 05:13 AM
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Thanks for the welcome. Good to be here!

Yes, you're right. The system has a boiler and an air handler. Sorry.

I already have an Amazon Drive account, so let's try that instead of Photobucket. Here's a larger version of that image: https://www.amazon.com/clouddrive/sh...hare_link_copy

It seems like the systems do connect:

The R from the boiler connects to the Rh from thermostat and to the R terminal on the air handler.

The W from the boiler connects to the W on the thermostat.

Here are links to the 'junction' of the thermostat line, boiler line, and air handler line.

With notes: https://www.amazon.com/clouddrive/sh...hare_link_copy

Without notes:
https://www.amazon.com/clouddrive/sh...hare_link_copy
 
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Old 01-12-16, 12:13 PM
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Updated Photobucket images

Just following up with the Photobucket folder that has the photos, in case Amazon Drive didn't work well:
  • Air handler connections and notes
  • Junction (air handler, boiler, thermostat lines) with and without notes

jstricks's Library | Photobucket
 
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Old 01-12-16, 05:28 PM
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The pictures are excellent. I'm trying to figure out why there are so many splices. It makes things tough to keep straight.

Just some FYI points....

The cable at the air handler with the green wrapped wired is the outdoor A/C compressor.
The white goes to air handler Common and the red connects to the condensate pump white. The condensate red goes to the stat Yellow. This is done so that if condensate tank overfills..... 24vac is removed from the A/C compressor. Remember this !

The white cable from the boiler/thermostat splice......
Leave red on R. This is 24vac
Move blue to C. There will be two wires on C now. (blue and white) This is your common.
Yellow connects to condensor thru condensate switch.... ok.
Green on G is blower control and is ok.
White hanging out is ok as we don't have anything else for it to do.

From the boiler......
R to Rh on stat is ok
W to W on stat is ok.

To the stat..... obviously the two preceding wires.
The blue from the air handler that we just moved to C should connect to C on the nest.
The red wire from the air handler goes to Rc
The yellow goes to Y1.
The green goes to G.

Name:  Nest_backplate_with_wires.jpg
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The four pink circled wires are from the air handler.
The two purple circled wires are from the boiler.

All wiring should be done with both units powered down.
 
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Old 01-12-16, 06:21 PM
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Excellent news! I'm glad the solution is intuitive. Thanks so, so much for taking the time to help.

The junctions really do complicate things, but I'm glad I made the post because now it's well documented for future reference.

So when the condensate pump fills, the switch in the pump opens, meaning the series circuit isn't complete anymore, and thus current is cut off to the compressor? Is that right? Can you just explain a bit more why you make such a strong point to remember that?

I read somewhere that odd wiring can cause the boiler's gas valve to rattle/vibrate. Anything you see here that would make you suspicious? My valve does rattle internally, but obviously that's for a separate post to fully diagnose. Just wanted to mention it at this point since you've gotten such a close look at these wiring pictures.
 
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Old 01-12-16, 06:31 PM
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Sorry, just realized that you're suggesting changing a bit more than I expected.

Right now, boiler R and air handler R are connected to each other at the junction and to the thermostat line going to Rh.

There isn't a clean way to run the air handler's R to Rc at the thermostat because the line running to the thermostat has only 5 wires (B, Y, G, W and R). So boiler and air handler share the Rh.

Are you sure I can't keep both the air handler's R and the boiler's R connected to Rh at the Nest? The reason I think it's okay is this Nest support post: https://nest.com/support/article/How...ting-Rh-and-Rc
 
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Old 01-12-16, 06:40 PM
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No.... rewire like I illustrated. The boiler and the air handler do NOT and can NOT share any connections. Break the blue and red wire apart. It is very risky to connect the transformers of two different pieces of equipment together,

You need six wires to the nest. Two from the boiler and four from the air handler.
Is that the problem.... not enough wires ?
 
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Old 01-12-16, 07:05 PM
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Couple of problems. 1 - not enough wires running to the Nest. 2 - the junction is behind a finished ceiling, so I'd have to open it up. Maybe that's not the end of the world, I probably should have an access panel there anyway. But having to run a 6th wire is challenging.

If the Nest is capable of powering both heat and cool via a single Rh, how could both be powered that way without being connected together?
 
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Old 01-12-16, 07:12 PM
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Your boiler should supply enough current to run the nest without using a C wire and without combining the R's together. Right now..... combining the Rc and Rh is doing nothing for the system other than possibly hurting the transformers.
 
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Old 01-12-16, 07:25 PM
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Sorry, maybe we're not exactly on the same page. Currently, and in the junction pictures I uploaded, the boiler's R and the air handler's R are already connected to each other via a wire nut. A third wire in the wire nut runs to the Rh on the Nest via the thermostat line. Do you see that connection in the junction photo?

You're right, the Nest is functioning just fine. I just figured since I have a 5th wire at the thermostat, and that 5th Blue wire runs to the air handler where it could easily be connected to the C terminal, is there harm in moving the Blue from the R terminal to the C terminal. That way the Nest can charge its battery without using the Rh circuit.
 
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Old 01-13-16, 06:38 AM
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I do totally understand what you're saying though ... that the R's shouldn't be connected. Separating the Rh and Rc though, which requires running a 6th line to the Nest and opening up the drywall, might be more than I can take on at this point. But I say that only because the two have been connected for so long already, and so I have my guard down.

Another option is to forget the C altogether, open up the ceiling and use the Blue wire in the thermostat line for the Rc; and the Red wire in the thermostat line for the Rh. I'm not sure which path is best at this point.

But again I still wonder: If the Nest is capable of powering both heat and cool via a single Rh connection, how could both be powered that way without being connected together?

What a mess! Thanks so much for help though. I'm sorry to drag you into this, but really appreciate the help.
 
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Old 01-13-16, 08:46 AM
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the boiler's R and the air handler's R are already connected to each other via a wire nut.
I understand that 100% and it's wrong.

You're getting away with it because only one half of the transformer is connected. When you try to connect a C connection.... that ties the transformers completely together.

The reason they designed stats to have an Rc and a Rh terminal is to keep the two systems independent of each other.

That's it..... the way I illustrated connecting it is the proper way.
 
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Old 01-13-16, 09:21 AM
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Got it, okay. Just wanted to make sure.

I still don't understand how stats are capable of powering both heat and cool via a single Rh connection without the two systems being connected. But at this point, I may just need to run the 6th wire, and do it the proper way.
 
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Old 01-14-16, 07:35 PM
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Would anything change if the boiler and air handler shared a transformer? Or is it obvious we can rule that out?
 
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Old 01-14-16, 08:10 PM
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At this point.... you do need to leave the R's combined as you are correct that the stat needs to use them both to operate the boiler and the A/C.

You are lucky that you have no interaction between the transformers. It's called luck of the draw. If you do decide you'd want to connect a C wire..... then you'd need to increase the amount of wires to the stat.
 
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Old 01-14-16, 08:22 PM
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Amazing. Well, it shouldn't be too hard to run a new Rc line from the air handler to the thermostat.

I'll remove the existing line from the R terminal on the air handler (thus, separating the two systems). And then connect the CC and R(air handler)Rc (therm).

Thanks again for sticking around to help.
 
 

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