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Old 12-06-2012, 10:22 PM   #1
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Question about adding "C" wire for new Nest (with pics)


Hello folks.

I'm in the process of replacing an aging battery-powered programmable t-stat with a new Nest unit. I have read some reviews stating that the best results are had by using a "C" wire to keep the unit with enough power.

Nest says you don't have to use one, since the unit scavenges power out of the system when it cycles. However, you can use a C wire to feed it constant power. Many have reported the rechargeable battery not keeping up, so I have decided to avoid this issue and use a C wire.

The wire bundle running from the unit to the t-stat has three extra wires (brown, orange and blue) that aren't being used. Notice I already labeled the blue one C, I'll get into why later.



I have opened the air handler (Carrier A/C, natural gas furnace) and found where the wiring connects. I found the same brown, orange and blue wires disconnected in there.



There is another wiring bundle that (I'm assuming) goes outside to the condenser. It has only the yellow and blue wires connected. The blue one connects to the common (Com) terminal.





I'm wondering if it is okay to connect the extra blue wire (from the bundle running to the t-stat) to the common (Com) terminal even though there is already a line connected there. I just want to make double sure I'm not going to mess anything up with the system or the new t-stat.

Thanks for any help!

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Old 12-06-2012, 11:32 PM   #2
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Question about adding "C" wire for new Nest (with pics)


Yellow and blue are going to condenser. You can connect the blue coming from the thermostat together with the another blue at the position C on the control board.

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Old 12-07-2012, 03:45 AM   #3
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Question about adding "C" wire for new Nest (with pics)


Yep, connect it to the common where the other blue wire is.
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Old 12-07-2012, 08:05 AM   #4
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Question about adding "C" wire for new Nest (with pics)


Post a follow up on how your Nest works for you, since we are getting regular questions about them now on this forum. As for yours, if it is a version 1, there has been some problems that people are having with the wifi side, but seems that Nest is rolling out updates as they can get them, when people update with issues.

If you have questions about the wifi side, continue to post into this thread, and screenshots if you have questions regarding your wifi router settings, along with Nest setup.
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Old 12-07-2012, 11:27 AM   #5
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Question about adding "C" wire for new Nest (with pics)


Cool, thanks for the quick responses.

I'm going to finish up the prep work on the wall today and hopefully get it installed over the weekend. I'll post pics of the process as I go. I'll also give some follow up reports on the functioning of the Nest (it's a 2nd gen).
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Old 12-09-2012, 08:48 AM   #6
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Question about adding "C" wire for new Nest (with pics)


Heres an update:

Got the C wire installed that runs to the t-stat


Got the wall prep done
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Old 12-09-2012, 08:54 AM   #7
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Question about adding "C" wire for new Nest (with pics)


I unpacked the Nest, which is an experience by unto itself. The thing is packed like a high end smartphone, which makes sense considering its ties with Apple.





It comes with its own hardware and multi-bit screwdriver.


They also give you a trim plate if you're not going to the trouble of repairing drywall.
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Old 12-09-2012, 09:02 AM   #8
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Question about adding "C" wire for new Nest (with pics)


Mounting the bracket was simple since it has its own level. The wire brackets are also incredibly simple. Just push down on the tab and feed the wire in. That's it.



It walks you through a short setup process and then you're finished.


Heres the old t-stat.
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Old 12-09-2012, 09:10 AM   #9
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Question about adding "C" wire for new Nest (with pics)


Overall first impressions:

I was surprised by how easy it was to install this thing. The reviews I read online were dead-on when they said that almost anyone can install a Nest. It looks very nice on the wall, much better than the outgoing plastic box. The tech that comes with it is very nice, especially the smartphone and tablet apps.

It cycled the heating and cooling just fine, so I can at least say that it does it's job as a regular thermostat. Of course the real test is how well it intelligently learns our patterns, and only time will tell that. I will stop back in periodically and give some updates.

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