"It can't be that hard", I thought when I finally bought one. I mean, it's just wires to a furnace. They're low voltage.

I was wrong. Well, they are low voltage, but I can't really figure out what's going on.

On my old thermostat there's the following wires:
W = White
G = Green
R = Red

Simple, right? There's also two other wires in the bundle, yellow and blue which aren't used. I know I will end up needing a C wire, so I figured I'd just use the yellow or blue and connect those on the furnace end.

This is where it gets complicated. On the furnace side, it's also like this:
R = Red
G = Green

But, here's where it goes strange. There's another set of furnace wires that tie into those wires.
White from thermostat ties into Green in the new wire
Red from the new wire goes into W on the furnace
White from the new wire goes into C on the furnace

Here's what I mean:


So I follow this new wire out of the furnace to another box. I have no clue what this thing does.


Anyone have any ideas how to get the C wire back to the thermostat? I'm tempted to use the unused blue wire.

Edit: I made the images a little smaller, but you can open them in a new tab if you want to see a larger size.

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Edited 2 time(s). Last edit at 01/16/2022 09:51PM by M A V I C.

you don't need a furnace, your wife's hot!

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There's another set of furnace wires that tie into those wires.

That is probably for the AC compressor outside.

I know it doesn't help you, but that's one of the reasons we went with an ecobee instead of the Nest. It seemed to be an easier hookup, at least for our furnace. I'm guessing Nest has phone support too, but I will say from experience that ecobee support is great.

You should be able to find docs on your old thermostat, if its not labeled directly on the device. Perhaps you can ignore the colors of the wires and simply follow the markings on the controller and thermostat.

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I'm not sure its the nest thats complicated, but your own questionable hvac wiring

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Edited 1 time(s). Last edit at 01/16/2022 11:01PM by mattkime.

Quote
wurm
I know it doesn't help you, but that's one of the reasons we went with an ecobee instead of the Nest. It seemed to be an easier hookup, at least for our furnace. I'm guessing Nest has phone support too, but I will say from experience that ecobee support is great.

That was my experience too.

Quote
gadje
There's another set of furnace wires that tie into those wires.

That is probably for the AC compressor outside.

No AC. I'm not sure what that box does. Seems to be connected to the furnace exhaust.

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Quote
Article Accelerator

Quote
wurm
I know it doesn't help you, but that's one of the reasons we went with an ecobee instead of the Nest. It seemed to be an easier hookup, at least for our furnace. I'm guessing Nest has phone support too, but I will say from experience that ecobee support is great.

That was my experience too.

Same here. Had a Nest but didn’t have a ‘c’ wire. Had to set the nest to run the fan every hour to charge the battery.
Switched to Ecobee and the support was great, Emailed them photos of the furnace control board and the old thermostat wiring and they walked me through installing a c wire and then the ecobee.
Can’t say enough good things about their support. Perhaps Nest support can do the same for you.

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richorlin

Quote
mattkime
You should be able to find docs on your old thermostat, if its not labeled directly on the device. Perhaps you can ignore the colors of the wires and simply follow the markings on the controller and thermostat.

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I'm not sure its the nest thats complicated, but your own questionable hvac wiring

It's not really related to the old thermostat. It didn't use the c wire, at least not directly. You're right it's not Nest's fault.

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Quote
richorlin

Quote
Article Accelerator

Quote
wurm
I know it doesn't help you, but that's one of the reasons we went with an ecobee instead of the Nest. It seemed to be an easier hookup, at least for our furnace. I'm guessing Nest has phone support too, but I will say from experience that ecobee support is great.

That was my experience too.

Same here. Had a Nest but didn’t have a ‘c’ wire. Had to set the nest to run the fan every hour to charge the battery.
Switched to Ecobee and the support was great, Emailed them photos of the furnace control board and the old thermostat wiring and they walked me through installing a c wire and then the ecobee.
Can’t say enough good things about their support. Perhaps Nest support can do the same for you.

My experience with Nest. Downstairs thermostat? Pretty much plug and play. So I did the upstairs one too. How hard could it be? Well hard enough when wires and diagrams don't match. Emailed Nest. They asked for pics of my heat pump model and my thermostat. Within an hour or so, I had a full color custom diagram emailed to me that said exactly which wire was what and where it went. Took about 5 minutes after that. Easy peasy.

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Use the markings on the furnace circuit board to denote what the wires do. Not sure about the bottom connection because the capacitors are in the way. Looks like the furnace board is labeled:
R
W
Y
G
O (C?)

My guess, if:

R,G,W, (Y), (B) goes to thermostat...

R,G,W cable is power/signal from a flue pressure relay.

Assuming the shiny pipe connects to something on either the inlet or outlet of the burner...

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In tha 360. MRF User Map

Quote
Filliam H. Muffman
Use the markings on the furnace circuit board to denote what the wires do. Not sure about the bottom connection because the capacitors are in the way. Looks like the furnace board is labeled:
R
W
Y
G
O (C?)

The last one is a C.

My guess, if:

R,G,W, (Y), (B) goes to thermostat...

R,G,W cable is power/signal from a flue pressure relay.

Assuming the shiny pipe connects to something on either the inlet or outlet of the burner...
[/quote]

Yeah, that shiny pipe you have circled comes back into the picture on the top left. Flue pressure relay sounds about right.

The question is, can I use the unused blue wire to connect to C and then that to the thermostat, while it's still connected to whatever the relay is using C for. Since C is supposedly just for power, it seems like that should work.

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Edited 1 time(s). Last edit at 01/17/2022 10:01AM by M A V I C.

Quote
bazookaman
Emailed Nest. They asked for pics of my heat pump model and my thermostat. Within an hour or so, I had a full color custom diagram emailed to me that said exactly which wire was what and where it went. Took about 5 minutes after that. Easy peasy.

I didn't realize that was even an option. Thanks for pointing that out.

Edit: they're not giving me an option for email, but do list Twitter, so I'm trying that.

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Edited 1 time(s). Last edit at 01/17/2022 10:09AM by M A V I C.

Quote
M A V I C

Quote
bazookaman
Emailed Nest. They asked for pics of my heat pump model and my thermostat. Within an hour or so, I had a full color custom diagram emailed to me that said exactly which wire was what and where it went. Took about 5 minutes after that. Easy peasy.

I didn't realize that was even an option. Thanks for pointing that out.

Edit: they're not giving me an option for email, but do list Twitter, so I'm trying that.

It's been awhile since I did mine. They certainly could have changed the policy. I may have even called them. Not sure.

Just remembered. This was even before they got bought out. So it's entirely possible that their level of CS dropped off a cliff.

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Edited 1 time(s). Last edit at 01/17/2022 10:18AM by bazookaman.

If all else fails just get an HVAC person/company that knows what they are doing. After looking at my heat pump thermostat and its wires I had a heat/air company that installed my heat pump and sells Nests and/or installation if you provide you own do it. Guy came and went over the install with a fine tooth comb. Climbed up in the attic to verify unit was doing what it is suppose to do after setup. He did not leave until we were sure it working correctly.