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I absolutely need to use a common wire to install my Nest...which is the common?

I absolutely need to use a common wire to install my Nest...which is the common?

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  #1  
Old 12-07-16, 06:52 PM
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I absolutely need to use a common wire to install my Nest...which is the common?

There's a lone blue wire upstairs where the thermostat is that is unused; the other four wires are used. Downstairs, at the furnace, there are three blue wires. I've labeled 'em for you. I have no idea which blue wire is the wire that's going upstairs. I'm assuming it is wire 1, because the insulated bundle of wires that wire 1 is in are all going to terminals, so that makes me think those are all the wires going upstairs to the thermostat. It could be wire 2, but for some reason I feel like it's not because it appears there are three unused wires in that bundle. I dont think its wire 3 since that one goes into a wire nut, but I don't know. I just can't make heads or tails of this. Is there any possible way to figure out which wire is the one I need to connect to the C terminal? Here are a bunch of other assorted pics of the furnace in case it helps. THANK YOU SO MUCH!
 
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Old 12-07-16, 11:13 PM
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It is wire 1...... you're hired.

Here's how you can be sure. Your stat MUST have a Y, R, G, and W connection.
Since there is only one wire on G and one wire on W..... just follow them to the proper cable.
 
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Old 12-08-16, 07:15 AM
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Can i just hook that last wire onto the screw too or do I have to splice it? They're pretty thin wires. And thanks a ton! I do believe that's wire #1. Weird problem I had with my old thermostat that I thought was working fine last night: seems to be doing what the new thermostat was doing and turning on and off. It almost like the system doesn't wanna get past 68 degrees. Odd.
 
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Old 12-08-16, 09:00 AM
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The cable you have circled is the one going to the stat. Yes you can put it on the C terminal.
 

Last edited by skaggsje; 12-08-16 at 09:04 AM. Reason: added
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Old 12-08-16, 09:11 AM
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No problem with several wires under one screw terminal..... just work carefully.

Some of the hifi stats state that they are power robbing and don't need a C connection but I haven't found them to be reliable running without the common connection.
 
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