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Old 02-24-2013, 02:04 PM   #1
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Wiring new Nest thermostat


Hi Everyone,

I'm new to the forum and am looking for a little help. I bought a nest 2nd generation thermostat and will be installing it in a few days. I have a gas furnace with access and 4 wires coming into the old state. This is how it's connected: Red - rc, white - w, blue - y, green - g. No common. Nest says no common wire is needed, but I have read issues with the system stealing power causing problems with furnaces. Is this a valid concern? I am thinking I need the common wire to avoid any issues.

Now I also have 2 wires coming in to the stat area from a humidifier we don't use. Red and white.

Down at the furnace, I have the connections above plus c for common. There are 2 red wires connected there. I'm pretty sure they are for the humidifier and ac.

So my question is does it make sense to run a new 5 wire from the stat to the furnace or steal the humidifier red wire for the common after disconnecting it from the humidifier entirely? Running new wire is a clear shot and doesn't look too bad. Just concerned about messing up the connections at the furnace.

Thanks!

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Old 02-24-2013, 02:48 PM   #2
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Wiring new Nest thermostat


I'd pull new 8 conductor 18 gauge wire to the thermostat. This would allow for control of a humidifier or future upgrades.

Remove power to the furnace and write down the existing connections before removing any wires. I'd also snap a few pictures of the wiring.

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Old 02-24-2013, 03:19 PM   #3
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Wiring new Nest thermostat


Thanks for the advice Houston. I will see about upgrading the wire.
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Old 02-24-2013, 04:57 PM   #4
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Wiring new Nest thermostat


As long as you try to keep some symmetry to the colors (R-Red, G-Green, Y-Yellow, W-White, so on). It is when you use different colors for different terminals, the next person may end up confused if they change it out.
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Old 02-25-2013, 01:38 PM   #5
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Wiring new Nest thermostat


Quote:
Originally Posted by nateonator View Post
Hi Everyone,

I'm new to the forum and am looking for a little help. I bought a nest 2nd generation thermostat and will be installing it in a few days. I have a gas furnace with access and 4 wires coming into the old state. This is how it's connected: Red - rc, white - w, blue - y, green - g. No common. Nest says no common wire is needed, but I have read issues with the system stealing power causing problems with furnaces. Is this a valid concern? I am thinking I need the common wire to avoid any issues.

Now I also have 2 wires coming in to the stat area from a humidifier we don't use. Red and white.

Down at the furnace, I have the connections above plus c for common. There are 2 red wires connected there. I'm pretty sure they are for the humidifier and ac.

So my question is does it make sense to run a new 5 wire from the stat to the furnace or steal the humidifier red wire for the common after disconnecting it from the humidifier entirely? Running new wire is a clear shot and doesn't look too bad. Just concerned about messing up the connections at the furnace.

Thanks!
If your new thermostat is using batteries.. you won't have to use the common wire ( typically blue )
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Old 02-25-2013, 08:47 PM   #6
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Wiring new Nest thermostat


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Originally Posted by rydan View Post
If your new thermostat is using batteries.. you won't have to use the common wire ( typically blue )
You may want to read up on the Nest. It uses a built in Lithium battery, but has problems with charging on certain hvac systems, so you need the C-Common wire to charge the battery in the device.

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