Wiring new Nest thermostat

nateonator

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!

Houston204

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.

nateonator

Thanks for the advice Houston. I will see about upgrading the wire.

gregzoll

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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rydan

If your new thermostat is using batteries.. you won't have to use the common wire ( typically blue )

gregzoll

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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