I'm installing a wifi-capable thermostat in my house which for obvious reasons requires a C wire. The old school mercury thermostat I'm replacing didnt have a C wire connected but I've checked the furnace schematic and connections and the C is there.

Can someone please take a look at the attachments and verify that the C wire exists / that I'm not losing my mind?

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Here are some additional pics: http://m.imgur.com/gallery/u1QJQ

It looks like I should be able to add the unused blue wire to the C terminal on the furnace. Thoughts?

I've been referencing this post: How can I add a "C" wire to my thermostat?


Yes. Connect the unused blue wire to the C terminal in the furnace, then connect the other end to the C terminal on the thermostat. As long as there's no funny business between the furnace and the thermostat, it should work just fine.

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It's not you. This is a mess. They've reused symbols in a way I don't like.

Here's NOT your C-wire: the five-wire harness and white connector labeled 4-3-2-1-C. (in your first picture) That appears to be a motor drive and is probably 120VAC. That would make a big bang.

I think this is your C wire: The schematic (second picture) shows a group of terminals labeled R-Gh-W-Gc-Y-C. (bottom area, left edge). (They are fed off a transformer just as you'd expect R and C to be. C is connected internally as you'd expect.) Look for a cluster of six terminals with exactly those labels. It may be those terminals on the top edge of your top photo.

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  • Using the blue wire worked perfectly. Thanks! – J. Smith Aug 2 '16 at 15:45

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