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I'd like to install an ecobee3 thermostat, but I'm not entirely sure about my wiring. The current thermostat seems to only have a W and RC wire coming into it.

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There is also what appears to be a blue wire buried in the wall, but I haven't tested to see if it's a live C wire. I've had an HVAC person out here before, and he told me we didn't have a C wire, so presumably this is something else.

On my furnace, I have two Honeywell R845A boxes, and also an RA832A box. Not sure if this is standard or not, or what exactly it means. I'm hoping it means running a C wire from the furnace, if I need to run it, will be easy.

I also have a weird bundle of wires above one of the two R845As, as follows:

enter image description here

There seems to also be a green wire merged here, about which I'm not sure at all what it does since it doesn't seem to come out of wall upstairs.

My question is: can anyone estimate how difficult/expensive it would be to convert this to a unit that could support a thermostat like the ecobee3 or Nest that need a C wire? (I know some of these power steal and don't need a C wire per se, but I'd rather go the most reliable route here.)

Many thanks in advance for any insights.

EDIT: Opened up the transformer box, and the blue wire is indeed there. But it is snipped off/not connected. Is it just a matter of stripping the wire, connecting an extension, and hooking it up? What do I connect it to? And what sort of wire should I use for the connection? (Picture below.)

enter image description here

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    Is there a blue wire coming out of either cable at the furnace end? You may have to look closely at the end of the cable jacket to find it... – ThreePhaseEel Nov 26 '16 at 1:57
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    The old wire that goes into the Honeywell box below the splice in the second picture looks to be your thermostat wire. Take the tape off of it, and or open the Honeywell box, to see if the blue wire is there. If so, you may be able to use it for your C (common) connection. – Connor Bredin Nov 26 '16 at 2:20
  • Awesome, thanks. Opened up the transformer box, and the blue wire is indeed there. But it is snipped off/not connected. Is it just a matter of stripping the wire, connecting an extension, and hooking it up? What do I connect it to? And what sort of wire should I use for the connection? Many thanks. – Skeptic Nov 26 '16 at 16:18
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    Yes it's a matter of hooking the blue up, as to where to hook it if you can take a picture it might help. If you see wiring diagram on the inside of the cover you opened a picture of that close up as well. What "C" is is Common and it boils down to the other side of the transformer. Be sure to edit it into the question (instead of just commenting) as the edit will get your question renewed attention that a comment won't get. – Tyson Nov 27 '16 at 1:33
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    Can you get the board out of the R845A and post a photo of the back? There's not a C terminal on the thing by default, but you should be able to hack one in with a bit of soldering... – ThreePhaseEel Dec 17 '16 at 13:57
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I don't know if you ever got this worked out or not, but I'm in the same boat. Bought an Ecobee3 and I have the RA832A relay. It doesn't appear as though it has any sort of C wire capability.

Awhile back before I bought the unit or knew what my boiler was using in any sort of detail I reached out to Ecobee and this is what they said (I was planning to merge the boiler and AC into one thermostat as they are two separate now).

They said:

The boiler would use Rh and W. The AC would use Rc C G and Y

On their support site they have an article talking about what you can do if you don't have a C wire and one of the options was use a transformer and isolation relay.

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