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A friend of mine wants to upgrade his thermostats to WiFi thermostats but the thermostat wires ran to the thermostats are missing the C wire. There are 2 thermostat cables running from 2 units to 2 different thermostats. We can run a new 18-5 cable from one of the furnaces to one of the thermostats. But we can't get the other unit open because of the way it was installed. Is it possible to split the common wire down at the thermostats and use it for both new thermostats?

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  • It would help if we knew what these units were: furnaces, air handlers, or some other sort of widget you'd hook a t-stat up to Mar 13, 2020 at 4:54
  • Both are connected to furnaces Mar 13, 2020 at 5:05
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    Did you ever get this resolved? If so, please give a check-mark to the answer or write up your own answer explaining what you did to get it fixed and give yourself a check mark. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer.
    – FreeMan
    Aug 10, 2020 at 18:03

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I think NO. The C-wire connects to the transformers on the individual units. It's not like a common neutral or ground wire. You'll need to get a C-wire to both furnaces as there is no real electrical connection between the two on the low voltage side. There might be other solutions that the "big three" might be able to come up with, but I think your stuck with getting a C wire to each furnace independently.

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  • Actually, often it is a ground wire.
    – Jasen
    Mar 13, 2020 at 8:28
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    Although I would agree without more information , if the secondary’s of both transformers are grounded it may work but would not be proper.
    – Ed Beal
    Mar 13, 2020 at 17:27

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