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I'm trying to wire a new wifi thermostat and only have two wires, R and W. The stack switch (transformer - pictured) only has two wire slots as well. What is the easiest way to add a common C wire? There are 4 wires currently running to the thermostat with 2 unused. Can I do this myself or need an electrician?

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    You'd need access to the transformers secondary terminals, which typically are not exposed in these types of systems. – Tester101 Oct 30 '16 at 20:22
  • The newest generation 3 of the "Nest Learning Thermostat" will likely work in this case. They have figured out how to do normal heat cool fan furnaces with no C wire. Chat with Nest first and ask if it will work if you're interested in that, they have good support. I'm guessing that the Nest board just uses so little current they can leak it back through the relay coil without energizing it, but I don't officially know if that's how it works. Nest Learning is full wifi and nice. I just installed a gen3 last week. – Tyson Oct 30 '16 at 21:09
  • Unfortunately I've already purchased a Honeywell WiFi Smart Thermostat. It says it must have a C wire to operate. – Martin W Oct 30 '16 at 22:26
  • Can the thermostat be powered from a second transformer? Some can some can't. If there is documentation for doing that, then you could use those extra wires for that. If not, then you may need to return that thermostat and get a model that will work without C wire – Tyson Oct 31 '16 at 0:29

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