I am interested in replacing an older thermostat with a modern networked 'smart' thermostat \ humidity monitor. That being said, does the photographed thermostat have a C-wire?

The thermostat controls an air conditioner in a Florida storage facility. Fortunately, the AC handler is proximal to the thermostat controller

As I understand it, the common wire is often necessary to provide a return path, one of two wires necessary to power the electronics in a smart thermostat.

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UPDATE: upper thermostat panel removed. Blue wire not connected:

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If there is 24V AC between the RED and BLUE wire, is it reasonable to conclude that the BLUE wire is the 'C Wire' required for smart thermostats?

  • 1
    Can you post photos of the wiring at the air handler end? Jan 19 '19 at 3:35
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    What ThreePhase said, but also please take a new photo here or tell us what wires are present in the wire behind this plate (of this thermostat)
    – noybman
    Jan 19 '19 at 5:15
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    you can install a C wire by taking your red or white thermostat wires and hooking them up to a snake, then pulling the wires out at the furnace if your furnace has a transformer, Then hook up an extra 18 gauge wire to one of your 24volt pins on the transformer and pull the snake back through with the extra wire
    – user70085
    Jan 19 '19 at 7:20
  • @noybman Photo posted per request. Blue wire not connected
    – gatorback
    Jan 23 '19 at 4:19
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    Perfect. Your wiring supports it, you will want to use the blue, and looking at what you have there, all wires will plug and play with the new thermostat, but you do need to connect the blue on the other end. Just as @QuocVu shows below.
    – noybman
    Jan 23 '19 at 5:15

There is no such thing as C-wire per-se. What we call the C-wire is a wire that is connecting your thermostat to the C-pin of your AC. It provides the return path for the power from your thermostat.

From this picture, I don't see any free wire to use as C-wire. But once you remove it from the wall, you might be pleasantly surprised to see an unused wire from the black cable concealed behind your unit. Then connect that free wire to your AC C-pin and follow the wiring our your new smart thermostat.

Look at my furnace, you see that blue free wire. I connected it to the C-pin currently connected to the yellow wire (the leftmost of the 5 screws) and it becomes my C-Wire.

enter image description here

  • To clarify, the C-wire does not actually provide power TO the thermostat. It provides the return (neutral) path for the power already at your thermostat via the red wire. Also, the C wire in your picture is grey isn't it? The Yellow is on the "Y"
    – noybman
    Jan 19 '19 at 5:12
  • Thanks @noybman for the correction of the C-wire. I updated my answer. In my picture the leftmost wire is yellow, but it does appear as gray in the picture due to the low light. That wire is coming from my AC. The big wire cable that splits into green-blue-yello-red-wire is the one connected to the thermostat.
    – Quoc Vu
    Jan 19 '19 at 5:25
  • @QuocVu There are two sets of wires with yellow. Can you annotate in the photo which set is relevant to the discussion or label the set in the photo? Thank you
    – gatorback
    Jan 19 '19 at 12:57

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