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Has anyone ever seen the common wire connected to the red wire??

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  • Welcome to Home Improvement! A picture or more explanation might help.
    – IronEagle
    Apr 8 '20 at 15:13
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    This sounds like classic XY problem. Please let us know why this is important to you. Apr 8 '20 at 15:17
  • Can you post photos of the wiring at both the thermostat and furnace/air-handler ends of the equation? Apr 8 '20 at 23:44
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In the case of a switch leg on a multi wire branch circuit the white should be connected to the red and the white reidentified as a hot wire at both ends. Yes I have seen it and wired them this way in the past.

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    I think someone did a stealth edit to change this to being about thermostats. I also thought it was about mains wiring until rereading just now. Apr 8 '20 at 17:44
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This may be about terms and names.

The craze of smart thermostats like the Nest has made everybody interested in a heretofore-unused-by-thermostats wire called the "C" wire. And C is for common; in thermostat wiring it's analogous to mains Neutral or electronics GND. 98% of the time when someone says "common" they're after that.

However, old steam thermostats like yours also have a wire which is the power source, and is called the R wire. The thermostat connects R to call-for-heat (W), fan (G), call-for-AC (Y), etc. It is, well, a kind of common... though you would be unwise to call it that today, due to the confusion it would create with smart 'stat wiring.

And the conventional color for the R wire is, in fact, red.

However these are only conventions. Given that every thermostat has R, I would expect them to use Red for R. However I suppose if someone was eccentric they could use red for C. I wouldn't, though.

Your thermostat does not need and cannot use the C wire, so that's not an issue for you. Prepare for the day it is!

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