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

  • Welcome to Home Improvement! A picture or more explanation might help.
    – IronEagle
    Apr 8, 2020 at 15:13
  • 1
    This sounds like classic XY problem. Please let us know why this is important to you. Apr 8, 2020 at 15:17
  • Can you post photos of the wiring at both the thermostat and furnace/air-handler ends of the equation? Apr 8, 2020 at 23:44

2 Answers 2


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.

  • 1
    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, 2020 at 17:44

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!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.