I recently purchased a new property, and I noticed the thermostat's blue wire is in the Y terminal. Reading online, the blue wire is typically in C. Is my thermostat wired correctly?

For reference, my furnace's instruction manual is this one. However, I don't know how to read the thermostat wiring starting page 20 of the pdf. The thermostat shown below is the Emerson 1F86-0471 Blue 4" Thermostat.

(By the way, if it's not obvious based on the picture, the green wire is going into G, and there is a red wire going into RH)

enter image description here

  • Note that the colors are irrelevant and while there are some general conventions, this is by no means a standard of any kind. Installers are free to do whatever they please. What's important is what is that blue (i.e. Y) wire connected to at the other end.
    – jwh20
    Commented Mar 3, 2022 at 17:42

1 Answer 1


Does the heating and/or cooling system work correctly? Then it's wired right. "Correct" it at your peril. If it ain't broke, don't fix it.

Do people often do things that are non-standard since there are a bunch of conflicting "standards" in thermostat wiring? Yes, of course. Since I don't see an unused yellow wire in the wall, could be as simple as they needed the Y terminals connected, and the thermostat really doesn't care if the insulation color is blue.

Can we tell you anything beyond that without knowing how the other end of the wires are connected, and to what, exactly? No, of course not.

If you want to change it (the usual reason people ask this question, due to vast marketing money poured into convincing people they need a "smart" thermostat or the Jones's will have bragging rights, and we can't let that happen) you had better sort out "what's connected to what, & how" before you start fiddling with it.

  • 1
    Expanding on this answer - it looks like the builder used a 4 conductor cable which doesn't have a yellow. They also left the 'C' wire disconnected (presumably because there aren't enough conductors and the 'C' wire commonly provides power to the thermostat, which can also be self-powered by batteries). As long as the terminals match on both sides, it's true that it doesn't really matter what color the conductors are, except for the fact that it makes future troubleshooting easier if they are correct.
    – Chris O
    Commented Mar 3, 2022 at 17:14
  • 2
    To reiterate: now that you've got a "before" picture of the wiring at the thermostat, make sure you take a "before" picture of the wiring at the furnace. Then, and only then is it safe to start disconnecting wires.
    – FreeMan
    Commented Mar 3, 2022 at 17:41

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