Got a free smart tstat from the electric company. Took off old tstat took picture, went to basement to check colors on the unit and r, w, and g matches up but the blue wire is connected to c on hvac board and y on tstat. Y is unused on hvac board.

Installed a c wire kit that came with tstat, which connected y and g at tstat to a diode that returned to hvac on blue to connect to a box with other wires. Hooked everything up per instructions got beat working but for ac, the compressor doesn't come on, the fan does.

Could I remove the c wire kit revert all wires back to normal and jump y to c on tstat since y on tstat is going to common on hvac and all works that way.? enter image description here

enter image description here

Could I just g for common instead? I work with industrial hvac units but this small thing is baffling the hell out of me.

*** EDIT *** @ThreePhaseEel @jwh20 @Harper - Reinstate Monica

enter image description here 2 wires - red and white exiting to the right on lower wire are going to the condenser outside. 4 wires - red, white, green, blue exiting to the right on top wire are going to tstat. 4 wires - red, white, green, blue exiting bottom conduit go to air handler board

So, blue wire from c terminal on handler goes to white wire on condenser, blue wire from y terminal on tstat goes to red wire going to condenser.

red, green, white from tstat go to red green white on handler.

Only 4 wires to and from tstat 2 wires to condenser 5 wires going to air handler (Hey, theres that yellow wire)

How can I hook up my emerson sensi smart tstat?

Right now I am using green as common to get power to tstat, heat and fan work, condenser works but no fan.

How to I wire tstat as is or use the included c wire kit?

  • Can you post a wiring diagram for your furnace please, as well as photos of where your outdoor unit wiring is joined in with the wires to the thermostat? Commented Nov 4, 2020 at 3:31
  • Connecting the furnace "C" wire to the thermostat's "Y" terminal makes to sense to me. Are you sure there isn't another unit in play here? I see that your furnace has a "Y" terminal, that's for cooling and it's not connected to anything. How is your home cooled?
    – jwh20
    Commented Nov 4, 2020 at 10:52
  • @ThreePhaseEel I'll get those pics after work.
    – Gunner
    Commented Nov 4, 2020 at 11:24
  • @jwh20 the house is cooled just fine with the condenser outside. No y wire on the board and cooling still worked with old tstat is what is bugging me out. Tstat wire goes through 2 junction boxes, I'll have to open em and trace the wires... It's my inlaws house and he "knows how to do everything" and his brother is a supposed "electrician" from back on the day so I wouldn't be surprised to find some kind of Rube Goldberg type of crap going on somewhere.
    – Gunner
    Commented Nov 4, 2020 at 11:30
  • Where's the yellow wire? Usually if there's a blue, there's a yellow. Commented Nov 5, 2020 at 0:02

2 Answers 2


Running the new thermostat wire was a good call...

Whoever Rube Goldberged your installation had used some 4-wire cable they had laying around for the run from the thermostat to the junction point, leaving you short a wire, so replacing the thermostat cable was a good call, although I would have used a fatter cable personally so that the next upgrade doesn't require another cable changeout.

...but there's another problem here

The other issue with your existing setup that needs cleaning up is the 5-wire cable to the furnace being run in the same conduit as the furnace power circuit, as that violates NEC 725.136(A) due to the thermostat wiring being a Class 2 control circuit. The easiest fix, and one I suspect you've already done with your new run down to the air handler, is simply to run that run strapped to the outside of said conduit to the furnace, as permitted by NEC 300.11(C) point 2, and move the thermostat-wiring splices outside the box.

  • We were out of tstat wire in the shop so I went to a big box store. I was going to get 18/7, but they only had 100' and bigger spools.... Way too much. Of course it's all ran properly. I was actually feeling lazy and was going to drop cat6 and use a twisted pair for the common but I like to do things right.
    – Gunner
    Commented Nov 6, 2020 at 1:16

Ran new 5 wire from tstat to junction box, replaced the spice going to condensor with with yellow then new 5 wire down to air handler. If course using blue for common.

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