Looking to install an ecobee thermostat and decided to measure my common wire before unhooking anything. I tested the R, C wires and see they are only reading 14v, whereas R,Y, and R,W are reading 28v.

I traced the wires from the HVAC board and do see a blue and red wire coming from the C terminal. I then traced the red wire and it looks to be going to the condensing unit, the blue wire does lead towards the thermostat, however, each end is coiled around itself at the splice.

Would it be okay to splice the two blue wires together to make connectivity? Or, is there a specific reason that it would be configured this way.

enter image description here

enter image description here

enter image description here

  • The circuit more than likely reading the potential voltage across the circuit Not the actual transformer voltage
    – Jim
    Dec 25, 2023 at 6:08

2 Answers 2


Go for it

Any modern furnace will have enough capacity on the C wire to support a "smart" thermostat as well as powering the control board and the compressor contactor, so I would simply take the coiled-up blue wire ends at the splice, nut them together, install your new thermostat, and enjoy!

  • Thanks, will give it a shot!
    – Eric
    Dec 21, 2019 at 19:18
  • @Eric -- we thank people around here by upvoting and/or accepting their answers :) Dec 21, 2019 at 19:23
  • I just joined, my upvote isn’t reflected for public view :)
    – Eric
    Dec 21, 2019 at 19:43

There's a decent chance your C-wire isn't actually connected at the furnace: that "14VAC" reading could just be inductive pickup.

One way to test this is to bridge C-R with a resistor and measure the voltage drop using a multimeter (for a 1/4-watt resistor use 1kOhm or higher resistance; for 1/8-watt use 2.2kOhm or higher). If the voltage drop (measured voltage across the resistor's legs while it's completing the C-R circuit) is zero, then the circuit isn't capable of delivering current (likely because the voltage you see on it is due to inductive pickup and isn't actually connected at the furnace). If the voltage drop remains at ~14V, then something is up with your transformer.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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