I'm installing a new thermostat (ecobee Smart Si) and it requires "C" wire. I checked before I bought, and saw an unused blue wire behind my current thermostat. However after connecting everything the unit didn't power on. I checked the system in the attic, and found that the blue wire isn't connected. Here is a picture of the system. I planned to connect the blue wire to the "Com 24V" but there's already a red wire there. Can I connect it anyway? The red wire that's there goes to the outside unit I think.

wires in hvac system


1 Answer 1


Yes, tapping along with the red wire under the (C)OM is where it goes. If you'll notice, your (Y) is already double taped.

  • OK, I'll give this a shot. Thank you very much!
    – Joel
    Commented Nov 1, 2014 at 2:53
  • No luck. I connected the blue wire to Com 24V along the red wire, but the new thermostat still didn't power on. I also tried removing the red, but still nothing. So I grabbed my multimeter, set it to ACV, and tested between R and Com24V, but it didn't register anything. Shouldn't it register as 24volts? What could cause it to not register?
    – Joel
    Commented Nov 2, 2014 at 23:45
  • @Joel Did you put the cover back on the furnace when you tested the new Tstat?
    – Mazura
    Commented Nov 3, 2014 at 0:23
  • No. I'll try that. Just so I know, what does that do?
    – Joel
    Commented Nov 3, 2014 at 0:32
  • 2
    It works! I started from scratch, disconnecting and reconnecting every wire, and poof!, it works. I believe that I didn't have a good seat of the blue wire when I initially connected it to the Com24V. Either way, everything works now. Thank you very much for your time! (ps, I figured out what putting the cover back on does, where it pushes in a little tab to power on the unit. Thanks!)
    – Joel
    Commented Nov 3, 2014 at 2:12

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.