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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

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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.

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  • 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
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    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

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