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I got a new ecobee thermostat that requires a c wire. my current thermostat has 4 wires plugged in, and there are 2 extra wires not being used coming out of the wall one blue labled y2, and a brown one lableled w2 aux. they both have current flowing. Can I use either of these and connect to the c terminal?

[this is the view on the furnace

this is the thermostat

  • i went to the furnace and checked, the C terminal is being used by a yellow wire going to the outside unit.. can i unplug the blue y2 wire and plug it in to the c terminal on the furnace along with the outdoor unit? – Chiefster Dec 28 '17 at 20:28
  • Where are your wires coming from? A y2 is for a second stage of cooling and the w2 aux indicates you have a heat pump and the w2 aux is for the auxiliary heat in the FCU. Regardless they should not be carrying any current unless they are connected to the T'stat. Then they would be carrying control current to the second stage cooling and the aux heat only when the T'stat calls for it. – Retired Master Electrician Dec 28 '17 at 20:28
  • the wires coming out of the wall are all connected to the control board on the furnace. my existing tstat only is using 4 of those wires. – Chiefster Dec 28 '17 at 20:34
  • I do have one more question though, since prior to the new tstat I only had the 4 wires connected to tstat. But on the furnace the y2 and w2 aux we're connected, is it possible I have those additional "features" and are now not operational?? – Chiefster Jan 1 '18 at 7:35
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It looks to me that you can connect either wire to the same terminal as the yellow wire. I would use the brown simply because it is as close to black than any other color. To be honest T'stat control wiring really doesn't have a color code but everyone sticks to the color that the letter indicates for clarity's sake. So disconnect the brown from your control panel and connect it to the C terminal and then connect it to the C terminal in the T'stat.

Good luck

  • Blue is the semi-conventional choice for a C wire. You know C isn't akin to hot, it's neutral. R is hot. – Harper Dec 28 '17 at 21:14
  • Thanks for your help, I ended up using the blue wire since it was easier/closer to move – Chiefster Jan 1 '18 at 7:33
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Yes you can use either the blue or the brown. Just make sure that it is connected to the 'C' terminal on control board in furnace.

  • Thank you, sorry for late reply but I took your advice and I got it working! Plugged in the blue wire to the c termin – Chiefster Jan 1 '18 at 7:32

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