My HVAC control board has two bundles of wires. One has 5 wires which runs to the thermostat but the 5th wire which I want to use as C is currently unused. The second set runs to the unit outside (condenser?).

My vague understanding is that I could just plug the unused blue wire into C beside the existing white wire and use it as C.

Here's some picture

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You're spot on

Hooking the existing blue wire up to the C terminal alongside the white wire that's already there will do what you want and get you that C wire for your Ecobee.

  • Thanks for the quick reply. Do you think you could explain a bit why doing this won't interfere with the existing condenser wiring? – Fletcher Moore Jul 26 '17 at 0:01
  • @FletcherMoore -- look at your Y terminal and how it's double tapped already – ThreePhaseEel Jul 26 '17 at 0:22

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