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I recently received a wifi enabled thermostat which I thought I could install easily since I saw an unused wire (Blue) which I assumed would be the C wire behind the current thermostat.

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When I went to install it, I found out the blue wire isn't actually connected to anything but there is a red wire connected the C terminal which I believe runs out to the ac unit.

furnace wiring

Can I just connect the blue wire to the C terminal in addition to leaving the red wire or will this short something? Or does this just need to be completely rewired?

Thanks!

marked as duplicate by gregmac, Community Jul 21 '16 at 18:34

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You should be fine connecting the unused blue wire to the C terminal in the furnace/air handler.

You'll notice that on the Y terminal that there's a yellow wire from the thermostat, and a white wire that presumably goes to the outdoor unit. The red wire connected to the C terminal, simply completes the circuit through the outdoor units contactor coil.

The full circuit path for cooling would be as follows:

L1 of transformer secondary -> R terminal in furnace -> red wire -> R terminal on thermostat -> Y terminal on thermostat -> yellow wire -> Y terminal in furnace -> white wire -> contactor coil -> red wire -> C terminal in furnace -> L2 of transformer secondary.

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