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Trying to install a WiFi thermostat. I have the unused blue wire which I believe is the common wire. It is unused both and the furnace level as well as on my current thermostat. However there is already a white wire in the COM: connection at the furnace. So I have no open terminal.

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HVAC

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There's nothing wrong with adding another wire to the COM terminal at your furnace.
Just use the spare blue wire there along with the existing white wire from the other cable.

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    do leave a small note behind the thermostat so as not to confuse the next person that replaces the thermostat
    – dandavis
    Commented Jun 20, 2019 at 19:06

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