Installing an Ecobee3 - Wiring Question
I'm assuming the terminal on the end is C, though the label is blocked in the photo.



If it is marked C, then the installation is simple.


Connect the unused blue wire to the terminal marked C in the furnace/air handler.
Connect the other end of the unused blue wire to the C terminal on the thermostat.




If that is not a C terminal. Please post the make and model number of the furnace/air handler, and/or a schematic for the unit.