New ecobee3 lite, have heat but no cool option

Question

I recently installed a new ecobee3 lite. When I completed the setup process, I found that there was no option to enable cooling. My original setup had both heat and cool options. I believe the option is missing because I connected the Y wire to C, which is what my schematic suggested. Right now, I have:

R wire -> ecobee Rc and Rh via jumper

W wire -> ecobee W terminal

G wire -> ecobee G terminal

Y wire -> ecobee C terminal

Is there any way I can get the option to enable cool back? There are 2 extra unused wires that run from the thermostat to the HVAC system in the below diagram. I was thinking about using one of them and connecting it to the BRN wire between the transformer and the left COM port, and wiring the Y wire to the ecobee Y port, would that work?

The photos are my old thermostat, new ecobee, and my HVAC system diagram.

The last picture is the wiring. The top left contains the connections to the thermostat, and the wires running downward on the left are the contractor seen in the system diagram connected to Y.

ecobee wiring

Answer 1

You skipped the compressor contactor

When you were reading your furnace's wiring diagram, you skipped over the symbol between the thermostat Y terminal and the furnace C connection that represents the coil on your air conditioner's compressor contactor. That coil is what is energized by the thermostat to turn the air conditioning on, so it's kind of important that it not be left out of the circuit, as you seem to have assumed so far.

Fortunately, you can fix this rather readily since you have a spare wire handy from the looks of things. Strip the end of the spare (blue) wire at the furnace end, and join it in with the junction of the fat brown and thin red wires (by the wiring diagram, that's your C wire connection); then you can wire yellow to Y and blue to C at the thermostat end. From there, finish wiring the thermostat as you had it before, button everything back up, and enjoy your new thermostat!