I'm installing a new thermostat (ecobee Smart Si) and it requires "C" wire. I checked before I bought, and saw an unused blue wire behind my current thermostat. However after connecting everything the unit didn't power on. I checked the system in the attic, and found that the blue wire isn't connected. Here is a picture of the system. I planned to connect the blue wire to the "Com 24V" but there's already a red wire there. Can I connect it anyway? The red wire that's there goes to the outside unit I think.