Secondly, not only did I not have a wire labeled “C”, but I also did not have an associated furnace control board with any labeled wires. Yikes!
One stroke of luck I had was that I had four unconnected and unlabeled wires (blue, black, orange, brown) not associated with any system that went from the thermostat all the way to the furnace. Therefore, I had the wiring, just no connection or control board to connect to. But fear not; here are some steps we took to get things working.
WORKING WITH HVAC WIRING IS HAZARDOUS! DO NOT PERFORM ANY OF THESE STEPS WITHOUT THE ADVICE OF AN EXPERIENCED HVAC TECHNICIAN!
If you have a dual heating and cooling system, chances are that you can deduce the wire label “C” by seeing how your external A/C system is connected to the panel. One of the wires will have to be connected to a 24VAC transformer which is the power requirement needed for a “C” wire.
- Turn the furnace breakers off.
- Trace the wiring from the thermostat to the furnace. Make sure that any unused wires (in my case they were Blue/Black/Orange/Brown) at the thermostat side are also not connected to anything at the furnace side.
- Trace the 2-conductor wiring from your OUTSIDE AC unit to the furnace. One of the conductors will be connected to the YELLOW conductor from the thermostat (most likely using a marrete). The other conductor will be connected to C side of the 24VAC transformer. My 24VAC transformer was in the very back behind all of the wired mess. But once I pushed the wires aside (DOUBLE CHECKING THE BREAKERS WERE OFF!) I could visibly see a rectangular transformer with it’s 24VAC specifications listed upon it.
- Pick any one of the unused conductors (the blue conductor its what I picked) from the thermostat wiring, and connect it along with the other conductor from the outside AC unit to the C side of the 24VAC transformer (they should all be in the same marrete).
- Make certain there are no exposed wires and turn the breakers back on.
- Test the connection with a multimeter at the thermostat end of the wiring by seeing if you detect 24VAC between the wire you just connected at the furnace (in my case blue) and the RH red wire.
- Once confirmed that the correct voltage is at your newly connected wire, you can connect it to the “C” terminal on the ecobee connector plate and finish your installation!
One last thing
If you do not have a furnace control board like I don’t, then you’ll need to ensure that the furnace blower fan is controlled by the ecobee thermostat and not the furnace. This can be done on the ecobee thermostat itself under preferences.
Do you have any “smart” installs-gone-wild stories you’d like to share? Let us know how you fixed them in the comments!
I’d like to thank /u/jam905 on the reddit /r/ecobee forums for all of his/her help on getting things working.