I'm trying to determine whether I can install a wifi thermostat (Ecobee or possibly Nest) with my existing gas furnace (dual stage) and electric air conditioner, ideally using existing wiring but able to run new wires if necessary.
My existing thermostat uses a five wire cable, but it's not obvious that any of the wires are a 'C' wire. Here is the existing thermostat wiring (Honeywell RTH6400D) - the existing wires are labeled G (green), W (white), Y (orange), R (red), Rc (black):
In looking for a 'C' wire, I've used a multimeter to measure the current per some online advice and both the R and Rc wires appear to carry 27 volts (note 27 not 24).
At the furnace, the wires are mapped as follows:
Thermostat Furnace G -> G W -> W1 Y -> Y R -> R Rc -> B/C
Here are photos of the wiring at the furnace:
The Ecobee website indicates that if I had an sixth 'C' wire then my existing wiring would be compatible however I'm wondering if my existing wiring can be used (with modification) instead. If the sixth wire is needed, then where would I connect it on the furnace? The 'B/C' wire might suggest "common wire" but it is already used for the Rc (black wire).