HVAC wiring for wifi thermostat installation (Ecobee) gas furnace + AC

Question

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).

Here's the wiring diagram from the furnace panel:

Answer 1

It is normal for a 24vac control transformer to measure somewhat high under no load (i.e. 27v vs 24v). It is 24vac system.

"B/C" on the furnace should be the common. I am surprised the existing thermostat works connected like it is. The only thing I can think of is that the thermostat is internally configured for a single power supply system and the "Rc" terminal is open instead of being shunted to Rh as is the case with many thermostats. In this case if Rc were shunted to Rh it would pop the overload protection on the furnace board or the transformer.

Should be good to go on connecting the black wire to C on your new thermostat. Do not connect any of the wires to Rc on the new thermostat.

Edit:

Schematic for Goodman AMV9/ACV9_XA furnace:

Answer 2

I ended up finding the problem - the black (Rc) and red (R) wires going to my thermostat were joined together in a junction box between the furnace and the thermostat (why I don't know).

I've now separated the black from the red, and installed the new thermostat with black as 'C' and red as 'Rh'. It's working great!.

Thanks to @chris and @ThreePhaseEei for their suggestions.