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):

Existing Honeywell RTH6400D thermostat wiring

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:

Existing wiring at furnace Existing wiring at furnace view 2

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).

Ecobee website wiring with extra C wire

Here's the wiring diagram from the furnace panel: Furnace wiring diagram

  • Can you measure the voltage from R to B/C on your furnace? Jan 6, 2017 at 3:11
  • @ThreePhaseEel: R to B/C on the furnace board measures 27.5v. I've also added a photo of the wiring diagram from the furnace.
    – JJ.
    Jan 6, 2017 at 3:27
  • What voltage do you measure from Rh to Rc at the thermostat end? Jan 6, 2017 at 4:25
  • @ThreePhaseEel: The measurement between R and Rc on the thermostat wires is 0v. the measurement between R and any other wire (G, W or Y) or Rc and any other wire is always ~27v.
    – JJ.
    Jan 6, 2017 at 15:19
  • 1
    I highlighted the schematic for you, which might help.
    – Tester101
    Jan 9, 2017 at 14:17

2 Answers 2


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.


Schematic for Goodman AMV9/ACV9_XA furnace: enter image description here

  • Thanks @chris - I tried that (black wire B/C in C, nothing in Rc) but the Ecobee thermostat wouldn't power on and made a clicking noise. According to the Ecobee website that indicates insufficient voltage.
    – JJ.
    Jan 6, 2017 at 3:56
  • @JJ - I suppose it is possible that the B/C terminal on the board is damaged from the the prior thermostat being miswired. Find the common connection on the secondary side of the transformer (the lead with 24v relative to the "R" terminal) and connect that directly to the black tstat wire going to the new thermostat C terminal. Leave the black tstat wire disconnected from the B/C terminal in this test.
    – user39367
    Jan 6, 2017 at 15:15
  • Caution - DO NOT connect the black wire back to the Rc terminal on the old thermostat
    – user39367
    Jan 6, 2017 at 15:21

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.

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