Replacing 2 Hunter thermostats with a Honeywell RTH9580 Wifi. Neither unit has a C wire - I'm comfortable with the upstairs high efficiency unit, but the downstairs older unit has thrown me for a loop.

The Hunter has cables connected to G-RH-Y-W, with proper color coding. There was a spare wire - so no problem, go downstairs, connect to C, and I'd have my 5 wires, good to go. But... when I got downstairs I found the "Y" connected to "C" on the unit, and the Y terminal had no wiring. I bought the house this winter, so its never run the A/C. I'm beginning to question what would happen if I flipped the switch to COOL!

There are secondary wires jumpered off that run to the compressors - but I'm not understanding how to fix the wiring. Can I just move the wire connected to "C-Y" to "Y-Y" and use the spare wire to connect to "C-C"? Or is there some reason a unit would be wired to "C" on the unit and "Y" on the thermostat??

  • A picture of the wiring would be useful. How are the control wires leading to the A/C unit connected? Are there two A/C units, as well as two heating units? What is the make and model of the heating unit you're connecting the thermostat to?
    – Tester101
    Feb 24, 2014 at 11:04
  • There are 2 wires running to the a/c unit. They are wire nutted to the RC and C wires from the main unit feed on the downstairs unit, and the upstairs unit is wire nutted to the RC and Y wires. Its really the unit that is tied into the C line that surprises me. The unit has a Y labeled lead, but nothing is connected to it.
    – Chris
    Feb 25, 2014 at 4:35

3 Answers 3


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The YELLOW or Y terminal at the furncae/ air handler is a dummy terminal for convenience, it is not required to be used, you could send Yellow directly to the AC unit out doors. Common will be used since the Common leg of power comes from the transformer which is in the furnace. The Yellow going to common likely if you trace it back it in fact is Common leg of the 24 v power supply.


No, DON'T splice the 'C' wire that goes to 'Y' -- without a clear picture of the continuity of the wires, that's not what that's asking for.

If your HVAC unit does not have a 'C' terminal, you will need to add the C (common) wire yourself. There is an answer here on Stack Exchange on adding a C wire for your thermostat. There's a wiring diagram about halfway down that answer that explains the entire process and what each wire is actually supposed to do -- in this case, the 'Y' wire that's connected to 'C' means that every time you ask for just the blower fan, the A/C compressor might come on. However, to get both the blower and the compressor fan to come on, you will need to have things wired up properly so that you ask the blower AND the compressor to come on when it's appropriate for each to come on.

You might consider getting a cheap remote continuity tester so that you can tell what wire is actually what all the way through. I've met plenty of A/C systems that have been spliced inside the wall and the wire color that is 'R' (say, a yellow one) in the HVAC unit isn't the wire that's 'R' (say, a red one) downstairs. A continuity tester ... which you should use only on wires that aren't connected to anything, will clear any potential for confusion.


enter image description here[![Below is a typical residential thermostat,it's terminals & their circuits.

Red - the 24 volt hot leg enters the thermostat White - is the Heat circuit. Yellow - is the Cool circuit. Green - is the fan circuit Common - is the neutral leg of the 24 volt power supplied by the transformer and the side of 24 volt power that every 24 volt circuit terminates to complete the circuit.

Between the Yellow,Cool & Green, Fan circuits inside the thermostat is a switch that closes connecting Yellow to Green if the thermostat is set in the Auto position. In the Auto position the fan circuit is energized when the Cool mode is energized. The fan in Heat mode is controlled by a temperature delay located in the furnace.

The transformer reduces 120 volts to 24 volts,a safe voltage. The 24 volt supply like every electric circuit has a "hot leg" and a "neutral" leg or "side" of power,like a car battery has a Positive and a Negative terminal,The neutral leg of 24 volt power is named"Common" because all 24 vol

The "hot leg" of 24 volt power supply is Red, it enters the thermostat on the Red terminal. The thermostat distributes the 24 volt hot leg to the desired circuit. Upon a call for heat the switch between Red and White will close sending 24 volts to the gas valve, from the gas valve it returns to Common to complete the circuit. Upon a call for Cool the switch between Red and Yellow closes. Upon a call for Fan the switch between Red and Green closes]2]2

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