Thermostat & HVAC C-Wire Setup

Old 09-02-14, 07:24 AM
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Thermostat & HVAC C-Wire Setup


Recently just purchased a new Honeywell WiFi Thermostat (RTH6580WF) that says it requires the C-Wire to be able to power on. When removing my old thermostat, I noticed that it does not have a C-Wire hookup and only has connections to the G, Y, W and RH wires (with a jumper wire between the RH and RC terminals). I proceeded to read on how to switch the G wire at both the thermostat and at the terminal block to the C-terminal to "correct" this, but when I got to my HVAC terminal block, I noticed that there is an extra wire that has a dual connection to the Y & C wires already, but I am not positive where it is running to (I can follow the wire into the wall, but it is not the same wire that my thermostat is using). I added two photos below. The first is of the old thermostat connection showing the G, Y, W and RH connections (in that order). The second shows the HVAC terminal block connection where I have 2 separate wires running from the block (the first wire has the Y, G, W and R connections that runs to the thermostat while the second wire has a connection the the Y & C wires...this is the wire that I cannot find where it goes after it gets into the wall). I am not positive on what to do or if I can do anything to setup this new thermostat? Any suggestions?

Thanks in advance.

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Old 09-02-14, 08:10 AM
firedawgsatx's Avatar
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To better assist you can you post the model numbers of your inside and outside units.

It is difficult to determine the exact colors on a couple of the control board terminals.Can you post the color of the wires on each terminal located next to the two wire nuts in your second photo such as:


The C wire originates from the low voltage side of your transformer and goes from the C terminal on the control board through the thermostat cable to the outdoors unit and if needed, to the C terminal on the thermostat.

Your photo shows the blue wire connected to the Y terminal on your thermostat which should go from the Y connection on your control board to the compressor outside. That is why you have two wires connected to the Y terminal at the control board; one from the thermostat and one to the condenser outside. It appears to have a red and a blue wire on the Y terminal. That would mean if the blue wire maintains color from the Y terminal on the thermostat the second wire (red?) on that terminal should continue to the condenser.

The dual wires going to the C terminal on the control board would be one wire going to the outside condenser and normally one would go to the thermostat.

Once we get the model numbers we can hopefully find a wiring diagram to see how the low voltage wiring is supposed to be connected.

Have you been able to determine where the wire connected to the C terminal at the control board goes?

I have attached typical basic low voltage wiring diagrams. The second diagram is based on your original terminal designations.

Do you have any unused wires in the thermostat cable going to the thermostat?
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Last edited by firedawgsatx; 09-02-14 at 10:00 AM.
Old 09-02-14, 09:35 PM
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There is only a four wire cable to the thermostat. The ideal thing to do would be pull a new cable. That two wire cable you are looking for goes to the outside condensor unit.

C-- (white to outside condensor)
Y--Blue (and red to condensor)

change to...
G-- short jumper to Y
C-- Green (white to outside condensor)
Y--Blue (and red to condensor)
Move green wire to C at thermostat.

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