Honeywell Wi-Fi Thermostat - Need "C" Wire


Old 12-22-14, 12:05 PM
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Honeywell Wi-Fi Thermostat - Need "C" Wire

Hi All,

A common issue, I am sure. I am trying to install a newly purchased Honeywell Model RTH6580WF Wi-Fi thermostat. I have W R Y & G installed at my old thermostat, and now need a "C" wire.

My furnace is a Lennox G20Q3/4E-100-1, and on the control board, I have terminals labeled W R Y T G.

On page #16 of the installation manual (URL below), I see the terminals labeled both W R Y T G as well as W R Y C G.

Can someone "in the know" please verify for me that the "C" wire of the new thermostat should be connected to the "T" terminal on the control board of the furnace.

Also, can I use 24AWG Cat-5 (Ethernet) wiring (I have lots of 4-pair wire lying around) to go from the control board of the furnace to the thermostat?

Many thanks!

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Last edited by Johnnyboy; 12-22-14 at 01:11 PM.
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Old 12-22-14, 01:05 PM
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There should be two wires going outside to the a/c condenser. One wire will be connected to the Y terminal on the terminal strip and the other one will be connected to the common wire (which is what you are looking for) terminal. From the photo you posted it looks like it is probably the T terminal. If you trace that wire on the T terminal to see if it goes outside to the a/c condenser that is the terminal you will connect your C wire for the thermostat you want to install.

I would not recommend using the 24AWG wire for the thermostat. While it would probably work, it is not rated for that application. It would be much better to run an 18AWG wire. On page 15 (#6 under Electrical) of the installation guide you linked it states to use 18AWG or larger for thermostat connections.

Make sure to turn off power to furnace before making wiring connections.
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