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C wire in wifi Thermostat form Lennox gas furnace g8d2

8156 Views 4 Replies 3 Participants Last post by  yuri
I have a Lennox gas furnace G8D2 (produced in a long time ago) and I just purchased a new Wi-Fi thermostat Honeywell TRH 6580 WF 1006/W.
The installation instruction says that wire C must be connected (to provide 20V power for the Wi-Fi thermostat, but the furnace does not have wire C (it only gas wire G).
Can anybody please advise how can I find wire C on the furnace that needs to be connected to wire C of the Thermostat?
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You will need to run a wire from the thermostat C to the C on the furnace transformer. That old furnace may not have a RWYG terminal strip.

If you have AC follow the 2 wires from outside back to the furnace. One goes to C and the other to Y from the thermostat.
R is hot, G is for the furnace fan and C is COM/common.

You can connect your C wire with the AC wire C or go back to the furnace transformer C ( max no difference ) and yes you need another wire to go to the tstat for C.
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