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Discussion Starter · #1 ·
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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Post pic of furnace wiring.

Your furnace has a common. The transformer will have a common post/or wire. Look at the wiring schematic on the furnace panel.
 

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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.
 

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Discussion Starter · #4 ·
is C the same as COM?

Hi,
Thanks for the advise. This is very helpful.
I found the cable coming back from AC and its connection points - there are 3 contacts, marked as COM, G and R.
Is COM the same as C?
So seems that I will need additional wire from COM (of furnace) to C of new Thermostat? (because my old thermostat does not have C at all - it has only G, RC, Y, W, RH).
 

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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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