C wire in wifi Thermostat form Lennox gas furnace g8d2

8164 Views 4 Replies 3 Participants Last post by yuri, Jan 10, 2016

tenteris

Discussion Starter · Jan 10, 2016

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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tenteris

Discussion Starter · #4 · Jan 10, 2016

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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