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Pretty common problem here but the posts I've seen are a little different. New Wi-Fi tstat, no C terminal at furnace but plenty of available wiring at both ends. Unlike others problems posted of unwiring and rewiring, couldn't I just take an available wire and combine it to my G connection at the terminal and then connect wire at tstat? If G controls the fan and I know there is power because I can turn on fan manually, is this the easiest fix?

Thanks

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No, because the G terminal gets power from the R. When you turn on the fan on ther thermostat, the thermostat is really just connecting the R to the G terminal.

Internally on the furnace control board, the other side of the relay that actually powers the fan is connected to C. R and C come from the two sides of a 24V AC transformer.

There's almost always a way to get C from any control board, it's just a matter of identifying a C wire from a schematic, or by finding the transformer and identifying how it's wired in.

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  • Thanks Greg. I've identified the 24v connectors coming from transformer. One is red of course but then there are two blue ones. One going to one of the available slots on the 9 pin connector, which I believe goes to ground and one blue going from transformer to some gas control. Could I wire available tstat wire to one of these?
    – NikM
    Commented Jul 13, 2015 at 13:37

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