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Discussion Starter · #1 ·
I'm trying to connect up a Honeywell WiFi thermostat to an older furnace. The old thermostat just had 4 wires connected to it (R, Y W, and G), but the WiFi thermostat also needs a C wire. The cable running from the furnace to the thermostat does have an unused Cyan wire. I assumed that I could use the COM 24V terminal for the C wire, but the thermostat didn't power up when I tried. So, how can I get this thermostat wired up?
 

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That "extra" wire could also have been broken or damaged as well. If you have 24 at C and R at the furnace board with the interlock switch pushed in, then see if you have it at the stat, if not it's either damaged or there's possibly another junction in between where they didn't bother carrying that wire through to the thermostat.

CC
 

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Discussion Starter · #6 ·
That "extra" wire could also have been broken or damaged as well. If you have 24 at C and R at the furnace board with the interlock switch pushed in, then see if you have it at the stat, if not it's either damaged or there's possibly another junction in between where they didn't bother carrying that wire through to the thermostat.

CC
I guess that I am a bit confused. I thought the C wire was to give 24V to provide power to the WiFi thermostat. At least that is what the install instructions for the thermostat seemed to be indicating. If that is the case and R is 24V, then a voltage potential between R and C would be zero.
 

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I guess that I am a bit confused. I thought the C wire was to give 24V to provide power to the WiFi thermostat. At least that is what the install instructions for the thermostat seemed to be indicating. If that is the case and R is 24V, then a voltage potential between R and C would be zero.
Yeah, they don't explain it well. It took me a while, too.

There should be 24VAC between R and C. You need both to power a thermostat.

The old-style mechanical thermostats didn't need power. They just sent the voltage from "R" back to the furnace (or whatever) on one of the other wires.

Maybe it's easier to think of R as "hot" and C as "neutral." It may not actually be this way, compared with your house wiring, but with AC it doesn't matter. All that matters is the potential between them.
 
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