My existing thermostat has an extra 5th wire. It's blue. We used that as the C wire. We have forced hot air so we checked the furnace and it has all the wires hooked into a clip that plugs in. It has switches designated for each wire. We turned the blue wire switch to the "on" position (it was off and all the others were on)and hooked up the blue wire to the C connector on the thermostat. Turned everything back on and no power to the thermostat. Any ideas?
Why doesn’t the extra C wire work when hooking up WiFi thermostat
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This is one possibility, from my distant past... In a house with hot-water heating that I lived in as a teen, the boiler and hot-water heating were controlled in a different way:
Whenever the water in the boiler dropped below a certain temperature, the burner came on and heated the water in the boiler jacket. This would go on as long as the system was turned on, winter, summer, whatever. None of this water circulated to the radiators, until...
When the house thermostat called for heat, the circulating pump came on and pumped the hot water from the boiler around to the radiators. Of course, if the pump returned cold water from the radiators to the boiler jacket, the burner would eventually sense this and come on to warm the circulating water.
Get rid of the external transformer, and pull a proper C wire.
It's not working because you have one side of the contactor coil connected to one transformer, and the other side connected to another transformer. There's not a complete circuit.
When the thermostat is calling for cool, your circuit looks like this
If you follow the circuit, you can see that it's not a complete circuit.
I've highlighted the R wire in red, and the C wire in blue.
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Best Answer
Check the voltage between the R wire and C wire at the thermostat. I bet it's 0.
It happens all the time... installers hook the wires backwards on the transformer. Historically, it didn't matter because transformers make AC power. Polarity doesn't matter on AC.** As long as one side of the transformer went to "thermostat" and the other side went to "furnace", it was good. The early transformers didn't even have R and C labels, that was added later as a "convention" for future expansion. And the convention is "R" to thermostat, and "C" to furnace.
Well, these smart thermostats need power for themselves. Since they're supposed to be getting "R", the instructions say connect "C". Of course if your transformer is wired backwards, the terminal labeled "C" on the transformer already goes to the thermostat, so connecting another wire does nothing.
So on "C", what the smart thermostat is looking for is the side of the transformer which goes to the furnace (and not to the thermostat). Whichever that may be.
** When I say polarity doesn't matter on AC, I mean for this little low-voltage stuff. Different deal on 120/240VAC household power because they do certain tricks to improve safety.