Since my local gas and electric supplier was giving them away, I acquired a couple Google Nest thermostats. I have a blue C wire that was not used in the thermostat wiring. The thermostat works but gives a no C wire warning. This is my furnace connection. Is it as easy as connecting the C wire to the C screw on the board? The red wire currently on the C screw goes out to the A/C unit.
C wire for thermostat
- Thread starter Leo99
- Start date
No, the C-wire provides voltage/power to the thermostat
Leo99:
I have a blue C wire that was not used in the thermostat wiring. The thermostat works but gives a no C wire warning.
The "C" wire should go back to the common side of the transformer.
If you hooked it to the red on the far right of your terminal strip.... that should be the same in your case....looking at your picture.
You can double check with a volt meter..... you should read 24v between the two reds on the right.
If you hooked it to the red on the far right of your terminal strip.... that should be the same in your case....looking at your picture.
You can double check with a volt meter..... you should read 24v between the two reds on the right.
From what I've read, the thermostat will get voltage via the red wire when the system powers up, turn on the system when it needs power, or take some power from the batteries if it needs it and causes the batteries to drain quicker. The C wire eliminates this.
Yes, the far right is marked "C". It's hard to see in the picture.
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The c supplies constant voltage, the red wire on the c goes to the 24v side of the ac contactor "when the thermostat calls for ac it supplies the voltage to pull in the contactor. You can use same c terminal for constant power to power the nest. Use the blue wire at both ends the c on the terminal board and c at the t-stat.
Many thermostats can use batteries or "power stealing" for their power. It's best to connect the C wire if one is available.
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Not advisable but Some people switch the green fan wire for a c wire but you'll lose the ability to run the fan independently it'll only run on ac.. I believe that method won't work on heat pump systems. And there is a nest I think the silver higher end unit will work with 2 wire boilers not furnaces.. I think it pulses the line to trickle charge itself but I read there was situations of unreliability and the system would falsely call for heat. Otherwise I think it'll only charge during a full heat call.
The color isn't important although it's normally red. I did the same as you, used a spare BLUE wire to connect to the "C" terminal on both ends.
Correct.
From the 5 wire running between the Tstat and furnace.
Red is 24v
White is 1st stage heat
Green is Fan
Yellow is 1st stage cooling.
Blue would be for 2nd stage cooling... using it as a common is fine.
Your condensing unit is probably wired incorrectly.
From the terminal strip where the yellow is tied down, the red from the 2 wire should be tied down.
The white should be tied down on the far right where the red is now.
Just opposite of what you have.
I know, the unit is working fine.
Why does it make a difference?
If the condensing unit is fused, you are putting the fuse after everything it is supposed to protect.
Anything that shorts out can blow your transformer.
Also, anyone working on the unit will except the red to be hot.
That is the wire you would disconnect first when working on the unit.
Could cause the tech to short something out.
Before you change anything... make sure you check the wiring in the condensing unit to verify.
Seems like many systems are installed with thermostat cable that has more wires than actually needed, so even if there doesn't appear to be any extra wires, look closely in the wall and you might find them. And one of these can be used for the C wire.
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Definitely not common to find 4-conductor in new construction unless a cheap electrician got the bid to pull the low volt cabling and said let the ac guys deal with it and slap a battery tstat up.
Right. A “Dumb” thermostat can get by with internal batteries. But if you want a “Smart” thermostat you can control or monitor via wifi, a C wire is typically needed. Nest thermostats suggest they can get by without one but reviews on that claim are very mixed.
Yep, mine was built in 96 and it had a 5-wire run. I pulled an 8-wire when I needed a 6 for future expansion possibilities.
